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| <title>SWIG:Examples:python:class</title> |
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| <tt>SWIG/Examples/python/class/</tt> |
| <hr> |
| |
| <H2>Wrapping a simple C++ class</H2> |
| |
| <p> |
| This example illustrates the most primitive form of C++ class wrapping performed |
| by SWIG. In this case, C++ classes are simply transformed into a collection of |
| C-style functions that provide access to class members. |
| |
| <h2>The C++ Code</h2> |
| |
| Suppose you have some C++ classes described by the following (and admittedly lame) |
| header file: |
| |
| <blockquote> |
| <pre> |
| /* File : example.h */ |
| |
| class Shape { |
| public: |
| Shape() { |
| nshapes++; |
| } |
| virtual ~Shape() { |
| nshapes--; |
| }; |
| double x, y; |
| void move(double dx, double dy); |
| virtual double area() = 0; |
| virtual double perimeter() = 0; |
| static int nshapes; |
| }; |
| |
| class Circle : public Shape { |
| private: |
| double radius; |
| public: |
| Circle(double r) : radius(r) { }; |
| virtual double area(); |
| virtual double perimeter(); |
| }; |
| |
| class Square : public Shape { |
| private: |
| double width; |
| public: |
| Square(double w) : width(w) { }; |
| virtual double area(); |
| virtual double perimeter(); |
| }; |
| </pre> |
| </blockquote> |
| |
| <h2>The SWIG interface</h2> |
| |
| A simple SWIG interface for this can be built by simply grabbing the header file |
| like this: |
| |
| <blockquote> |
| <pre> |
| /* File : example.i */ |
| %module example |
| |
| %{ |
| #include "example.h" |
| %} |
| |
| /* Let's just grab the original header file here */ |
| %include "example.h" |
| </pre> |
| </blockquote> |
| |
| Note: when creating a C++ extension, you must run SWIG with the <tt>-c++</tt> option like this: |
| <blockquote> |
| <pre> |
| % swig -c++ -python example.i |
| </pre> |
| </blockquote> |
| |
| <h2>A sample Python script</h2> |
| |
| Click <a href="example.py">here</a> to see a script that calls the C++ functions from Python. |
| |
| <h2>Key points</h2> |
| |
| <ul> |
| <li>To create a new object, you call a constructor like this: |
| |
| <blockquote> |
| <pre> |
| c = example.new_Circle(10.0) |
| </pre> |
| </blockquote> |
| |
| <p> |
| <li>To access member data, a pair of accessor functions are used. |
| For example: |
| |
| <blockquote> |
| <pre> |
| example.Shape_x_set(c,15) # Set member data |
| x = example.Shape_x_get(c) # Get member data |
| </pre> |
| </blockquote> |
| |
| Note: when accessing member data, the name of the class in which |
| the member data was must be used. In this case, <tt>Shape_x_get()</tt> |
| and <tt>Shape_x_set()</tt> are used since 'x' was defined in Shape. |
| |
| <p> |
| <li>To invoke a member function, you simply do this |
| |
| <blockquote> |
| <pre> |
| print "The area is ", example.Shape_area(c) |
| </pre> |
| </blockquote> |
| |
| <p> |
| <li>Type checking knows about the inheritance structure of C++. For example: |
| |
| <blockquote> |
| <pre> |
| example.Shape_area(c) # Works (c is a Shape) |
| example.Circle_area(c) # Works (c is a Circle) |
| example.Square_area(c) # Fails (c is definitely not a Square) |
| </pre> |
| </blockquote> |
| |
| <p> |
| <li>To invoke a destructor, simply do this |
| |
| <blockquote> |
| <pre> |
| example.delete_Shape(c) # Deletes a shape |
| </pre> |
| </blockquote> |
| |
| (Note: destructors are currently not inherited. This might change later). |
| |
| <p> |
| <li>Static member variables are wrapped as C global variables. For example: |
| |
| <blockquote> |
| <pre> |
| n = example.cvar.Shape_nshapes # Get a static data member |
| example.cvar.Shapes_nshapes = 13 # Set a static data member |
| </pre> |
| </blockquote> |
| |
| </ul> |
| |
| <h2>General Comments</h2> |
| |
| <ul> |
| <li>This low-level interface is not the only way to handle C++ code. |
| Proxy classes provide a much higher-level interface. |
| |
| <p> |
| <li>SWIG *does* know how to properly perform upcasting of objects in |
| an inheritance hierarchy (including multiple inheritance). Therefore |
| it is perfectly safe to pass an object of a derived class to any |
| function involving a base class. |
| |
| <p> |
| <li>A wide variety of C++ features are not currently supported by SWIG. Here is the |
| short and incomplete list: |
| |
| <p> |
| <ul> |
| <li>Overloaded methods and functions. SWIG wrappers don't know how to resolve name |
| conflicts so you must give an alternative name to any overloaded method name using the |
| %name directive like this: |
| |
| <blockquote> |
| <pre> |
| void foo(int a); |
| %name(foo2) void foo(double a, double b); |
| </pre> |
| </blockquote> |
| |
| <p> |
| <li>Overloaded operators. Not supported at all. The only workaround for this is |
| to write a helper function. For example: |
| |
| <blockquote> |
| <pre> |
| %inline %{ |
| Vector *vector_add(Vector *a, Vector *b) { |
| ... whatever ... |
| } |
| %} |
| </pre> |
| </blockquote> |
| |
| <p> |
| <li>Namespaces. Not supported at all. Won't be supported until SWIG2.0 (if at all). |
| |
| <p> |
| <li>Dave's snide remark: Like a large bottle of strong Tequilla, it's better to |
| use C++ in moderation. |
| |
| </ul> |
| |
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| </body> |
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