Examples/java/class/index.html - master - niceSOFT/swig

Examples/java/class/index.html
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Olly Betts
Improve the class example for several languages.
23 фев 2014, 09:24
23 фев 2014, 09:2434c97ff
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<title>SWIG:Examples:java:class</title>
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<tt>SWIG/Examples/java/class/</tt>
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<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>

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