| <html> |
| <head> |
| <title>SWIG:Examples:java:class</title> |
| </head> |
| |
| <body bgcolor="#ffffff"> |
| |
| |
| <tt>SWIG/Examples/java/class/</tt> |
| <hr> |
| |
| <H2>Wrapping a simple C++ class</H2> |
| |
| <p> |
| This example illustrates the high level form of C++ class wrapping performed |
| by SWIG. In this case, a C++ class has a proxy Java class, which |
| provides access to C++ 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++ -java example.i |
| </pre> |
| </blockquote> |
| |
| <h2>A sample Java program</h2> |
| |
| Click <a href="runme.java">here</a> to see a Java program that calls the C++ functions from Java. |
| |
| <h2>Key points</h2> |
| |
| <ul> |
| <li>To create a new object, you call a constructor like this: |
| |
| <blockquote> |
| <pre> |
| Circle c = new Circle(10); |
| </pre> |
| </blockquote> |
| |
| <p> |
| <li>To access member data, a pair of accessor functions are used. |
| For example: |
| |
| <blockquote> |
| <pre> |
| c.setX(15); // Set member data |
| x = c.getX(); // Get member data |
| </pre> |
| </blockquote> |
| |
| <p> |
| <li>To invoke a member function, you simply do this |
| |
| <blockquote> |
| <pre> |
| System.out.println( "The area is " + c.area() ); |
| </pre> |
| </blockquote> |
| |
| <p> |
| <li>To invoke a destructor, simply do this |
| |
| <blockquote> |
| <pre> |
| c.delete(); // Deletes a shape |
| </pre> |
| </blockquote> |
| |
| <p> |
| <li>Static member variables are wrapped with java static get and set access functions. For example: |
| |
| <blockquote> |
| <pre> |
| n = Shape.getNshapes(); // Get a static data member |
| Shape.setNshapes(13); // Set a static data member |
| </pre> |
| </blockquote> |
| |
| </ul> |
| |
| <h2>General Comments</h2> |
| |
| <ul> |
| <li>This high-level interface using proxy classes is not the only way to handle C++ code. |
| A low level interface using C functions to access member variables and member functions is the alternative SWIG |
| approach. This entails passing around the C pointer or C++ 'this' pointer and as such it is not difficult to crash the JVM. |
| The abstraction of the underlying pointer by the java proxy classes far better fits the java programming paradigm. |
| |
| <p> |
| <li>SWIG <b>does</b> know how to properly perform upcasting of objects in an inheritance |
| hierarchy (including multiple inheritance). However Java classes can only derive from one base class so multiple inheritance |
| is not implemented. Java classes can implement more than one interface so there is scope for improvement in the future. |
| |
| </ul> |
| |
| <hr> |
| </body> |
| </html> |
