SWIG Myths

Here are some common misconceptions about SWIG that have occasionally appeared on other web pages and in articles--especially those that describe older SWIG versions.

Myth: SWIG only really works with ISO C

Fact:

SWIG does provide wrapping support for all of ISO C. However, SWIG has also included C++ support since its initial release in 1996. To be fair, early SWIG releases had limitations, but C++ support has continually improved with each SWIG release.

C++ support in current SWIG releases is quite advanced--providing support for nearly every C++ feature including templates, namespaces, operators, overloaded methods, nested classes and more. The vast majority of C++11 is even supported. See the Features page for more information.

Myth: SWIG wraps C++ into this unusable low-level "C-tran" interface.

Fact:

When wrapping C++, SWIG does generate a set of low-level procedural wrappers. However, these are only used as building blocks for creating a more advanced high-level wrappers (in fact, users rarely interact with this layer directly). In nearly all target languages, SWIG wraps C++ classes into a nice object-oriented interface that mirrors the underlying C++ code. In Python, C++ classes are wrapped as Python classes, in Perl, C++ classes are wrapped as Perl classes, and so forth. The existence of procedural wrappers is only an artifact of SWIG's layered approach to wrapper code generation.

Myth: SWIG doesn't support overloaded methods/functions

Fact:

SWIG provides full support for C++ overloaded methods and functions. For example, if you have declarations like this:

class Foo {
public:
     Foo();
     Foo(const Foo &);
     void spam(int x);
     void spam(double x);
     void spam(char *x, int n);
};

They can be used in a completely natural way from most SWIG language modules. For example, in Python:

>>> f = Foo()
>>> f.spam(3)
>>> f.spam(3.5)
>>> f.spam("hello",5)
>>> g = Foo(f)           # Make a copy

Or in Tcl:

% Foo f
% f spam 3
% f spam 3.5
% f spam "hello" 5
% Foo g f

The SWIG implementation of overloading utilizes type categories and a type precedence scheme that results in the generation of dynamic dispatch functions. The order in which declarations appear does not matter nor does SWIG rely on unreliable mechanisms like trial execution (i.e., trying methods one by one until one happens to execute successfully). Overloading support in SWIG is more advanced than that.

Due to the dynamic nature of scripting languages, it is not possible to fully disambiguate overloaded methods to the same degree as in C++. For example, you might have declarations like this:

int foo(int);
long foo(long);

In this case, SWIG provides a number of directives to either rename or ignore one of the declarations. For example:

%ignore foo(long);

%rename(foo_long) foo(long);

Myth: SWIG doesn't support basic idiomatic C++ constructs like templates and smart pointers

Fact:

SWIG fully supports templates. However, to wrap a template class, you have to tell SWIG about a particular instantiation. This is easy, just make sure SWIG knows about the template definition and include a special directive like this:

%template(deque_int) std::deque<int>;

In this case, "deque_int" becomes the target language name for the wrapped object. You would use it just like a normal class in the extension module:

>>> d = deque_int(100)
>>> d.size()
100
>>>

On the subject of smart-pointers, those are supported too. For example, suppose you had some code like this:

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

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

typedef SmartPtr<Foo> FooPtr;

FooPtr make_Foo();

To wrap this with SWIG, just tell it about a template instantiation. For example:

%template(FooPtr) SmartPtr<Foo>;

Now, in the target language, the wrapper object works just like you would expect:

>>> f = make_Foo()
>>> f.x = 3
>>> f.spam(42)

Myth: SWIG is just too confusing to use

Fact:

Most users find SWIG to be relatively easy to use. However, confusion can arise from the following:

C programming. To effectively use SWIG, you should have a good grasp of basic C programming concepts like pointers, arrays, memory management, data structures, functions, parameter passing semantics, and so forth. SWIG tries to make it easy for C programmers to build extension modules, but it does not try to make C programming easy.
C++. This language is so large and complicated that certain parts of SWIG may not make any sense unless you are also familiar with the underlying C++ concepts.
Dynamic linking and shared libraries. To build extension modules, you usually have to create DLLs. If you've never done this before, there is a small learning curve associated with finding the right compiler and linker options.
Customization features. SWIG provides a large number of customization features and options that can be used to control almost all parts of the wrapper generation process. These are provided to better support power users and to provide the flexibility needed to solve difficult real-world wrapping problems. However, none of this is needed to get started.

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Last modified : Thu Apr 18 20:05:46 2019