32 SWIG and Ruby

This chapter describes SWIG's support of Ruby.

32.1 Preliminaries

SWIG 1.3 is known to work with Ruby versions 1.6 and later. Given the choice, you should use the latest stable version of Ruby. You should also determine if your system supports shared libraries and dynamic loading. SWIG will work with or without dynamic loading, but the compilation process will vary.

This chapter covers most SWIG features, but in less depth than is found in earlier chapters. At the very least, make sure you also read the "SWIG Basics" chapter. It is also assumed that the reader has a basic understanding of Ruby.

32.1.1 Running SWIG

To build a Ruby module, run SWIG using the -ruby option:

$ swig -ruby example.i

If building a C++ extension, add the -c++ option: 

$ swig -c++ -ruby example.i

This creates a file example_wrap.c (example_wrap.cxx if compiling a C++ extension) that contains all of the code needed to build a Ruby extension module. To finish building the module, you need to compile this file and link it with the rest of your program.

32.1.2 Getting the right header files

In order to compile the wrapper code, the compiler needs the ruby.h header file. This file is usually contained in a directory such as 

/usr/lib/ruby/1.8/x86_64-linux-gnu/ruby.h
/usr/local/lib/ruby/1.6/i686-linux/ruby.h

The exact location may vary on your machine, but the above location is typical. If you are not entirely sure where Ruby is installed, you can run Ruby to find out. For example: 

$ ruby -e 'puts $:.join("\n")'
/usr/local/lib/ruby/site_ruby/1.6 /usr/local/lib/ruby/site_ruby/1.6/i686-linux
/usr/local/lib/ruby/site_ruby /usr/local/lib/ruby/1.6 /usr/local/lib/ruby/1.6/i686-linux .

32.1.3 Compiling a dynamic module

Ruby extension modules are typically compiled into shared libraries that the interpreter loads dynamically at runtime. Since the exact commands for doing this vary from platform to platform, your best bet is to follow the steps described in the README.EXT file from the Ruby distribution:

  1. Create a file called extconf.rb that looks like the following:

    require 'mkmf'
    create_makefile('example')

  2. Type the following to build the extension:

    $ ruby extconf.rb
    $ make
    $ make install

Of course, there is the problem that mkmf does not work correctly on all platforms, e.g, HPUX. If you need to add your own make rules to the file that extconf.rb produces, you can add this: 

open("Makefile", "a") { |mf|
 puts <<EOM
 # Your make rules go here
 EOM
}

to the end of the extconf.rb file. If for some reason you don't want to use the standard approach, you'll need to determine the correct compiler and linker flags for your build platform. For example, a typical sequence of commands for the Linux operating system would look something like this: 

$ swig -ruby example.i
$ gcc -c example.c
$ gcc -c example_wrap.c -I/usr/local/lib/ruby/1.6/i686-linux 
$ gcc -shared example.o example_wrap.o -o example.so

For other platforms it may be necessary to compile with the -fPIC option to generate position-independent code. If in doubt, consult the manual pages for your compiler and linker to determine the correct set of options. You might also check the SWIG Wiki for additional information.

32.1.4 Using your module

Ruby module names must be capitalized, but the convention for Ruby feature names is to use lowercase names. So, for example, the Etc extension module is imported by requiring the etc feature: 

# The feature name begins with a lowercase letter...
require 'etc'

# ... but the module name begins with an uppercase letter
puts "Your login name: #{Etc.getlogin}"

To stay consistent with this practice, you should always specify a lowercase module name with SWIG's %module directive. SWIG will automatically correct the resulting Ruby module name for your extension. So for example, a SWIG interface file that begins with: 

%module example

will result in an extension module using the feature name "example" and Ruby module name "Example".

32.1.5 Static linking

An alternative approach to dynamic linking is to rebuild the Ruby interpreter with your extension module added to it. In the past, this approach was sometimes necessary due to limitations in dynamic loading support on certain machines. However, the situation has improved greatly over the last few years and you should not consider this approach unless there is really no other option.

The usual procedure for adding a new module to Ruby involves finding the Ruby source, adding an entry to the ext/Setup file, adding your directory to the list of extensions in the file, and finally rebuilding Ruby.

32.1.6 Compilation of C++ extensions

On most machines, C++ extension modules should be linked using the C++ compiler. For example: 

$ swig -c++ -ruby example.i
$ g++ -c example.cxx
$ g++ -c example_wrap.cxx -I/usr/local/lib/ruby/1.6/i686-linux
$ g++ -shared example.o example_wrap.o -o example.so

If you've written an extconf.rb script to automatically generate a Makefile for your C++ extension module, keep in mind that (as of this writing) Ruby still uses gcc and not g++ as its linker. As a result, the required C++ runtime library support will not be automatically linked into your extension module and it may fail to load on some platforms. A workaround for this problem is use the mkmf module's append_library() method to add one of the C++ runtime libraries to the list of libraries linked into your extension, e.g. 

require 'mkmf'
$libs = append_library($libs, "supc++")
create_makefile('example')

32.2 Building Ruby Extensions under Windows 95/NT

Building a SWIG extension to Ruby under Windows 95/NT is roughly similar to the process used with Unix. Normally, you will want to produce a DLL that can be loaded into the Ruby interpreter. For all recent versions of Ruby, the procedure described above (i.e. using an extconf.rb script) will work with Windows as well; you should be able to build your code into a DLL by typing: 

C:\swigtest> ruby extconf.rb
C:\swigtest> nmake
C:\swigtest> nmake install

The remainder of this section covers the process of compiling SWIG-generated Ruby extensions with Microsoft Visual C++ 6 (i.e. within the Developer Studio IDE, instead of using the command line tools). In order to build extensions, you may need to download the source distribution to the Ruby package, as you will need the Ruby header files.

32.2.1 Running SWIG from Developer Studio

If you are developing your application within Microsoft developer studio, SWIG can be invoked as a custom build option. The process roughly follows these steps :

Now, assuming all went well, SWIG will be automatically invoked when you build your project. Any changes made to the interface file will result in SWIG being automatically invoked to produce a new version of the wrapper file. To run your new Ruby extension, simply run Ruby and use the require command as normal. For example if you have this ruby file run.rb:

# file: run.rb
require 'Example'

# Call a c function
print "Foo = ", Example.Foo, "\n"

Ensure the dll just built is in your path or current directory, then run the Ruby script from the DOS/Command prompt: 

C:\swigtest> ruby run.rb
Foo = 3.0

32.3 The Ruby-to-C/C++ Mapping

This section describes the basics of how SWIG maps C or C++ declarations in your SWIG interface files to Ruby constructs.

32.3.1 Modules

The SWIG %module directive specifies the name of the Ruby module. If you specify: 

%module example

then everything is wrapped into a Ruby module named Example that is nested directly under the global module. You can specify a more deeply nested module by specifying the fully-qualified module name in quotes, e.g. 

%module "foo::bar::spam"

An alternate method of specifying a nested module name is to use the -prefix option on the SWIG command line. The prefix that you specify with this option will be prepended to the module name specified with the %module directive in your SWIG interface file. So for example, this declaration at the top of your SWIG interface file:

%module "foo::bar::spam"

will result in a nested module name of Foo::Bar::Spam, but you can achieve the same effect by specifying:

%module spam

and then running SWIG with the -prefix command line option:

$ swig -ruby -prefix "foo::bar::" example.i

Starting with SWIG 1.3.20, you can also choose to wrap everything into the global module by specifying the -globalmodule option on the SWIG command line, i.e. 

$ swig -ruby -globalmodule example.i

Note that this does not relieve you of the requirement of specifying the SWIG module name with the %module directive (or the -module command-line option) as described earlier.

When choosing a module name, do not use the same name as a built-in Ruby command or standard module name, as the results may be unpredictable. Similarly, if you're using the -globalmodule option to wrap everything into the global module, take care that the names of your constants, classes and methods don't conflict with any of Ruby's built-in names.

32.3.2 Functions

Global functions are wrapped as Ruby module methods. For example, given the SWIG interface file example.i: 

%module example

int fact(int n);

and C source file example.c: 

int fact(int n) {
 if (n == 0)
 return 1;
 return (n * fact(n-1));
}

SWIG will generate a method fact in the Example module that can be used like so: 

$ irb
irb(main):001:0> require 'example'
true
irb(main):002:0> Example.fact(4)
24

32.3.3 Variable Linking

C/C++ global variables are wrapped as a pair of singleton methods for the module: one to get the value of the global variable and one to set it. For example, the following SWIG interface file declares two global variables: 

// SWIG interface file with global variables
%module example
...
%inline %{
extern int variable1;
extern double Variable2;
%}
...

Now look at the Ruby interface:

$ irb
irb(main):001:0> require 'Example'
true
irb(main):002:0> Example.variable1 = 2
2
irb(main):003:0> Example.Variable2 = 4 * 10.3
41.2
irb(main):004:0> Example.Variable2
41.2

If you make an error in variable assignment, you will receive an error message. For example: 

irb(main):005:0> Example.Variable2 = "hello"
TypeError: no implicit conversion to float from string
from (irb):5:in `Variable2='
from (irb):5

If a variable is declared as const, it is wrapped as a read-only variable. Attempts to modify its value will result in an error.

To make ordinary variables read-only, you can also use the %immutable directive. For example: 

%immutable;
%inline %{
extern char *path;
%}
%mutable;

The %immutable directive stays in effect until it is explicitly disabled using %mutable.

32.3.4 Constants

C/C++ constants are wrapped as module constants initialized to the appropriate value. To create a constant, use #define or the %constant directive. For example: 

#define PI 3.14159
#define VERSION "1.0"

%constant int FOO = 42;
%constant const char *path = "/usr/local";

const int BAR = 32;

Remember to use the :: operator in Ruby to get at these constant values, e.g. 

$ irb
irb(main):001:0> require 'Example'
true
irb(main):002:0> Example::PI
3.14159

32.3.5 Pointers

"Opaque" pointers to arbitrary C/C++ types (i.e. types that aren't explicitly declared in your SWIG interface file) are wrapped as data objects. So, for example, consider a SWIG interface file containing only the declarations: 

Foo *get_foo();
void set_foo(Foo *foo);

For this case, the get_foo() method returns an instance of an internally generated Ruby class: 

irb(main):001:0> foo = Example::get_foo()
#<SWIG::TYPE_p_Foo:0x402b1654>

A NULL pointer is always represented by the Ruby nil object.

32.3.6 Structures

C/C++ structs are wrapped as Ruby classes, with accessor methods (i.e. "getters" and "setters") for all of the struct members. For example, this struct declaration: 

struct Vector {
 double x, y;
};

gets wrapped as a Vector class, with Ruby instance methods x, x=, y and y=. These methods can be used to access structure data from Ruby as follows: 

$ irb
irb(main):001:0> require 'Example'
true
irb(main):002:0> f = Example::Vector.new
#<Example::Vector:0x4020b268>
irb(main):003:0> f.x = 10
nil
irb(main):004:0> f.x
10.0

Similar access is provided for unions and the public data members of C++ classes.

const members of a structure are read-only. Data members can also be forced to be read-only using the %immutable directive (in C++, private may also be used). For example: 

struct Foo {
 ...
 %immutable;
 int x; /* Read-only members */
 char *name;
 %mutable;
 ...
};

When char * members of a structure are wrapped, the contents are assumed to be dynamically allocated using malloc or new (depending on whether or not SWIG is run with the -c++ option). When the structure member is set, the old contents will be released and a new value created. If this is not the behavior you want, you will have to use a typemap (described shortly).

Array members are normally wrapped as read-only. For example, this code: 

struct Foo {
 int x[50];
};

produces a single accessor function like this: 

int *Foo_x_get(Foo *self) {
 return self->x;
};

If you want to set an array member, you will need to supply a "memberin" typemap described in the section on typemaps. As a special case, SWIG does generate code to set array members of type char (allowing you to store a Ruby string in the structure).

When structure members are wrapped, they are handled as pointers. For example, 

struct Foo {
 ...
};

struct Bar {
 Foo f;
};

generates accessor functions such as this: 

Foo *Bar_f_get(Bar *b) {
 return &b->f;
}

void Bar_f_set(Bar *b, Foo *val) {
 b->f = *val;
}

32.3.7 C++ classes

Like structs, C++ classes are wrapped by creating a new Ruby class of the same name with accessor methods for the public class member data. Additionally, public member functions for the class are wrapped as Ruby instance methods, and public static member functions are wrapped as Ruby singleton methods. So, given the C++ class declaration: 

class List {
public:
 List();
 ~List();
 int search(char *item);
 void insert(char *item);
 void remove(char *item);
 char *get(int n);
 int length;
 static void print(List *l);
};

SWIG would create a List class with:

In Ruby, these functions are used as follows: 

require 'Example'

l = Example::List.new

l.insert("Ale")
l.insert("Stout")
l.insert("Lager")
Example.print(l)
l.length()
----- produces the following output 
Lager
Stout
Ale
3

32.3.8 C++ Inheritance

The SWIG type-checker is fully aware of C++ inheritance. Therefore, if you have classes like this: 

class Parent {
 ...
};

class Child : public Parent {
 ...
};

those classes are wrapped into a hierarchy of Ruby classes that reflect the same inheritance structure. All of the usual Ruby utility methods work normally: 

irb(main):001:0> c = Child.new
#<Bar:0x4016efd4>
irb(main):002:0> c.instance_of? Child
true
irb(main):003:0> b.instance_of? Parent
false
irb(main):004:0> b.is_a? Child
true
irb(main):005:0> b.is_a? Parent
true
irb(main):006:0> Child < Parent
true
irb(main):007:0> Child > Parent
false

Furthermore, if you have a function like this: 

void spam(Parent *f);

then the function spam() accepts Parent* or a pointer to any class derived from Parent.

Until recently, the Ruby module for SWIG didn't support multiple inheritance, and this is still the default behavior. This doesn't mean that you can't wrap C++ classes which inherit from multiple base classes; it simply means that only the first base class listed in the class declaration is considered, and any additional base classes are ignored. As an example, consider a SWIG interface file with a declaration like this: 

class Derived : public Base1, public Base2
{
 ...
};

For this case, the resulting Ruby class (Derived) will only consider Base1 as its superclass. It won't inherit any of Base2's member functions or data and it won't recognize Base2 as an "ancestor" of Derived (i.e. the is_a? relationship would fail). When SWIG processes this interface file, you'll see a warning message like: 

example.i:5: Warning(802): Warning for Derived: Base Base2 ignored.
Multiple inheritance is not supported in Ruby.

Starting with SWIG 1.3.20, the Ruby module for SWIG provides limited support for multiple inheritance. Because the approach for dealing with multiple inheritance introduces some limitations, this is an optional feature that you can activate with the -minherit command-line option: 

$ swig -c++ -ruby -minherit example.i

Using our previous example, if your SWIG interface file contains a declaration like this: 

class Derived : public Base1, public Base2
{
 ...
};

and you run SWIG with the -minherit command-line option, then you will end up with a Ruby class Derived that appears to "inherit" the member data and functions from both Base1 and Base2. What actually happens is that three different top-level classes are created, with Ruby's Object class as their superclass. Each of these classes defines a nested module named Impl, and it's in these nested Impl modules that the actual instance methods for the classes are defined, i.e. 

class Base1
 module Impl
 # Define Base1 methods here
 end
 include Impl
end

class Base2
 module Impl
 # Define Base2 methods here
 end
 include Impl
end

class Derived
 module Impl
 include Base1::Impl
 include Base2::Impl
 # Define Derived methods here
 end
 include Impl
end

Observe that after the nested Impl module for a class is defined, it is mixed-in to the class itself. Also observe that the Derived::Impl module first mixes-in its base classes' Impl modules, thus "inheriting" all of their behavior.

The primary drawback is that, unlike the default mode of operation, neither Base1 nor Base2 is a true superclass of Derived anymore: 

obj = Derived.new
obj.is_a? Base1 # this will return false...
obj.is_a? Base2 # ... and so will this

In most cases, this is not a serious problem since objects of type Derived will otherwise behave as though they inherit from both Base1 and Base2 (i.e. they exhibit "Duck Typing").

32.3.9 C++ Overloaded Functions

C++ overloaded functions, methods, and constructors are mostly supported by SWIG. For example, if you have two functions like this: 

void foo(int);
void foo(char *c);

You can use them in Ruby in a straightforward manner: 

irb(main):001:0> foo(3) # foo(int)
irb(main):002:0> foo("Hello") # foo(char *c)

Similarly, if you have a class like this,

class Foo {
public:
 Foo();
 Foo(const Foo &);
 ...
};

you can write Ruby code like this:

irb(main):001:0> f = Foo.new # Create a Foo
irb(main):002:0> g = Foo.new(f) # Copy f

Overloading support is not quite as flexible as in C++. Sometimes there are methods that SWIG can't disambiguate. For example: 

void spam(int);
void spam(short);

or

void foo(Bar *b);
void foo(Bar &b);

If declarations such as these appear, you will get a warning message like this: 

example.i:12: Warning(509): Overloaded spam(short) is shadowed by spam(int)
at example.i:11.

To fix this, you either need to ignore or rename one of the methods. For example: 

%rename(spam_short) spam(short);
...
void spam(int); 
void spam(short); // Accessed as spam_short

or

%ignore spam(short);
...
void spam(int); 
void spam(short); // Ignored

SWIG resolves overloaded functions and methods using a disambiguation scheme that ranks and sorts declarations according to a set of type-precedence rules. The order in which declarations appear in the input does not matter except in situations where ambiguity arises--in this case, the first declaration takes precedence.

Please refer to the "SWIG and C++" chapter for more information about overloading.

32.3.10 C++ Operators

For the most part, overloaded operators are handled automatically by SWIG and do not require any special treatment on your part. So if your class declares an overloaded addition operator, e.g. 

class Complex {
 ...
 Complex operator+(Complex &);
 ...
};

the resulting Ruby class will also support the addition (+) method correctly.

For cases where SWIG's built-in support is not sufficient, C++ operators can be wrapped using the %rename directive (available on SWIG 1.3.10 and later releases). All you need to do is give the operator the name of a valid Ruby identifier. For example: 

%rename(add_complex) operator+(Complex &, Complex &);
...
Complex operator+(Complex &, Complex &);

Now, in Ruby, you can do this:

a = Example::Complex.new(2, 3)
b = Example::Complex.new(4, -1)
c = Example.add_complex(a, b)

More details about wrapping C++ operators into Ruby operators is discussed in the section on operator overloading.

32.3.11 C++ namespaces

SWIG is aware of C++ namespaces, but namespace names do not appear in the module nor do namespaces result in a module that is broken up into submodules or packages. For example, if you have a file like this, 

%module example

namespace foo {
 int fact(int n);
 struct Vector {
 double x,y,z;
 };
};

it works in Ruby as follows:

irb(main):001:0> require 'example'
true
irb(main):002:0> Example.fact(3)
6
irb(main):003:0> v = Example::Vector.new
#<Example::Vector:0x4016f4d4>
irb(main):004:0> v.x = 3.4
3.4
irb(main):004:0> v.y
0.0

If your program has more than one namespace, name conflicts (if any) can be resolved using %rename For example: 

%rename(Bar_spam) Bar::spam;

namespace Foo {
 int spam();
}

namespace Bar {
 int spam();
}

If you have more than one namespace and your want to keep their symbols separate, consider wrapping them as separate SWIG modules. For example, make the module name the same as the namespace and create extension modules for each namespace separately. If your program utilizes thousands of small deeply nested namespaces each with identical symbol names, well, then you get what you deserve.

32.3.12 C++ templates

C++ templates don't present a huge problem for SWIG. However, in order to create wrappers, you have to tell SWIG to create wrappers for a particular template instantiation. To do this, you use the %template directive. For example: 

%module example

%{
#include "pair.h"
%}

template<class T1, class T2>
struct pair {
 typedef T1 first_type;
 typedef T2 second_type;
 T1 first;
 T2 second;
 pair();
 pair(const T1&, const T2&);
 ~pair();
};

%template(Pairii) pair<int,int>;

In Ruby:

irb(main):001:0> require 'example'
true
irb(main):002:0> p = Example::Pairii.new(3, 4)
#<Example:Pairii:0x4016f4df>
irb(main):003:0> p.first
3
irb(main):004:0> p.second
4

32.3.13 C++ Standard Template Library (STL)

On a related note, the standard SWIG library contains a number of modules that provide typemaps for standard C++ library classes (such as std::pair, std::string and std::vector). These library modules don't provide wrappers around the templates themselves, but they do make it convenient for users of your extension module to pass Ruby objects (such as arrays and strings) to wrapped C++ code that expects instances of standard C++ templates. For example, suppose the C++ library you're wrapping has a function that expects a vector of floats: 

%module example

float sum(const std::vector<float>& values);

Rather than go through the hassle of writing an "in" typemap to convert an array of Ruby numbers into a std::vector<float>, you can just use the std_vector.i module from the standard SWIG library: 

%module example

%include std_vector.i
float sum(const std::vector<float>& values);

Ruby's STL wrappings provide additional methods to make them behave more similarly to Ruby's native classes.

Thus, you can do, for example:

v = IntVector.new
v << 2
v << 3
v << 4
v.each { |x| puts x }

=> 2
3
4
v.delete_if { |x| x == 3 }
=> [2,4]

The SWIG Ruby module provides also the ability for all the STL containers to carry around Ruby native objects (Fixnum, Classes, etc) making them act almost like Ruby's own Array, Hash, etc.   To do that, you need to define a container that contains a swig::GC_VALUE, like:

%module nativevector

%{
std::vector< swig::GC_VALUE > NativeVector;
%}

%template(NativeVector) std::vector< swig::GC_VALUE >;

This vector can then contain any Ruby object, making them almost identical to Ruby's own Array class.

require 'nativevector'
include NativeVector

v = NativeVector.new
v << 1
v << [1,2]
v << 'hello'

class A; end

v << A.new

puts v
=> [1, [1,2], 'hello', #<A:0x245325>]

Obviously, there is a lot more to template wrapping than shown in these examples. More details can be found in the SWIG and C++ chapter.

32.3.14 C++ STL Functors

Some containers in the STL allow you to modify their default behavior by using so called functors or function objects.  Functors are often just a very simple struct with operator() redefined or an actual C/C++ function.  This allows you, for example, to always keep the sort order of a STL container to your liking.

The Ruby STL mappings allows you to modify those containers that support functors using Ruby procs or methods, instead.  Currently, this includes std::set, set::map, std::multiset and std::multimap.

The functors in swig are called swig::UnaryFunction and swig::BinaryFunction.
For C++ predicates (ie. functors that must return bool as a result) swig::UnaryPredicate and swig::BinaryPredicate are provided.

As an example, if given this swig file:

%module intset;

%include <std_set.i>

%typemap(IntSet)  std::set< int, swig::BinaryPredicate >;

You can then use the set from Ruby with or without a proc object as a predicate:

require 'intset'
include Intset

# Default sorting behavior defined in C++
a = IntSet.new
a << 1
a << 2
a << 3
a
=>  [1,2,3]

# Custom sorting behavior defined by a Ruby proc
b = IntSet.new( proc { |a,b| a > b } )
b << 1
b << 2
b << 3
b
=>  [3,2,1]

32.3.15 C++ STL Iterators

The STL is well known for the use of iterators.  There are a number of iterators possible with different properties, but in general there are two main categories: const iterators and non-const iterators.  The const iterators can access and not modify the values they point at, while the non-const iterators can both read and modify the values.

The Ruby STL wrappings support both type of iterators by using a proxy class in-between.  This proxy class is swig::Iterator or swig::ConstIterator.  Derived from them are template classes that need to be initialized with the actual iterator for the container you are wrapping and often times with the beginning and ending points of the iteration range. 

The SWIG STL library already provides typemaps to all the standard containers to do this wrapping automatically for you, but if you have your own STL-like iterator, you will need to write your own typemap for them.  For out typemaps, the special functions make_const_iterator and make_nonconst_iterator are provided.

These can be used either like:

make_const_iterator( iterator, rubyclass );
make_const_iterator( iterator, iterator_begin, iterator_end, rubyclass );

The iterators support a next() and previous() member function to just change the iterator without returning anything.  previous() should obviously only be used for bidirectional iterators.  You can also advance the iterator multiple steps by using standard math operations like +=.

The value the iterator points at can be accessed with value() -- this is equivalent to dereferencing it with *i.   For non-const iterators, a value=() function is also provided which allows you to change the value pointed by the iterator.  This is equivalent to the C++ construct of dereferencing and assignment, like *i = something. 

Thus, given say a vector class of doubles defined as:

%module doublevector

%include std_vector.i

%template(DoubleVector) std::vector<double>;

Its iterator can then be used from Ruby like:

require 'doublevector'
include Doublevector

v = DoubleVector.new
v << 1
v << 2
v << 3

#
# an elaborate and less efficient way of doing v.map! { |x| x+2 }
#
i = v.begin
e = v.end
while i != e
  val = i.value
  val += 2
  i.value = val
  i.next
end
i
>> [3, 4, 5 ]

If you'd rather have STL classes without any iterators, you should define -DSWIG_NO_EXPORT_ITERATOR_METHODS when running swig.

32.3.16 C++ Smart Pointers

In certain C++ programs, it is common to use classes that have been wrapped by so-called "smart pointers." Generally, this involves the use of a template class that implements operator->() like this: 

template<class T> class SmartPtr {
 ...
 T *operator->();
 ...
}

Then, if you have a class like this,

class Foo {
public:
 int x;
 int bar();
};

A smart pointer would be used in C++ as follows:

SmartPtr<Foo> p = CreateFoo(); // Created somehow (not shown)
...
p->x = 3; // Foo::x
int y = p->bar(); // Foo::bar

To wrap this in Ruby, simply tell SWIG about the SmartPtr class and the low-level Foo object. Make sure you instantiate SmartPtr using %template if necessary. For example: 

%module example
...
%template(SmartPtrFoo) SmartPtr<Foo>;
...

Now, in Ruby, everything should just "work":

irb(main):001:0> p = Example::CreateFoo() # Create a smart-pointer somehow
#<Example::SmartPtrFoo:0x4016f4df>
irb(main):002:0> p.x = 3 # Foo::x
3
irb(main):003:0> p.bar() # Foo::bar

If you ever need to access the underlying pointer returned by operator->() itself, simply use the __deref__() method. For example: 

irb(main):004:0> f = p.__deref__() # Returns underlying Foo *

32.3.17 Cross-Language Polymorphism

SWIG's Ruby module supports cross-language polymorphism (a.k.a. the "directors" feature) similar to that for SWIG's Python module. Rather than duplicate the information presented in the Python chapter, this section just notes the differences that you need to be aware of when using this feature with Ruby.

32.3.17.1 Exception Unrolling

Whenever a C++ director class routes one of its virtual member function calls to a Ruby instance method, there's always the possibility that an exception will be raised in the Ruby code. By default, those exceptions are ignored, which simply means that the exception will be exposed to the Ruby interpreter. If you would like to change this behavior, you can use the %feature("director:except") directive to indicate what action should be taken when a Ruby exception is raised. The following code should suffice in most cases: 

%feature("director:except") {
 throw Swig::DirectorMethodException($error);
}

When this feature is activated, the call to the Ruby instance method is "wrapped" using the rb_rescue2() function from Ruby's C API. If any Ruby exception is raised, it will be caught here and a C++ exception is raised in its place.

32.4 Naming

Ruby has several common naming conventions. Constants are generally in upper case, module and class names are in camel case and methods are in lower case with underscores. For example:

Prior to version 1.3.28, SWIG did not support these Ruby conventions. The only modifications it made to names was to capitalize the first letter of constants (which includes module and class names).

SWIG 1.3.28 introduces the new -autorename command line parameter. When this parameter is specified, SWIG will automatically change constant, class and method names to conform with the standard Ruby naming conventions. For example: 

$ swig -ruby -autorename example.i

To disable renaming use the -noautorename command line option.

Since this change significantly changes the wrapper code generated by SWIG, it is turned off by default in SWIG 1.3.28. However, it is planned to become the default option in future releases.

32.4.1 Defining Aliases

It's a fairly common practice in the Ruby built-ins and standard library to provide aliases for method names. For example, Array#size is an alias for Array#length. If you would like to provide an alias for one of your class' instance methods, one approach is to use SWIG's %extend directive to add a new method of the aliased name that calls the original function. For example: 

class MyArray {
public:
 // Construct an empty array
 MyArray();
 
 // Return the size of this array
 size_t length() const;
};

%extend MyArray {
 // MyArray#size is an alias for MyArray#length
 size_t size() const {
 return $self->length();
 }
}

A better solution is to use the %alias directive (unique to SWIG's Ruby module). The previous example could then be rewritten as: 

// MyArray#size is an alias for MyArray#length
%alias MyArray::length "size";

class MyArray {
public:
 // Construct an empty array
 MyArray();
 
 // Return the size of this array
 size_t length() const;
};

Multiple aliases can be associated with a method by providing a comma-separated list of aliases to the %alias directive, e.g. 

%alias MyArray::length "amount,quantity,size";

From an end-user's standpoint, there's no functional difference between these two approaches; i.e. they should get the same result from calling either MyArray#size or MyArray#length. However, when the %alias directive is used, SWIG doesn't need to generate all of the wrapper code that's usually associated with added methods like our MyArray::size() example.

Note that the %alias directive is implemented using SWIG's "features" mechanism and so the same name matching rules used for other kinds of features apply (see the chapter on "Customization Features") for more details).

32.4.2 Predicate Methods

Ruby methods that return a boolean value and end in a question mark are known as predicate methods. Examples of predicate methods in standard Ruby classes include Array#empty? (which returns true for an array containing no elements) and Object#instance_of? (which returns true if the object is an instance of the specified class). For consistency with Ruby conventions, methods that return boolean values should be marked as predicate methods.

One cumbersome solution to this problem is to rename the method (using SWIG's %rename directive) and provide a custom typemap that converts the function's actual return type to Ruby's true or false. For example: 

%rename("is_it_safe?") is_it_safe();

%typemap(out) int is_it_safe 
 "$result = ($1 != 0) ? Qtrue : Qfalse;";

int is_it_safe();

A better solution is to use the %predicate directive (unique to SWIG's Ruby module) to designate a method as a predicate method. For the previous example, this would look like: 

%predicate is_it_safe();

int is_it_safe();

This method would be invoked from Ruby code like this:

irb(main):001:0> Example::is_it_safe?
true

The %predicate directive is implemented using SWIG's "features" mechanism and so the same name matching rules used for other kinds of features apply (see the chapter on "Customization Features") for more details).

32.4.3 Bang Methods

Ruby methods that modify an object in-place and end in an exclamation mark are known as bang methods. An example of a bang method is Array#sort! which changes the ordering of items in an array. Contrast this with Array#sort, which returns a copy of the array with the items sorted instead of modifying the original array. For consistency with Ruby conventions, methods that modify objects in place should be marked as bang methods.

Bang methods can be marked using the %bang directive which is unique to the Ruby module and was introduced in SWIG 1.3.28. For example:

%bang sort!(arr);

int sort(int arr[]);

This method would be invoked from Ruby code like this:

irb(main):001:0> Example::sort!(arr)

The %bang directive is implemented using SWIG's "features" mechanism and so the same name matching rules used for other kinds of features apply (see the chapter on "Customization Features") for more details).

32.4.4 Getters and Setters

Often times a C++ library will expose properties through getter and setter methods. For example:

class Foo {
	Foo() {}

 int getValue() { return value_; }

 void setValue(int value) { value_ = value; }

private:
 int value_;
};

By default, SWIG will expose these methods to Ruby as get_value and set_value. However, it more natural for these methods to be exposed in Ruby as value and value=. That allows the methods to be used like this:

irb(main):001:0> foo = Foo.new()
irb(main):002:0> foo.value = 5
irb(main):003:0> puts foo.value

This can be done by using the %rename directive:

%rename("value") Foo::getValue();
%rename("value=") Foo::setValue(int value);

 

32.5 Input and output parameters

A common problem in some C programs is handling parameters passed as simple pointers. For example: 

void add(int x, int y, int *result) {
 *result = x + y;
}
or
int sub(int *x, int *y) {
 return *x-*y;
}

The easiest way to handle these situations is to use the typemaps.i file. For example: 

%module Example
%include "typemaps.i"

void add(int, int, int *OUTPUT);
int sub(int *INPUT, int *INPUT);

In Ruby, this allows you to pass simple values. For example:

a = Example.add(3,4)
puts a
7
b = Example.sub(7,4)
puts b
3

Notice how the INPUT parameters allow integer values to be passed instead of pointers and how the OUTPUT parameter creates a return result.

If you don't want to use the names INPUT or OUTPUT, use the %apply directive. For example: 

%module Example
%include "typemaps.i"

%apply int *OUTPUT { int *result };
%apply int *INPUT { int *x, int *y};

void add(int x, int y, int *result);
int sub(int *x, int *y);

If a function mutates one of its parameters like this, 

void negate(int *x) {
 *x = -(*x);
}

you can use INOUT like this:

%include "typemaps.i"
...
void negate(int *INOUT);

In Ruby, a mutated parameter shows up as a return value. For example:

a = Example.negate(3)
print a
-3

The most common use of these special typemap rules is to handle functions that return more than one value. For example, sometimes a function returns a result as well as a special error code: 

/* send message, return number of bytes sent, success code, and error_code */
int send_message(char *text, int *success, int *error_code);

To wrap such a function, simply use the OUTPUT rule above. For example: 

%module example
%include "typemaps.i"
...
int send_message(char *, int *OUTPUT, int *OUTPUT);

When used in Ruby, the function will return an array of multiple values. 

bytes, success, error_code = send_message("Hello World")
if not success
 print "error #{error_code} : in send_message"
else
 print "Sent", bytes
end

Another way to access multiple return values is to use the %apply rule. In the following example, the parameters rows and columns are related to SWIG as OUTPUT values through the use of %apply 

%module Example
%include "typemaps.i"
%apply int *OUTPUT { int *rows, int *columns };
...
void get_dimensions(Matrix *m, int *rows, int*columns);

In Ruby:

r, c = Example.get_dimensions(m)

32.6 Exception handling

32.6.1 Using the %exception directive

The SWIG %exception directive can be used to define a user-definable exception handler that can convert C/C++ errors into Ruby exceptions. The chapter on Customization Features contains more details, but suppose you have a C++ class like the following : 

class DoubleArray {
 private:
 int n;
 double *ptr;
 public:
 // Create a new array of fixed size
 DoubleArray(int size) {
 ptr = new double[size];
 n = size;
 }

 // Destroy an array
 ~DoubleArray() {
 delete ptr;
 } 

 // Return the length of the array
 int length() {
 return n;
 }

 // Get an array item and perform bounds checking.
 double getitem(int i) {
 if ((i >= 0) && (i < n))
 return ptr[i];
 else
 throw RangeError();
 }

 // Set an array item and perform bounds checking.
 void setitem(int i, double val) {
 if ((i >= 0) && (i < n))
 ptr[i] = val;
 else {
 throw RangeError();
 }
 }
 };

Since several methods in this class can throw an exception for an out-of-bounds access, you might want to catch this in the Ruby extension by writing the following in an interface file: 

%exception {
 try {
 $action
 }
 catch (const RangeError&) {
 static VALUE cpperror = rb_define_class("CPPError", rb_eStandardError);
 rb_raise(cpperror, "Range error.");
 }
}

class DoubleArray {
 ...
};

The exception handling code is inserted directly into generated wrapper functions. When an exception handler is defined, errors can be caught and used to gracefully raise a Ruby exception instead of forcing the entire program to terminate with an uncaught error.

As shown, the exception handling code will be added to every wrapper function. Because this is somewhat inefficient, you might consider refining the exception handler to only apply to specific methods like this: 

%exception getitem {
 try {
 $action
 }
 catch (const RangeError&) {
 static VALUE cpperror = rb_define_class("CPPError", rb_eStandardError);
 rb_raise(cpperror, "Range error in getitem.");
 }
}

%exception setitem {
 try {
 $action
 }
 catch (const RangeError&) {
 static VALUE cpperror = rb_define_class("CPPError", rb_eStandardError);
 rb_raise(cpperror, "Range error in setitem.");
 }
}

In this case, the exception handler is only attached to methods and functions named getitem and setitem.

Since SWIG's exception handling is user-definable, you are not limited to C++ exception handling. See the chapter on Customization Features for more examples.

32.6.2 Handling Ruby Blocks

One of the highlights of Ruby and most of its standard library is the use of blocks, which allow the easy creation of continuations and other niceties.  Blocks in ruby are also often used to simplify the passing of many arguments to a class.

In order to make your class constructor support blocks, you can take advantage of the %exception directive, which will get run after the C++ class' constructor was called. 

For example, this yields the class over after its construction: 

class Window
{
public:
 Window(int x, int y, int w, int h);
// .... other methods here ....
};

// Add support for yielding self in the Class' constructor.
%exception Window::Window {
 $action
 if (rb_block_given_p()) {
 rb_yield(self);
 }
}

Then, in ruby, it can be used like:

Window.new(0,0,360,480) { |w|
    w.color = Fltk::RED
    w.border = false
}

For other methods, you can usually use a dummy parameter with a special in typemap, like:

//
// original function was:
//
// void func(int x);

%typemap(in,numinputs=0) int RUBY_YIELD_SELF {
     if ( !rb_block_given_p() )
            rb_raise("No block given");
     return rb_yield(self);
}

%extend {
        void func(int x, int RUBY_YIELD_SELF );
}

For more information on typemaps, see Typemaps.

32.6.3 Raising exceptions

There are three ways to raise exceptions from C++ code to Ruby.

The first way is to use SWIG_exception(int code, const char *msg). The following table shows the mappings from SWIG error codes to Ruby exceptions:

SWIG_MemoryError
rb_eNoMemError
SWIG_IOError
rb_eIOError
SWIG_RuntimeError
rb_eRuntimeError
SWIG_IndexError
rb_eIndexError
SWIG_TypeError
rb_eTypeError
SWIG_DivisionByZero
rb_eZeroDivError
SWIG_OverflowError
rb_eRangeError
SWIG_SyntaxError
rb_eSyntaxError
SWIG_ValueError
rb_eArgError
SWIG_SystemError
rb_eFatal
SWIG_AttributeError
rb_eRuntimeError
SWIG_NullReferenceError
rb_eNullReferenceError*
SWIG_ObjectPreviouslyDeletedError
rb_eObjectPreviouslyDeleted*
SWIG_UnknownError
rb_eRuntimeError
* These error classes are created by SWIG and are not built-in Ruby exception classes

The second way to raise errors is to use SWIG_Raise(obj, type, desc). Obj is a C++ instance of an exception class, type is a string specifying the type of exception (for example, "MyError") and desc is the SWIG description of the exception class. For example:

%raise(SWIG_NewPointerObj(e, SWIGTYPE_p_AssertionFailedException, 0), ":AssertionFailedException", SWIGTYPE_p_AssertionFailedException);

This is useful when you want to pass the current exception object directly to Ruby, particularly when the object is an instance of class marked as an %exceptionclass (see the next section for more information).

Last, you can raise an exception by directly calling Ruby's C api. This is done by invoking the rb_raise() function. The first argument passed to rb_raise() is the exception type. You can raise a custom exception type or one of the built-in Ruby exception types.

32.6.4 Exception classes

Starting with SWIG 1.3.28, the Ruby module supports the %exceptionclass directive, which is used to identify C++ classes that are used as exceptions. Classes that are marked with the %exceptionclass directive are exposed in Ruby as child classes of rb_eRuntimeError. This allows C++ exceptions to be directly mapped to Ruby exceptions, providing for a more natural integration between C++ code and Ruby code.

	%exceptionclass CustomError;
	
	%inline %{
	class CustomError { };
	
	class Foo { 
	public:
	void test() { throw CustomError; }
	};
	}

From Ruby you can now call this method like this: 

foo = Foo.new
begin
 foo.test()
rescue CustomError => e
 puts "Caught custom error"
end

For another example look at swig/Examples/ruby/exception_class.

32.7 Typemaps

This section describes how you can modify SWIG's default wrapping behavior for various C/C++ datatypes using the %typemap directive. This is an advanced topic that assumes familiarity with the Ruby C API as well as the material in the "Typemaps" chapter. 

Before proceeding, it should be stressed that typemaps are not a required part of using SWIG---the default wrapping behavior is enough in most cases. Typemaps are only used if you want to change some aspect of the primitive C-Ruby interface.

32.7.1 What is a typemap?

A typemap is nothing more than a code generation rule that is attached to a specific C datatype. The general form of this declaration is as follows ( parts enclosed in [...] are optional ):    

%typemap( method [, modifiers...] ) typelist code;

method is a simply a name that specifies what kind of typemap is being defined. It is usually a name like "in", "out", or "argout" (or its director variations). The purpose of these methods is described later.

modifiers is an optional comma separated list of name="value" values. These are sometimes to attach extra information to a typemap and is often target-language dependent.

typelist is a list of the C++ type patterns that the typemap will match. The general form of this list is as follows:

typelist : typepattern [, typepattern, typepattern, ... ] ;

typepattern : type [ (parms) ]
 | type name [ (parms) ]
 | ( typelist ) [ (parms) ]

Each type pattern is either a simple type, a simple type and argument name, or a list of types in the case of multi-argument typemaps. In addition, each type pattern can be parameterized with a list of temporary variables (parms). The purpose of these variables will be explained shortly.

code specifies the C code used in the typemap. It can take any one of the following forms:

code : { ... }
 | " ... "
 | %{ ... %}

For example, to convert integers from Ruby to C, you might define a typemap like this: 

%module example

%typemap(in) int {
 $1 = (int) NUM2INT($input);
 printf("Received an integer : %d\n",$1);
}

%inline %{
extern int fact(int n);
%}

Typemaps are always associated with some specific aspect of code generation. In this case, the "in" method refers to the conversion of input arguments to C/C++. The datatype int is the datatype to which the typemap will be applied. The supplied C code is used to convert values. In this code a number of special variables prefaced by a $ are used. The $1 variable is placeholder for a local variable of type int. The $input variable is the input Ruby object.

When this example is compiled into a Ruby module, the following sample code: 

require 'example'

puts Example.fact(6)

prints the result:

Received an integer : 6
720

In this example, the typemap is applied to all occurrences of the int datatype. You can refine this by supplying an optional parameter name. For example: 

%module example

%typemap(in) int n {
 $1 = (int) NUM2INT($input);
 printf("n = %d\n",$1);
}

%inline %{
extern int fact(int n);
%}

In this case, the typemap code is only attached to arguments that exactly match "int n".

The application of a typemap to specific datatypes and argument names involves more than simple text-matching--typemaps are fully integrated into the SWIG type-system. When you define a typemap for int, that typemap applies to int and qualified variations such as const int. In addition, the typemap system follows typedef declarations. For example: 

%typemap(in) int n {
 $1 = (int) NUM2INT($input);
 printf("n = %d\n",$1);
}

typedef int Integer;
extern int fact(Integer n); // Above typemap is applied

However, the matching of typedef only occurs in one direction. If you defined a typemap for Integer, it is not applied to arguments of type int.

Typemaps can also be defined for groups of consecutive arguments. For example: 

%typemap(in) (char *str, int len) {
 $1 = STR2CSTR($input);
 $2 = (int) RSTRING($input)->len;
};

int count(char c, char *str, int len);

When a multi-argument typemap is defined, the arguments are always handled as a single Ruby object. This allows the function count to be used as follows (notice how the length parameter is omitted): 

puts Example.count('o','Hello World')
2

32.7.2 Typemap scope

Once defined, a typemap remains in effect for all of the declarations that follow. A typemap may be redefined for different sections of an input file. For example:

// typemap1
%typemap(in) int {
...
}

int fact(int); // typemap1
int gcd(int x, int y); // typemap1

// typemap2
%typemap(in) int {
...
}

int isprime(int); // typemap2

One exception to the typemap scoping rules pertains to the %extend declaration. %extend is used to attach new declarations to a class or structure definition. Because of this, all of the declarations in an %extend block are subject to the typemap rules that are in effect at the point where the class itself is defined. For example:

class Foo {
 ...
};

%typemap(in) int {
 ...
}

%extend Foo {
 int blah(int x); // typemap has no effect. Declaration is attached to Foo which 
 // appears before the %typemap declaration.
};

32.7.3 Copying a typemap

A typemap is copied by using assignment. For example:

%typemap(in) Integer = int;

or this:

%typemap(in) Integer, Number, int32_t = int;

Types are often managed by a collection of different typemaps. For example:

%typemap(in) int { ... }
%typemap(out) int { ... }
%typemap(varin) int { ... }
%typemap(varout) int { ... }

To copy all of these typemaps to a new type, use %apply. For example:

%apply int { Integer }; // Copy all int typemaps to Integer
%apply int { Integer, Number }; // Copy all int typemaps to both Integer and Number

The patterns for %apply follow the same rules as for %typemap. For example:

%apply int *output { Integer *output }; // Typemap with name
%apply (char *buf, int len) { (char *buffer, int size) }; // Multiple arguments

32.7.4 Deleting a typemap

A typemap can be deleted by simply defining no code. For example:

%typemap(in) int; // Clears typemap for int
%typemap(in) int, long, short; // Clears typemap for int, long, short
%typemap(in) int *output;

The %clear directive clears all typemaps for a given type. For example:

%clear int; // Removes all types for int
%clear int *output, long *output;

Note: Since SWIG's default behavior is defined by typemaps, clearing a fundamental type like int will make that type unusable unless you also define a new set of typemaps immediately after the clear operation.

32.7.5 Placement of typemaps

Typemap declarations can be declared in the global scope, within a C++ namespace, and within a C++ class. For example:

%typemap(in) int {
 ...
}

namespace std {
 class string;
 %typemap(in) string {
 ...
 }
}

class Bar {
public:
 typedef const int & const_reference;
 %typemap(out) const_reference {
 ...
 }
};

When a typemap appears inside a namespace or class, it stays in effect until the end of the SWIG input (just like before). However, the typemap takes the local scope into account. Therefore, this code

namespace std {
 class string;
 %typemap(in) string {
 ...
 }
}

is really defining a typemap for the type std::string. You could have code like this:

namespace std {
 class string;
 %typemap(in) string { /* std::string */
 ...
 }
}

namespace Foo {
 class string;
 %typemap(in) string { /* Foo::string */
 ...
 }
}

In this case, there are two completely distinct typemaps that apply to two completely different types (std::string and Foo::string).

It should be noted that for scoping to work, SWIG has to know that string is a typename defined within a particular namespace. In this example, this is done using the class declaration class string .

32.7.6 Ruby typemaps

The following list details all of the typemap methods that can be used by the Ruby module:

32.7.6.1  "in" typemap

Converts Ruby objects to input function arguments. For example: 

%typemap(in) int {
 $1 = NUM2INT($input);
}

The following special variables are available:

$input Input object holding value to be converted.
$symname Name of function/method being wrapped
$1...n Argument being sent to the function
$1_name Name of the argument (if provided)
$1_type The actual C datatype matched by the typemap.
$1_ltype The assignable version of the C datatype matched by the typemap.

This is probably the most commonly redefined typemap because it can be used to implement customized conversions.

In addition, the "in" typemap allows the number of converted arguments to be specified. For example:

// Ignored argument.
%typemap(in, numinputs=0) int *out (int temp) {
 $1 = &temp;
}

At this time, only zero or one arguments may be converted.

32.7.6.2 "typecheck" typemap

The "typecheck" typemap is used to support overloaded functions and methods. It merely checks an argument to see whether or not it matches a specific type. For example:

%typemap(typecheck,precedence=SWIG_TYPECHECK_INTEGER) int {
 $1 = FIXNUM_P($input) ? 1 : 0;
}

For typechecking, the $1 variable is always a simple integer that is set to 1 or 0 depending on whether or not the input argument is the correct type.

If you define new "in" typemaps and your program uses overloaded methods, you should also define a collection of "typecheck" typemaps. More details about this follow in a later section on "Typemaps and Overloading."

32.7.6.3  "out" typemap

Converts return value of a C function to a Ruby object.


%typemap(out) int {
   $result = INT2NUM( $1 );
}

The following special variables are available.

$result Result object returned to target language.
$symname Name of function/method being wrapped
$1...n Argument being wrapped
$1_name Name of the argument (if provided)
$1_type The actual C datatype matched by the typemap.
$1_ltype The assignable version of the C datatype matched by the typemap.

32.7.6.4 "arginit" typemap

The "arginit" typemap is used to set the initial value of a function argument--before any conversion has occurred. This is not normally necessary, but might be useful in highly specialized applications. For example:

// Set argument to NULL before any conversion occurs
%typemap(arginit) int *data {
 $1 = NULL;
}

32.7.6.5 "default" typemap

The "default" typemap is used to turn an argument into a default argument. For example:

%typemap(default) int flags {
 $1 = DEFAULT_FLAGS;
}
...
int foo(int x, int y, int flags);

The primary use of this typemap is to either change the wrapping of default arguments or specify a default argument in a language where they aren't supported (like C). Target languages that do not support optional arguments, such as Java and C#, effectively ignore the value specified by this typemap as all arguments must be given.

Once a default typemap has been applied to an argument, all arguments that follow must have default values. See the Default/optional arguments section for further information on default argument wrapping.

32.7.6.6 "check" typemap

The "check" typemap is used to supply value checking code during argument conversion. The typemap is applied after arguments have been converted. For example:

%typemap(check) int positive {
 if ($1 <= 0) {
 SWIG_exception(SWIG_ValueError,"Expected positive value.");
 }
}

32.7.6.7 "argout" typemap

The "argout" typemap is used to return values from arguments. This is most commonly used to write wrappers for C/C++ functions that need to return multiple values. The "argout" typemap is almost always combined with an "in" typemap---possibly to ignore the input value. For example:

/* Set the input argument to point to a temporary variable */
%typemap(in, numinputs=0) int *out (int temp) {
 $1 = &temp;
}

%typemap(argout, fragment="output_helper") int *out {
 // Append output value $1 to $result (assuming a single integer in this case)
 $result = output_helper( $result, INT2NUM(*$1) );
}

The following special variables are available.

$result Result object returned to target language.
$input The original input object passed.
$symname Name of function/method being wrapped.

The code supplied to the "argout" typemap is always placed after the "out" typemap. If multiple return values are used, the extra return values are often appended to return value of the function.

Output helper is a fragment that usually defines a macro to some function like SWIG_Ruby_AppendOutput.

See the typemaps.i library for examples.

32.7.6.8 "freearg" typemap

The "freearg" typemap is used to cleanup argument data. It is only used when an argument might have allocated resources that need to be cleaned up when the wrapper function exits. The "freearg" typemap usually cleans up argument resources allocated by the "in" typemap. For example:

// Get a list of integers
%typemap(in) int *items {
 int nitems = Length($input); 
 $1 = (int *) malloc(sizeof(int)*nitems);
}
// Free the list 
%typemap(freearg) int *items {
 free($1);
}

The "freearg" typemap inserted at the end of the wrapper function, just before control is returned back to the target language. This code is also placed into a special variable $cleanup that may be used in other typemaps whenever a wrapper function needs to abort prematurely.

32.7.6.9 "newfree" typemap

The "newfree" typemap is used in conjunction with the %newobject directive and is used to deallocate memory used by the return result of a function. For example:

%typemap(newfree) string * {
 delete $1;
}
%typemap(out) string * {
 $result = PyString_FromString($1->c_str());
}
...

%newobject foo;
...
string *foo();

See Object ownership and %newobject for further details.

32.7.6.10 "memberin" typemap

The "memberin" typemap is used to copy data from an already converted input value into a structure member. It is typically used to handle array members and other special cases. For example:

%typemap(memberin) int [4] {
 memmove($1, $input, 4*sizeof(int));
}

It is rarely necessary to write "memberin" typemaps---SWIG already provides a default implementation for arrays, strings, and other objects.

32.7.6.11 "varin" typemap

The "varin" typemap is used to convert objects in the target language to C for the purposes of assigning to a C/C++ global variable. This is implementation specific.

32.7.6.12 "varout" typemap

The "varout" typemap is used to convert a C/C++ object to an object in the target language when reading a C/C++ global variable. This is implementation specific.

32.7.6.13 "throws" typemap

The "throws" typemap is only used when SWIG parses a C++ method with an exception specification or has the %catches feature attached to the method. It provides a default mechanism for handling C++ methods that have declared the exceptions they will throw. The purpose of this typemap is to convert a C++ exception into an error or exception in the target language. It is slightly different to the other typemaps as it is based around the exception type rather than the type of a parameter or variable. For example:

%typemap(throws) const char * %{
 rb_raise(rb_eRuntimeError, $1);
 SWIG_fail;
%}
void bar() throw (const char *);

As can be seen from the generated code below, SWIG generates an exception handler with the catch block comprising the "throws" typemap content.

...
try {
 bar();
}
catch(char const *_e) {
 rb_raise(rb_eRuntimeError, _e);
 SWIG_fail;
}
...

Note that if your methods do not have an exception specification yet they do throw exceptions, SWIG cannot know how to deal with them. For a neat way to handle these, see the Exception handling with %exception section.

32.7.6.14 directorin typemap

Converts C++ objects in director member functions to ruby objects. It is roughly the opposite of the "in" typemap, making its typemap rule often similar to the "out" typemap.


%typemap(directorin) int {
     $result = INT2NUM($1);
}

The following special variables are available.

$result Result object returned to target language.
$symname Name of function/method being wrapped
$1...n Argument being wrapped
$1_name Name of the argument (if provided)
$1_type The actual C datatype matched by the typemap.
$1_ltype The assignable version of the C datatype matched by the typemap.
this C++ this, referring to the class itself.

32.7.6.15 directorout typemap

Converts Ruby objects in director member functions to C++ objects.  It is roughly the opposite of the "out" typemap, making its rule often similar to the "in" typemap.


%typemap(directorout) int {
   $result = NUM2INT($1);
}

The following special variables are available:

$symname Name of function/method being wrapped
$1...n Argument being sent to the function
$1_name Name of the argument (if provided)
$1_type The actual C datatype matched by the typemap.
$1_ltype The assignable version of the C datatype matched by the typemap.
this C++ this, referring to the class itself.

Currently, the directorout nor the out typemap support the option numoutputs, but the Ruby module provides that functionality through a %feature directive.  Thus, a function can be made to return "nothing" if you do:

%feature("numoutputs","0") MyClass::function;

This feature can be useful if a function returns a status code, which you want to discard but still use the typemap to raise an exception.

32.7.6.16 directorargout typemap

Output argument processing in director member functions.

%typemap(directorargout, fragment="output_helper") int {
$result = output_helper( $result, NUM2INT($1) );
}

The following special variables are available:

$result Result that the director function returns
$symname name of the function/method being wrapped
$1...n Argument being sent to the function
$1_name Name of the argument (if provided)
$1_type The actual C datatype matched by the typemap
$1_ltype The assignable version of the C datatype matched by the typemap
this C++ this, referring to the instance of the class itself

32.7.6.17 ret typemap

Cleanup of function return values

32.7.6.18 globalin typemap

Setting of C global variables

32.7.7 Typemap variables

Within a typemap, a number of special variables prefaced with a $ may appear. A full list of variables can be found in the "Typemaps" chapter. This is a list of the most common variables:

$1

A C local variable corresponding to the actual type specified in the %typemap directive. For input values, this is a C local variable that is supposed to hold an argument value. For output values, this is the raw result that is supposed to be returned to Ruby.

$input

A VALUE holding a raw Ruby object with an argument or variable value.

$result

A VALUE that holds the result to be returned to Ruby.

$1_name

The parameter name that was matched.

$1_type

The actual C datatype matched by the typemap.

$1_ltype

An assignable version of the datatype matched by the typemap (a type that can appear on the left-hand-side of a C assignment operation). This type is stripped of qualifiers and may be an altered version of $1_type. All arguments and local variables in wrapper functions are declared using this type so that their values can be properly assigned.

$symname

The Ruby name of the wrapper function being created.

32.7.8 Useful Functions

When you write a typemap, you usually have to work directly with Ruby objects. The following functions may prove to be useful. (These functions plus many more can be found in Programming Ruby, by David Thomas and Andrew Hunt.) 

In addition, we list equivalent functions that Swig defines, which provide a language neutral conversion (these functions are defined for each swig language supported).  If you are trying to create a swig file that will work under multiple languages, it is recommended you stick to the swig functions instead of the native Ruby functions.  That should help you avoid having to rewrite a lot of typemaps across multiple languages.

32.7.8.1 C Datatypes to Ruby Objects

RUBY Swig
INT2NUM(long or int) SWIG_From_int(int x) int to Fixnum or Bignum
INT2FIX(long or int) int to Fixnum (faster than INT2NUM)
CHR2FIX(char) SWIG_From_char(char x) char to Fixnum
rb_str_new2(char*) SWIG_FromCharPtrAndSize(char*, size_t) char* to String
rb_float_new(double) SWIG_From_double(double),
SWIG_From_float(float)		 float/double to Float

32.7.8.2 Ruby Objects to C Datatypes

Here, while the Ruby versions return the value directly, the SWIG versions do not, but return a status value to indicate success (SWIG_OK). While more akward to use, this allows you to write typemaps that report more helpful error messages, like:


%typemap(in) size_t (int ok)
 
  ok = SWIG_AsVal_size_t($input, &$1);
  if (!SWIG_IsOK(ok)) {
    SWIG_exception_fail(SWIG_ArgError(ok), Ruby_Format_TypeError( "$1_name", "$1_type","$symname", $argnum, $input ));
   }

}
  
int NUM2INT(Numeric) SWIG_AsVal_int(VALUE, int*)
int FIX2INT(Numeric) SWIG_AsVal_int(VALUE, int*)
unsigned int NUM2UINT(Numeric) SWIG_AsVal_unsigned_SS_int(VALUE, int*)
unsigned int FIX2UINT(Numeric) SWIG_AsVal_unsigned_SS_int(VALUE, int*)
long NUM2LONG(Numeric) SWIG_AsVal_long(VALUE, long*)
long FIX2LONG(Numeric) SWIG_AsVal_long(VALUE, long*)
unsigned long FIX2ULONG(Numeric) SWIG_AsVal_unsigned_SS_long(VALUE, unsigned long*)
char NUM2CHR(Numeric or String) SWIG_AsVal_char(VALUE, int*)
char * STR2CSTR(String) SWIG_AsCharPtrAndSize(VALUE, char*, size_t, int* alloc)
char * rb_str2cstr(String, int*length)
double NUM2DBL(Numeric) (double) SWIG_AsVal_int(VALUE) or similar

32.7.8.3 Macros for VALUE

RSTRING_LEN(str)

length of the Ruby string

RSTRING_PTR(str)

pointer to string storage

RARRAY_LEN(arr)

length of the Ruby array

RARRAY(arr)->capa

capacity of the Ruby array

RARRAY_PTR(arr)

pointer to array storage

32.7.8.4 Exceptions

void rb_raise(VALUE exception, const char *fmt, ...)

Raises an exception. The given format string fmt and remaining arguments are interpreted as with printf().

void rb_fatal(const char *fmt, ...)

Raises a fatal exception, terminating the process. No rescue blocks are called, but ensure blocks will be called. The given format string fmt and remaining arguments are interpreted as with printf().

void rb_bug(const char *fmt, ...)

Terminates the process immediately -- no handlers of any sort will be called. The given format string fmt and remaining arguments are interpreted as with printf(). You should call this function only if a fatal bug has been exposed.

void rb_sys_fail(const char *msg)

Raises a platform-specific exception corresponding to the last known system error, with the given string msg.

VALUE rb_rescue(VALUE (*body)(VALUE), VALUE args, VALUE(*rescue)(VALUE, VALUE), VALUE rargs)

Executes body with the given args. If a StandardError exception is raised, then execute rescue with the given rargs.

VALUE rb_ensure(VALUE(*body)(VALUE), VALUE args, VALUE(*ensure)(VALUE), VALUE eargs)

Executes body with the given args. Whether or not an exception is raised, execute ensure with the given rargs after body has completed.

VALUE rb_protect(VALUE (*body)(VALUE), VALUE args, int *result)

Executes body with the given args and returns nonzero in result if any exception was raised.

void rb_notimplement()

Raises a NotImpError exception to indicate that the enclosed function is not implemented yet, or not available on this platform.

void rb_exit(int status)

Exits Ruby with the given status. Raises a SystemExit exception and calls registered exit functions and finalizers.

void rb_warn(const char *fmt, ...)

Unconditionally issues a warning message to standard error. The given format string fmt and remaining arguments are interpreted as with printf().

void rb_warning(const char *fmt, ...)

Conditionally issues a warning message to standard error if Ruby was invoked with the -w flag. The given format string fmt and remaining arguments are interpreted as with printf().

32.7.8.5 Iterators

void rb_iter_break()

Breaks out of the enclosing iterator block.

VALUE rb_each(VALUE obj)

Invokes the each method of the given obj.

VALUE rb_yield(VALUE arg)

Transfers execution to the iterator block in the current context, passing arg as an argument. Multiple values may be passed in an array.

int rb_block_given_p()

Returns true if yield would execute a block in the current context; that is, if a code block was passed to the current method and is available to be called.

VALUE rb_iterate(VALUE (*method)(VALUE), VALUE args, VALUE (*block)(VALUE, VALUE), VALUE arg2)

Invokes method with argument args and block block. A yield from that method will invoke block with the argument given to yield, and a second argument arg2.

VALUE rb_catch(const char *tag, VALUE (*proc)(VALUE, VALUE), VALUE value)

Equivalent to Ruby's catch.

void rb_throw(const char *tag, VALUE value)

Equivalent to Ruby's throw.

32.7.9 Typemap Examples

This section includes a few examples of typemaps. For more examples, you might look at the examples in the Example/ruby directory.

32.7.10 Converting a Ruby array to a char **

A common problem in many C programs is the processing of command line arguments, which are usually passed in an array of NULL terminated strings. The following SWIG interface file allows a Ruby Array instance to be used as a char ** object. 

%module argv

// This tells SWIG to treat char ** as a special case
%typemap(in) char ** {
 /* Get the length of the array */
 int size = RARRAY($input)->len; 
 int i;
 $1 = (char **) malloc((size+1)*sizeof(char *));
 /* Get the first element in memory */
 VALUE *ptr = RARRAY($input)->ptr; 
 for (i=0; i < size; i++, ptr++)
 /* Convert Ruby Object String to char* */
 $1[i]= STR2CSTR(*ptr); 
 $1[i]=NULL; /* End of list */
}

// This cleans up the char ** array created before 
// the function call

%typemap(freearg) char ** {
 free((char *) $1);
}

// Now a test function
%inline %{
int print_args(char **argv) {
 int i = 0;
 while (argv[i]) {
 printf("argv[%d] = %s\n", i,argv[i]);
 i++;
 }
 return i;
}
%}

When this module is compiled, the wrapped C function now operates as follows : 

require 'Argv'
Argv.print_args(["Dave","Mike","Mary","Jane","John"])
argv[0] = Dave
argv[1] = Mike
argv[2] = Mary
argv[3] = Jane
argv[4] = John

In the example, two different typemaps are used. The "in" typemap is used to receive an input argument and convert it to a C array. Since dynamic memory allocation is used to allocate memory for the array, the "freearg" typemap is used to later release this memory after the execution of the C function.

32.7.11 Collecting arguments in a hash

Ruby's solution to the "keyword arguments" capability of some other languages is to allow the programmer to pass in one or more key-value pairs as arguments to a function. All of those key-value pairs are collected in a single Hash argument that's presented to the function. If it makes sense, you might want to provide similar functionality for your Ruby interface. For example, suppose you'd like to wrap this C function that collects information about people's vital statistics: 

void setVitalStats(const char *person, int nattributes, const char **names, int *values);

and you'd like to be able to call it from Ruby by passing in an arbitrary number of key-value pairs as inputs, e.g. 

setVitalStats("Fred",
 'weight' => 270,
	'age' => 42
	)

To make this work, you need to write a typemap that expects a Ruby Hash as its input and somehow extracts the last three arguments (nattributes, names and values) needed by your C function. Let's start with the basics: 

%typemap(in) (int nattributes, const char **names, const int *values)
 (VALUE keys_arr, int i, VALUE key, VALUE val) {
}

This %typemap directive tells SWIG that we want to match any function declaration that has the specified types and names of arguments somewhere in the argument list. The fact that we specified the argument names (nattributes, names and values) in our typemap is significant; this ensures that SWIG won't try to apply this typemap to other functions it sees that happen to have a similar declaration with different argument names. The arguments that appear in the second set of parentheses (keys_arr, i, key and val) define local variables that our typemap will need.

Since we expect the input argument to be a Hash, let's next add a check for that: 

%typemap(in) (int nattributes, const char **names, const int *values)
 (VALUE keys_arr, int i, VALUE key, VALUE val) {
 Check_Type($input, T_HASH);
}

Check_Type() is just a macro (defined in the Ruby header files) that confirms that the input argument is of the correct type; if it isn't, an exception will be raised.

The next task is to determine how many key-value pairs are present in the hash; we'll assign this number to the first typemap argument ($1). This is a little tricky since the Ruby/C API doesn't provide a public function for querying the size of a hash, but we can get around that by calling the hash's size method directly and converting its result to a C int value: 

%typemap(in) (int nattributes, const char **names, const int *values)
 (VALUE keys_arr, int i, VALUE key, VALUE val) {
 Check_Type($input, T_HASH);
 $1 = NUM2INT(rb_funcall($input, rb_intern("size"), 0, NULL));
}

So now we know the number of attributes. Next we need to initialize the second and third typemap arguments (i.e. the two C arrays) to NULL and set the stage for extracting the keys and values from the hash: 

%typemap(in) (int nattributes, const char **names, const int *values)
 (VALUE keys_arr, int i, VALUE key, VALUE val) {
 Check_Type($input, T_HASH);
 $1 = NUM2INT(rb_funcall($input, rb_intern("size"), 0, NULL));
 $2 = NULL;
 $3 = NULL;
 if ($1 > 0) {
 $2 = (char **) malloc($1*sizeof(char *));
 $3 = (int *) malloc($1*sizeof(int));
 }
}

There are a number of ways we could extract the keys and values from the input hash, but the simplest approach is to first call the hash's keys method (which returns a Ruby array of the keys) and then start looping over the elements in that array: 

%typemap(in) (int nattributes, const char **names, const int *values)
 (VALUE keys_arr, int i, VALUE key, VALUE val) {
 Check_Type($input, T_HASH);
 $1 = NUM2INT(rb_funcall($input, rb_intern("size"), 0, NULL));
 $2 = NULL;
 $3 = NULL;
 if ($1 > 0) {
 $2 = (char **) malloc($1*sizeof(char *));
 $3 = (int *) malloc($1*sizeof(int));
 keys_arr = rb_funcall($input, rb_intern("keys"), 0, NULL);
 for (i = 0; i < $1; i++) {
 }
}
}

Recall that keys_arr and i are local variables for this typemap. For each element in the keys_arr array, we want to get the key itself, as well as the value corresponding to that key in the hash: 

%typemap(in) (int nattributes, const char **names, const int *values)
 (VALUE keys_arr, int i, VALUE key, VALUE val) {
 Check_Type($input, T_HASH);
 $1 = NUM2INT(rb_funcall($input, rb_intern("size"), 0, NULL));
 $2 = NULL;
 $3 = NULL;
 if ($1 > 0) {
 $2 = (char **) malloc($1*sizeof(char *));
 $3 = (int *) malloc($1*sizeof(int));
 keys_arr = rb_funcall($input, rb_intern("keys"), 0, NULL);
 for (i = 0; i < $1; i++) {
 key = rb_ary_entry(keys_arr, i);
 val = rb_hash_aref($input, key);
}
}
}

To be safe, we should again use the Check_Type() macro to confirm that the key is a String and the value is a Fixnum: 

%typemap(in) (int nattributes, const char **names, const int *values)
 (VALUE keys_arr, int i, VALUE key, VALUE val) {
 Check_Type($input, T_HASH);
 $1 = NUM2INT(rb_funcall($input, rb_intern("size"), 0, NULL));
 $2 = NULL;
 $3 = NULL;
 if ($1 > 0) {
 $2 = (char **) malloc($1*sizeof(char *));
 $3 = (int *) malloc($1*sizeof(int));
 keys_arr = rb_funcall($input, rb_intern("keys"), 0, NULL);
 for (i = 0; i < $1; i++) {
 key = rb_ary_entry(keys_arr, i);
 val = rb_hash_aref($input, key);
 Check_Type(key, T_STRING);
 Check_Type(val, T_FIXNUM);
}
}
}

Finally, we can convert these Ruby objects into their C equivalents and store them in our local C arrays: 

%typemap(in) (int nattributes, const char **names, const int *values)
 (VALUE keys_arr, int i, VALUE key, VALUE val) {
 Check_Type($input, T_HASH);
 $1 = NUM2INT(rb_funcall($input, rb_intern("size"), 0, NULL));
 $2 = NULL;
 $3 = NULL;
 if ($1 > 0) {
 $2 = (char **) malloc($1*sizeof(char *));
 $3 = (int *) malloc($1*sizeof(int));
 keys_arr = rb_funcall($input, rb_intern("keys"), 0, NULL);
 for (i = 0; i < $1; i++) {
 key = rb_ary_entry(keys_arr, i);
 val = rb_hash_aref($input, key);
 Check_Type(key, T_STRING);
 Check_Type(val, T_FIXNUM);
 $2[i] = STR2CSTR(key);
 $3[i] = NUM2INT(val);
}
}
}

We're not done yet. Since we used malloc() to dynamically allocate the memory used for the names and values arguments, we need to provide a corresponding "freearg" typemap to free that memory so that there is no memory leak. Fortunately, this typemap is a lot easier to write: 

%typemap(freearg) (int nattributes, const char **names, const int *values) {
 free((void *) $2);
 free((void *) $3);
}

All of the code for this example, as well as a sample Ruby program that uses the extension, can be found in the Examples/ruby/hashargs directory of the SWIG distribution.

32.7.12 Pointer handling

Occasionally, it might be necessary to convert pointer values that have been stored using the SWIG typed-pointer representation. Since there are several ways in which pointers can be represented, the following two functions are used to safely perform this conversion:

int SWIG_ConvertPtr(VALUE obj, void **ptr, swig_type_info *ty, int flags)

Converts a Ruby object obj to a C pointer whose address is ptr (i.e. ptr is a pointer to a pointer). The third argument, ty, is a pointer to a SWIG type descriptor structure. If ty is not NULL, that type information is used to validate type compatibility and other aspects of the type conversion. If flags is non-zero, any type errors encountered during this validation result in a Ruby TypeError exception being raised; if flags is zero, such type errors will cause SWIG_ConvertPtr() to return -1 but not raise an exception. If ty is NULL, no type-checking is performed.

VALUE SWIG_NewPointerObj(void *ptr, swig_type_info *ty, int own)

Creates a new Ruby pointer object. Here, ptr is the pointer to convert, ty is the SWIG type descriptor structure that describes the type, and own is a flag that indicates whether or not Ruby should take ownership of the pointer (i.e. whether Ruby should free this data when the corresponding Ruby instance is garbage-collected).

Both of these functions require the use of a special SWIG type-descriptor structure. This structure contains information about the mangled name of the datatype, type-equivalence information, as well as information about converting pointer values under C++ inheritance. For a type of Foo *, the type descriptor structure is usually accessed as follows: 

Foo *foo;
SWIG_ConvertPtr($input, (void **) &foo, SWIGTYPE_p_Foo, 1);

VALUE obj;
obj = SWIG_NewPointerObj(f, SWIGTYPE_p_Foo, 0);

In a typemap, the type descriptor should always be accessed using the special typemap variable $1_descriptor. For example: 

%typemap(in) Foo * {
 SWIG_ConvertPtr($input, (void **) &$1, $1_descriptor, 1);
}

32.7.12.1 Ruby Datatype Wrapping

VALUE Data_Wrap_Struct(VALUE class, void (*mark)(void *), void (*free)(void *), void *ptr)

Given a pointer ptr to some C data, and the two garbage collection routines for this data (mark and free), return a VALUE for the Ruby object.

VALUE Data_Make_Struct(VALUE class, c-type, void (*mark)(void *), void (*free)(void *), c-type *ptr)

Allocates a new instance of a C data type c-type, assigns it to the pointer ptr, then wraps that pointer with Data_Wrap_Struct() as above.

Data_Get_Struct(VALUE obj, c-type, c-type *ptr)

Retrieves the original C pointer of type c-type from the data object obj and assigns that pointer to ptr.

32.7.13 Example: STL Vector to Ruby Array

Another use for macros and type maps is to create a Ruby array from a STL vector of pointers. In essence, copy of all the pointers in the vector into a Ruby array. The use of the macro is to make the typemap so generic that any vector with pointers can use the type map. The following is an example of how to construct this type of macro/typemap and should give insight into constructing similar typemaps for other STL structures: 

%define PTR_VECTOR_TO_RUBY_ARRAY(vectorclassname, classname)
%typemap(out) vectorclassname &, const vectorclassname & {
 VALUE arr = rb_ary_new2($1->size());
 vectorclassname::iterator i = $1->begin(), iend = $1->end();
 for ( ; i!=iend; i++ )
 rb_ary_push(arr, Data_Wrap_Struct(c ## classname.klass, 0, 0, *i));
 $result = arr;
}
%typemap(out) vectorclassname, const vectorclassname {
 VALUE arr = rb_ary_new2($1.size());
 vectorclassname::iterator i = $1.begin(), iend = $1.end();
 for ( ; i!=iend; i++ )
 rb_ary_push(arr, Data_Wrap_Struct(c ## classname.klass, 0, 0, *i));
 $result = arr;
}
%enddef

Note, that the "c ## classname.klass" is used in the preprocessor step to determine the actual object from the class name.

To use the macro with a class Foo, the following is used: 

PTR_VECTOR_TO_RUBY_ARRAY(vector<foo *="">, Foo)

It is also possible to create a STL vector of Ruby objects: 

%define RUBY_ARRAY_TO_PTR_VECTOR(vectorclassname, classname)
%typemap(in) vectorclassname &, const vectorclassname & {
 Check_Type($input, T_ARRAY);
 vectorclassname *vec = new vectorclassname;
 int len = RARRAY($input)->len;
 for (int i=0; i!=len; i++) {
 VALUE inst = rb_ary_entry($input, i);
 //The following _should_ work but doesn't on HPUX
 // Check_Type(inst, T_DATA);
 classname *element = NULL;
 Data_Get_Struct(inst, classname, element);
 vec->push_back(element);
 }
 $1 = vec;
}

%typemap(freearg) vectorclassname &, const vectorclassname & {
 delete $1;
}
%enddef

It is also possible to create a Ruby array from a vector of static data types: 

%define VECTOR_TO_RUBY_ARRAY(vectorclassname, classname)
%typemap(out) vectorclassname &, const vectorclassname & {
 VALUE arr = rb_ary_new2($1->size()); 
 vectorclassname::iterator i = $1->begin(), iend = $1->end();
 for ( ; i!=iend; i++ )
 rb_ary_push(arr, Data_Wrap_Struct(c ## classname.klass, 0, 0, &(*i)));
 $result = arr;
}
%typemap(out) vectorclassname, const vectorclassname {
 VALUE arr = rb_ary_new2($1.size()); 
 vectorclassname::iterator i = $1.begin(), iend = $1.end();
 for ( ; i!=iend; i++ )
 rb_ary_push(arr, Data_Wrap_Struct(c ## classname.klass, 0, 0, &(*i)));
 $result = arr;
}
%enddef

Note that this is mostly an example of typemaps. If you want to use the STL with ruby, you are advised to use the standard swig STL library, which does much more than this.  Refer to the section called the C++ Standard Template Library.

32.8 Docstring Features

Using ri and rdoc web pages in Ruby libraries is a common practice. Given the way that SWIG generates the extensions by default, your users will normally not get any documentation for it, even if they run 'rdoc' on the resulting .c or .cxx file.

The features described in this section make it easy for you to add rdoc strings to your modules, functions and methods that can then be read by Ruby's rdoc tool to generate html web pages, ri documentation, Windows chm file and an .xml description.

rdoc can then be run from a console or shell window on a swig generated file. 

For example, to generate html web pages from a C++ file, you'd do: 

$ rdoc -E cxx=c -f html file_wrap.cxx

To generate ri documentation from a c wrap file, you could do:

$ rdoc -r file_wrap.c

32.8.1 Module docstring

Ruby allows a docstring at the beginning of the file before any other statements, and it is typically used to give a general description of the entire module. SWIG supports this by setting an option of the %module directive. For example: 

%module(docstring="This is the example module's docstring") example

When you have more than just a line or so then you can retain the easy readability of the %module directive by using a macro. For example: 

%define DOCSTRING
"The `XmlResource` class allows program resources defining menus, 
layout of controls on a panel, etc. to be loaded from an XML file."
%enddef

%module(docstring=DOCSTRING) xrc

32.8.2 %feature("autodoc")

Since SWIG does know everything about the function it wraps, it is possible to generate an rdoc containing the parameter types, names and default values. Since Ruby ships with one of the best documentation systems of any language, it makes sense to take advantage of it.

SWIG's Ruby module provides support for the "autodoc" feature, which when attached to a node in the parse tree will cause an rdoc comment to be generated in the wrapper file that includes the name of the function, parameter names, default values if any, and return type if any. There are also several options for autodoc controlled by the value given to the feature, described below.

32.8.2.1 %feature("autodoc", "0")

When the "0" option is given then the types of the parameters will not be included in the autodoc string. For example, given this function prototype: 

%feature("autodoc", "0");
bool function_name(int x, int y, Foo* foo=NULL, Bar* bar=NULL);

Then Ruby code like this will be generated: 

function_name(x, y, foo=nil, bar=nil) -> bool
 ...

32.8.2.2 %feature("autodoc", "1")

When the "1" option is used then the parameter types will be used in the rdoc string. In addition, an attempt is made to simplify the type name such that it makes more sense to the Ruby user. Pointer, reference and const info is removed, %rename's are evaluated, etc. (This is not always successful, but works most of the time. See the next section for what to do when it doesn't.) Given the example above, then turning on the parameter types with the "1" option will result in rdoc code like this: 

function_name(int x, int y, Foo foo=nil, Bar bar=nil) -> bool
 ...

32.8.2.3 %feature("autodoc", "2")

When the "2" option is used then the parameter types will not be used in the rdoc string. However, they will be listed in full after the function.  Given the example above, then turning on the parameter types with the "2" option will result in Ruby code like this:

32.8.2.4 %feature("autodoc", "3")

When the "3" option is used then the function will be documented using a combination of "1" and "2" above.  Given the example above, then turning on the parameter types with the "2" option will result in Ruby code like this: 

function_name(int x, int y, Foo foo=nil, Bar bar=nil) -> bool

Parameters:
	x - int
	y - int
	foo - Foo
	bar - Bar

32.8.2.5 %feature("autodoc", "docstring")

Finally, there are times when the automatically generated autodoc string will make no sense for a Ruby programmer, particularly when a typemap is involved. So if you give an explicit value for the autodoc feature then that string will be used in place of the automatically generated string. For example: 

%feature("autodoc", "GetPosition() -> (x, y)") GetPosition;
void GetPosition(int* OUTPUT, int* OUTPUT);

32.8.3 %feature("docstring")

In addition to the autodoc strings described above, you can also attach any arbitrary descriptive text to a node in the parse tree with the "docstring" feature. When the proxy module is generated then any docstring associated with classes, function or methods are output. If an item already has an autodoc string then it is combined with the docstring and they are output together.

32.9 Advanced Topics

32.9.1 Operator overloading

SWIG allows operator overloading with, by using the %extend or %rename commands in SWIG and the following operator names (derived from Python):

General
__repr__ inspect
__str__ to_s
__cmp__ <=>
__hash__ hash
__nonzero__ nonzero?
Callable
__call__ call
Collection
__len__ length
__getitem__ []
__setitem__ []=
Numeric
__add__ +
__sub__ -
__mul__ *
__div__ /
__mod__ %
__divmod__ divmod
__pow__ **
__lshift__ <<
__rshift__ >>
__and__ &
__xor__ ^
__or__ |
__neg__ -@
__pos__ +@
__abs__ abs
__invert__ ~
__int__ to_i
__float__ to_f
__coerce__ coerce
Additions in 1.3.13
__lt__ <
__le__ <=
__eq__ ==
__gt__ >
__ge__ >=

Note that although SWIG supports the __eq__ magic method name for defining an equivalence operator, there is no separate method for handling inequality since Ruby parses the expression a != b as !(a == b).

32.9.2 Creating Multi-Module Packages

The chapter on Working with Modules discusses the basics of creating multi-module extensions with SWIG, and in particular the considerations for sharing runtime type information among the different modules.

As an example, consider one module's interface file (shape.i) that defines our base class: 

%module shape

%{
#include "Shape.h"
%}

class Shape {
protected:
 double xpos;
 double ypos;
protected:
 Shape(double x, double y);
public:
 double getX() const;
 double getY() const;
};

We also have a separate interface file (circle.i) that defines a derived class: 

%module circle

%{
#include "Shape.h"
#include "Circle.h"
%}

// Import the base class definition from Shape module
%import shape.i

class Circle : public Shape {
protected:
 double radius;
public:
 Circle(double x, double y, double r);
 double getRadius() const;
};

We'll start by building the Shape extension module: 

$ swig -c++ -ruby shape.i

SWIG generates a wrapper file named shape_wrap.cxx. To compile this into a dynamically loadable extension for Ruby, prepare an extconf.rb script using this template: 

require 'mkmf'

# Since the SWIG runtime support library for Ruby
# depends on the Ruby library, make sure it's in the list
# of libraries.
$libs = append_library($libs, Config::CONFIG['RUBY_INSTALL_NAME'])

# Create the makefile
create_makefile('shape')

Run this script to create a Makefile and then type make to build the shared library: 

$ ruby extconf.rb
creating Makefile
$ make
g++ -fPIC -g -O2 -I. -I/usr/local/lib/ruby/1.7/i686-linux \
-I. -c shape_wrap.cxx
gcc -shared -L/usr/local/lib -o shape.so shape_wrap.o -L. \
-lruby -lruby -lc

Note that depending on your installation, the outputs may be slightly different; these outputs are those for a Linux-based development environment. The end result should be a shared library (here, shape.so) containing the extension module code. Now repeat this process in a separate directory for the Circle module:

  1. Run SWIG to generate the wrapper code (circle_wrap.cxx);
  2. Write an extconf.rb script that your end-users can use to create a platform-specific Makefile for the extension;
  3. Build the shared library for this extension by typing make.

Once you've built both of these extension modules, you can test them interactively in IRB to confirm that the Shape and Circle modules are properly loaded and initialized: 

$ irb
irb(main):001:0> require 'shape'
true
irb(main):002:0> require 'circle'
true
irb(main):003:0> c = Circle::Circle.new(5, 5, 20)
#<Circle::Circle:0xa097208>
irb(main):004:0> c.kind_of? Shape::Shape
true
irb(main):005:0> c.getX()
5.0

32.9.3 Specifying Mixin Modules

The Ruby language doesn't support multiple inheritance, but it does allow you to mix one or more modules into a class using Ruby's include method. For example, if you have a Ruby class that defines an each instance method, e.g. 

class Set
 def initialize
 @members = []
 end
 
 def each
 @members.each { |m| yield m }
 end
end

then you can mix-in Ruby's Enumerable module to easily add a lot of functionality to your class: 

class Set
 include Enumerable
def initialize
@members = []
end
def each
@members.each { |m| yield m }
end
end

To get the same benefit for your SWIG-wrapped classes, you can use the %mixin directive to specify the names of one or more modules that should be mixed-in to a class. For the above example, the SWIG interface specification might look like this: 

%mixin Set "Enumerable";

class Set {
public:
 // Constructor
 Set();
 
 // Iterates through set members
 void each();
};

Multiple modules can be mixed into a class by providing a comma-separated list of module names to the %mixin directive, e.g. 

%mixin Set "Fee,Fi,Fo,Fum";

Note that the %mixin directive is implemented using SWIG's "features" mechanism and so the same name matching rules used for other kinds of features apply (see the chapter on "Customization Features") for more details).

32.10 Memory Management

One of the most common issues in generating SWIG bindings for Ruby is proper memory management. The key to proper memory management is clearly defining whether a wrapper Ruby object owns the underlying C struct or C++ class. There are two possibilities:

To complicate matters, object ownership may transfer from Ruby to C++ (or vice versa) depending on what function or methods are invoked. Clearly, developing a SWIG wrapper requires a thorough understanding of how the underlying library manages memory.

32.10.1 Mark and Sweep Garbage Collector

Ruby uses a mark and sweep garbage collector. When the garbage collector runs, it finds all the "root" objects, including local variables, global variables, global constants, hardware registers and the C stack. For each root object, the garbage collector sets its mark flag to true and calls rb_gc_mark on the object. The job of rb_gc_mark is to recursively mark all the objects that a Ruby object has a reference to (ignoring those objects that have already been marked). Those objects, in turn, may reference other objects. This process will continue until all active objects have been "marked." After the mark phase comes the sweep phase. In the sweep phase, all objects that have not been marked will be garbage collected. For more information about the Ruby garbage collector please refer to http://rubygarden.org/ruby/ruby?GCAndExtensions.

The Ruby C/API provides extension developers two hooks into the garbage collector - a "mark" function and a "sweep" function. By default these functions are set to NULL.

If a C struct or C++ class references any other Ruby objects, then it must provide a "mark" function. The "mark" function should identify any referenced Ruby objects by calling the rb_gc_mark function for each one. Unsurprisingly, this function will be called by the Ruby garbage during the "mark" phase.

During the sweep phase, Ruby destroys any unused objects. If any memory has been allocated in creating the underlying C struct or C++ struct, then a "free" function must be defined that deallocates this memory.

32.10.2 Object Ownership

As described above, memory management depends on clearly defining who is responsible for freeing the underlying C struct or C++ class. If the Ruby object is responsible for freeing the C++ object, then a "free" function must be registered for the object. If the Ruby object is not responsible for freeing the underlying memory, then a "free" function must not be registered for the object.

For the most part, SWIG takes care of memory management issues. The rules it uses are:

To make this clearer, let's look at an example. Assume we have a Foo and a Bar class. 

/* File "RubyOwernshipExample.h" */

class Foo
{
public:
 Foo() {}
 ~Foo() {}
};

class Bar
{
 Foo *foo_;
public:
 Bar(): foo_(new Foo) {}
 ~Bar() { delete foo_; }
 Foo* get_foo() { return foo_; }
 Foo* get_new_foo() { return new Foo; }
 void set_foo(Foo *foo) { delete foo_; foo_ = foo; }
};

First, consider this Ruby code: 

foo = Foo.new

In this case, the Ruby code calls the underlying Foo C++ constructor, thus creating a new foo object. By default, SWIG will assign the new Ruby object a "free" function. When the Ruby object is garbage collected, the "free" function will be called. It in turn will call Foo's destructor.

Next, consider this code: 

bar = Bar.new
foo = bar.get_foo()

In this case, the Ruby code calls a C++ member function, get_foo. By default, SWIG will not assign the Ruby object a "free" function. Thus, when the Ruby object is garbage collected the underlying C++ foo object is not affected.

Unfortunately, the real world is not as simple as the examples above. For example:

bar = Bar.new
foo = bar.get_new_foo()

In this case, the default SWIG behavior for calling member functions is incorrect. The Ruby object should assume ownership of the returned object. This can be done by using the %newobject directive. See Object ownership and %newobject for more information.

The SWIG default mappings are also incorrect in this case:

foo = Foo.new
bar = Bar.new
bar.set_foo(foo)

Without modification, this code will cause a segmentation fault. When the Ruby foo object goes out of scope, it will free the underlying C++ foo object. However, when the Ruby bar object goes out of scope, it will call the C++ bar destructor which will also free the C++ foo object. The problem is that object ownership is transferred from the Ruby object to the C++ object when the set_foo method is called. This can be done by using the special DISOWN type map, which was added to the Ruby bindings in SWIG-1.3.26.

Thus, a correct SWIG interface file correct mapping for these classes is:

/* File RubyOwnershipExample.i */

%module RubyOwnershipExample

%{
#include "RubyOwnershipExample.h"
%}

class Foo
{
public:
 Foo();
 ~Foo();
};

class Bar
{
 Foo *foo_;
public:
 Bar();
 ~Bar();
 Foo* get_foo();

 %newobject get_new_foo;
 Foo* get_new_foo();

 %apply SWIGTYPE *DISOWN {Foo *foo};
 void set_foo(Foo *foo);
 %clear Foo *foo;
};


This code can be seen in swig/examples/ruby/tracking.


32.10.3 Object Tracking

The remaining parts of this section will use the class library shown below to illustrate different memory management techniques. The class library models a zoo and the animals it contains. 

%module zoo

%{
#include <string>
#include <vector>

#include "zoo.h"
%}

class Animal
{
private:
 typedef std::vector<Animal*> AnimalsType;
 typedef AnimalsType::iterator IterType;
protected:
 AnimalsType animals;
protected:
 std::string name_;
public:
 // Construct an animal with this name
 Animal(const char* name) : name_(name) {}
 
 // Return the animal's name
 const char* get_name() const { return name.c_str(); }
};

class Zoo
{
protected:
 std::vector<animal *=""> animals;
 
public:
 // Construct an empty zoo
 Zoo() {}
 
 /* Create a new animal. */
 static Animal* Zoo::create_animal(const char* name)
 {
 return new Animal(name);
 }

 // Add a new animal to the zoo
 void add_animal(Animal* animal) {
 animals.push_back(animal); 
 }

 Animal* remove_animal(size_t i) {
 Animal* result = this->animals[i];
 IterType iter = this->animals.begin();
 std::advance(iter, i);
 this->animals.erase(iter);

 return result;
 }
 
 // Return the number of animals in the zoo
 size_t get_num_animals() const {
 return animals.size(); 
 }
 
 // Return a pointer to the ith animal
 Animal* get_animal(size_t i) const {
 return animals[i]; 
 }
};

Let's say you SWIG this code and then run IRB:

$ irb
irb(main):001:0> require 'example'
=> true

irb(main):002:0> tiger1 = Example::Animal.new("tiger1")
=> #<Example::Animal:0x2be3820>

irb(main):004:0> tiger1.get_name()
=> "tiger1"

irb(main):003:0> zoo = Example::Zoo.new()
=> #<Example::Zoo:0x2be0a60>

irb(main):006:0> zoo.add_animal(tiger)
=> nil

irb(main):007:0> zoo.get_num_animals()
=> 1

irb(main):007:0> tiger2 = zoo.remove_animal(0)
=> #<Example::Animal:0x2bd4a18>

irb(main):008:0> tiger2.get_name()
=> "tiger1"

irb(main):009:0> tiger1.equal?(tiger2)
=> false

Pay particular attention to the code tiger1.equal?(tiger2). Note that the two Ruby objects are not the same - but they reference the same underlying C++ object. This can cause problems. For example:

irb(main):010:0> tiger1 = nil
=> nil

irb(main):011:0> GC.start
=> nil

irb(main):012:0> tiger2.get_name()
(irb):12: [BUG] Segmentation fault

After the the garbage collector runs, as a result of our call to GC.start, callingtiger2.get_name() causes a segmentation fault. The problem is that when tiger1 is garbage collected, it frees the underlying C++ object. Thus, when tiger2 calls the get_name() method it invokes it on a destroyed object.

This problem can be avoided if SWIG enforces a one-to-one mapping between Ruby objects and C++ classes. This can be done via the use of the %trackobjects functionality available in SWIG-1.3.26. and later.

When the %trackobjects is turned on, SWIG automatically keeps track of mappings between C++ objects and Ruby objects. Note that enabling object tracking causes a slight performance degradation. Test results show this degradation to be about 3% to 5% when creating and destroying 100,000 animals in a row.

Since %trackobjects is implemented as a %feature, it uses the same name matching rules as other kinds of features (see the chapter on "Customization Features") . Thus it can be applied on a class-by-class basis if needed. To fix the example above:


%module example

%{
#include "example.h"
%}

/* Tell SWIG that create_animal creates a new object */
%newobject Zoo::create_animal;

/* Tell SWIG to keep track of mappings between C/C++ structs/classes. */
%trackobjects;

%include "example.h"

When this code runs we see:

$ irb
irb(main):001:0> require 'example'
=> true

irb(main):002:0> tiger1 = Example::Animal.new("tiger1")
=> #<Example::Animal:0x2be37d8>

irb(main):003:0> zoo = Example::Zoo.new()
=> #<Example::Zoo:0x2be0a18>

irb(main):004:0> zoo.add_animal(tiger1)
=> nil

irb(main):006:0> tiger2 = zoo.remove_animal(0)
=> #<Example::Animal:0x2be37d8>

irb(main):007:0> tiger1.equal?(tiger2)
=> true

irb(main):008:0> tiger1 = nil
=> nil

irb(main):009:0> GC.start
=> nil

irb(main):010:0> tiger.get_name()
=> "tiger1"
irb(main):011:0>

For those who are interested, object tracking is implemented by storing Ruby objects in a hash table and keying them on C++ pointers. The underlying API is:

static void SWIG_RubyAddTracking(void* ptr, VALUE object);
static VALUE SWIG_RubyInstanceFor(void* ptr) ;
static void SWIG_RubyRemoveTracking(void* ptr);
static void SWIG_RubyUnlinkObjects(void* ptr);

When an object is created, SWIG will automatically call the SWIG_RubyAddTracking method. Similarly, when an object is deleted, SWIG will call the SWIG_RubyRemoveTracking. When an object is returned to Ruby from C++, SWIG will use the SWIG_RubyInstanceFor method to ensure a one-to-one mapping from Ruby to C++ objects. Last, the RubyUnlinkObjects method unlinks a Ruby object from its underlying C++ object.

In general, you will only need to use the SWIG_RubyInstanceFor, which is required for implementing mark functions as shown below. However, if you implement your own free functions (see below) you may also have to call the SWIG_RubyRemoveTracking and RubyUnlinkObjects methods.

32.10.4 Mark Functions

With a bit more testing, we see that our class library still has problems. For example:

$ irb
irb(main):001:0> require 'example'
=> true

irb(main):002:0> tiger1 = Example::Animal.new("tiger1")
=> #<Example::Animal:0x2bea6a8>

irb(main):003:0> zoo = Example::Zoo.new()
=> #<Example::Zoo:0x2be7960>

irb(main):004:0> zoo.add_animal(tiger1)
=> nil

irb(main):007:0> tiger1 = nil
=> nil

irb(main):007:0> GC.start
=> nil

irb(main):005:0> tiger2 = zoo.get_animal(0)
(irb):12: [BUG] Segmentation fault

The problem is that Ruby does not know that the zoo object contains a reference to a Ruby object. Thus, when Ruby garbage collects tiger1 it frees the underlying C++ object.

This can be fixed by implementing a mark function as described above in the Mark and Sweep Garbage Collector section. You can specify a mark function by using the %markfunc directive. Since the %markfunc directive is implemented using SWIG's' "features" mechanism it uses the same name matching rules as other kinds of features (see the chapter on "Customization Features" for more details).

A mark function takes a single argument, which is a pointer to the C++ object being marked; it should, in turn, call rb_gc_mark() for any instances that are reachable from the current object. The mark function for our Zoo class should therefore loop over all of the C++ animal objects in the zoo object, look up their Ruby object equivalent, and then call rb_gc_mark(). One possible implementation is:

%module example

%{
#include "example.h"
%}

/* Keep track of mappings between C/C++ structs/classes
 and Ruby objects so we can implement a mark function. */
%trackobjects;

/* Specify the mark function */
%markfunc Zoo "mark_Zoo";

%include "example.h"

%header %{

static void mark_Zoo(void* ptr) {
 Zoo* zoo = (Zoo*) ptr;

 /* Loop over each object and tell the garbage collector
 that we are holding a reference to them. */
 int count = zoo->get_num_animals();

 for(int i = 0; i < count; ++i) {
 Animal* animal = zoo->get_animal(i);
 VALUE object = SWIG_RubyInstanceFor(animal);

 if (object != Qnil) {
 rb_gc_mark(object);
 }
 }
}
%}

Note the mark function is dependent on the SWIG_RUBY_InstanceFor method, and thus requires that %trackobjects is enabled. For more information, please refer to the track_object.i test case in the SWIG test suite.

When this code is compiled we now see:

$ irb
irb(main):002:0> tiger1=Example::Animal.new("tiger1")
=> #<Example::Animal:0x2be3bf8>

irb(main):003:0> Example::Zoo.new()
=> #<Example::Zoo:0x2be1780>

irb(main):004:0> zoo = Example::Zoo.new()
=> #<Example::Zoo:0x2bde9c0>

irb(main):005:0> zoo.add_animal(tiger1)
=> nil

irb(main):009:0> tiger1 = nil
=> nil

irb(main):010:0> GC.start
=> nil
irb(main):014:0> tiger2 = zoo.get_animal(0)
=> #<Example::Animal:0x2be3bf8>

irb(main):015:0> tiger2.get_name()
=> "tiger1"
irb(main):016:0>


This code can be seen in swig/examples/ruby/mark_function.

32.10.5 Free Functions

By default, SWIG creates a "free" function that is called when a Ruby object is garbage collected. The free function simply calls the C++ object's destructor.

However, sometimes an appropriate destructor does not exist or special processing needs to be performed before the destructor is called. Therefore, SWIG allows you to manually specify a "free" function via the use of the %freefunc directive. The %freefunc directive is implemented using SWIG's' "features" mechanism and so the same name matching rules used for other kinds of features apply (see the chapter on "Customization Features") for more details).

IMPORTANT ! - If you define your own free function, then you must ensure that you call the underlying C++ object's destructor. In addition, if object tracking is activated for the object's class, you must also call the SWIG_RubyRemoveTracking function (of course call this before you destroy the C++ object). Note that it is harmless to call this method if object tracking if off so it is advised to always call it.

Note there is a subtle interaction between object ownership and free functions. A custom defined free function will only be called if the Ruby object owns the underlying C++ object. This also to Ruby objects which are created, but then transfer ownership to C++ objects via the use of the disown typemap described above.

To show how to use the %freefunc directive, let's slightly change our example. Assume that the zoo object is responsible for freeing animal that it contains. This means that the Zoo::add_animal function should be marked with a DISOWN typemap and the destructor should be updated as below::

Zoo::~Zoo() {
 IterType iter = this->animals.begin();
 IterType end = this->animals.end();

 for(iter; iter != end; ++iter) {
 Animal* animal = *iter;
 delete animal;
 }
}

When we use these objects in IRB we see:

$irb
irb(main):002:0> require 'example'
=> true

irb(main):003:0> zoo = Example::Zoo.new()
=> #<Example::Zoo:0x2be0fe8>

irb(main):005:0> tiger1 = Example::Animal.new("tiger1")
=> #<Example::Animal:0x2bda760>

irb(main):006:0> zoo.add_animal(tiger1)
=> nil

irb(main):007:0> zoo = nil
=> nil

irb(main):008:0> GC.start
=> nil

irb(main):009:0> tiger1.get_name()
(irb):12: [BUG] Segmentation fault

The error happens because the C++ animal object is freed when the zoo object is freed. Although this error is unavoidable, we can at least prevent the segmentation fault. To do this requires enabling object tracking and implementing a custom free function that calls the SWIG_RubyUnlinkObjects function for each animal object that is destroyed. The SWIG_RubyUnlinkObjects function notifies SWIG that a Ruby object's underlying C++ object is no longer valid. Once notified, SWIG will intercept any calls from the existing Ruby object to the destroyed C++ object and raise an exception.

%module example

%{
#include "example.h"
%}

/* Specify that ownership is transferred to the zoo
	when calling add_animal */
%apply SWIGTYPE *DISOWN { Animal* animal };

/* Track objects */
%trackobjects;

/* Specify the mark function */
%freefunc Zoo "free_Zoo";

%include "example.h"

%header %{
 static void free_Zoo(void* ptr) {
 Zoo* zoo = (Zoo*) ptr;

 /* Loop over each animal */
 int count = zoo->get_num_animals();

 for(int i = 0; i < count; ++i) {
 /* Get an animal */
 Animal* animal = zoo->get_animal(i);

 /* Unlink the Ruby object from the C++ object */
 SWIG_RubyUnlinkObjects(animal);

 /* Now remove the tracking for this animal */
 SWIG_RubyRemoveTracking(animal);
 }

 /* Now call SWIG_RubyRemoveTracking for the zoo */
 SWIG_RubyRemoveTracking(ptr);
 
 /* Now free the zoo which will free the animals it contains */
 delete zoo;
 }
%}

Now when we use these objects in IRB we see:

$irb
irb(main):002:0> require 'example'
=> true

irb(main):003:0> zoo = Example::Zoo.new()
=> #<Example::Zoo:0x2be0fe8>

irb(main):005:0> tiger1 = Example::Animal.new("tiger1")
=> #<Example::Animal:0x2bda760>

irb(main):006:0> zoo.add_animal(tiger1)
=> nil

irb(main):007:0> zoo = nil
=> nil

irb(main):008:0> GC.start
=> nil

irb(main):009:0> tiger1.get_name()
RuntimeError: This Animal * already released
 from (irb):10:in `get_name'
 from (irb):10
irb(main):011:0>

Notice that SWIG can now detect the underlying C++ object has been freed, and thus raises a runtime exception.

This code can be seen in swig/examples/ruby/free_function.

32.10.6 Embedded Ruby and the C++ Stack

As has been said, the Ruby GC runs and marks objects before its sweep phase.  When the garbage collector is called, it will also try to mark any Ruby objects (VALUE) it finds in the machine registers and in the C++ stack.

The stack is basically the history of the functions that have been called and also contains local variables, such as the ones you define whenever you do inside a function:

VALUE obj;

For ruby to determine where its stack space begins, during initialization a normal Ruby interpreter will call the ruby_init() function which in turn will call a function called Init_stack or similar.  This function will store a pointer to the location where the stack points at at that point in time.

ruby_init() is presumed to always be called within the main() function of your program and whenever the GC is called, ruby will assume that the memory between the current location in memory and the pointer that was stored previously represents the stack, which may contain local (and temporary) VALUE ruby objects.   Ruby will then be careful not to remove any of those objects in that location.

So far so good.  For a normal Ruby session, all the above is completely transparent and magic to the extensions developer.   

However, with an embedded Ruby, it may not always be possible to modify main() to make sure ruby_init() is called there.   As such, ruby_init() will likely end up being called from within some other function.  This can lead Ruby to measure incorrectly where the stack begins and can result in Ruby incorrectly collecting those temporary VALUE objects that are created once another function is called.  The end result: random crashes and segmentation faults.

This problem will often be seen in director functions that are used for callbacks, for example.  

To solve the problem, SWIG can now generate code with director functions containing the optional macros SWIG_INIT_STACK and SWIG_RELEASE_STACK.   These macros will try to force Ruby to reinitiliaze the beginning of the stack the first time a director function is called.  This will lead Ruby to measure and not collect any VALUE objects defined from that point on.  

To mark functions to either reset the ruby stack or not, you can use:

%initstack   Class::memberfunction;  // only re-init the stack in this director method
%ignorestack Class::memberfunction;  // do not re-init the stack in this director method
%initstack   Class;               // init the stack on all the methods of this class
%initstack;   // all director functions will re-init the stack