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SWIG/Examples/java/class/index.html
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SWIG/Examples/java/class/index.html
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<html>
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<head>
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<title>SWIG:Examples:java:class</title>
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</head>
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<body bgcolor="#ffffff">
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<tt>SWIG/Examples/java/class/</tt>
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<hr>
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<H2>Wrapping a simple C++ class</H2>
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<tt>$Header$</tt><br>
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<p>
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This example illustrates the high level form of C++ class wrapping performed
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by SWIG. In this case, a C++ class has a proxy Java class, which
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provides access to C++ class members.
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<h2>The C++ Code</h2>
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Suppose you have some C++ classes described by the following (and admittedly lame)
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header file:
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<blockquote>
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<pre>
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/* File : example.h */
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class Shape {
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public:
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Shape() {
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nshapes++;
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}
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virtual ~Shape() {
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nshapes--;
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};
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double x, y;
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void move(double dx, double dy);
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virtual double area() = 0;
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virtual double perimeter() = 0;
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static int nshapes;
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};
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class Circle : public Shape {
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private:
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double radius;
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public:
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Circle(double r) : radius(r) { };
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virtual double area();
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virtual double perimeter();
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};
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class Square : public Shape {
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private:
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double width;
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public:
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Square(double w) : width(w) { };
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virtual double area();
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virtual double perimeter();
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};
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</pre>
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</blockquote>
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<h2>The SWIG interface</h2>
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A simple SWIG interface for this can be built by simply grabbing the header file
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like this:
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<blockquote>
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<pre>
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/* File : example.i */
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%module example
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%{
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#include "example.h"
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%}
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/* Let's just grab the original header file here */
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%include "example.h"
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</pre>
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</blockquote>
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Note: when creating a C++ extension, you must run SWIG with the <tt>-c++</tt> option like this:
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<blockquote>
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<pre>
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% swig -c++ -java example.i
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</pre>
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</blockquote>
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<h2>A sample Java program</h2>
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Click <a href="main.java">here</a> to see a Java program that calls the C++ functions from Java.
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<h2>Key points</h2>
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<ul>
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<li>To create a new object, you call a constructor like this:
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<blockquote>
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<pre>
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Circle c = new Circle(10);
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</pre>
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</blockquote>
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<p>
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<li>To access member data, a pair of accessor functions are used.
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For example:
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<blockquote>
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<pre>
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c.setX(15); // Set member data
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x = c.getX(); // Get member data
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</pre>
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</blockquote>
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<p>
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<li>To invoke a member function, you simply do this
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<blockquote>
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<pre>
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System.out.println( "The area is " + c.area() );
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</pre>
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</blockquote>
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<p>
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<li>To invoke a destructor, simply do this
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<blockquote>
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<pre>
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c.delete(); // Deletes a shape
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</pre>
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</blockquote>
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<p>
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<li>Static member variables are wrapped with java static get and set access functions. For example:
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<blockquote>
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<pre>
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n = Shape.getNshapes(); // Get a static data member
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Shape.setNshapes(13); // Set a static data member
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</pre>
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</blockquote>
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</ul>
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<h2>General Comments</h2>
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<ul>
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<li>This high-level interface using shadow classes is not the only way to handle C++ code.
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A low level interface using c functions to access member variables and member functions is the alternative SWIG
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approach. This entails passing around the c pointer or c++ 'this' pointer and as such it is not difficult to crash the JVM.
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The abstraction of the underlying pointer by the java shadow classes far better fits the java programming paradigm.
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<p>
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<li>SWIG *does* know how to properly perform upcasting of objects in an inheritance
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hierarchy (including multiple inheritance). However Java classes can only derive from one base class so multiple inheritance
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is not implemented. Java classes can implement more than one interface so there is scope for improvement in the future.
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<p>
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<li>A wide variety of C++ features are not currently supported by SWIG. Here is the
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short and incomplete list:
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<p>
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<ul>
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<li>Overloaded methods and functions. SWIG wrappers don't know how to resolve name
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conflicts so you must give an alternative name to any overloaded method name using the
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%name directive like this:
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<blockquote>
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<pre>
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void foo(int a);
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%name(foo2) void foo(double a, double b);
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</pre>
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</blockquote>
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<p>
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<li>Overloaded operators. Not supported at all. The only workaround for this is
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to write a helper function. For example:
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<blockquote>
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<pre>
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%inline %{
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Vector *vector_add(Vector *a, Vector *b) {
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... whatever ...
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}
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%}
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</pre>
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</blockquote>
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<p>
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<li>Namespaces. Not supported at all. Won't be supported until SWIG2.0 (if at all).
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</ul>
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</ul>
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<hr>
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</body>
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</html>
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