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  3. <title>SWIG:Examples:java:class</title>
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  7. 

  8. 

  9. <tt>SWIG/Examples/java/class/</tt>
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  10. <hr>
    11. 

  11. 

  12. <H2>Wrapping a simple C++ class</H2>
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  13. 

  14. <tt>$Header$</tt><br>
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  15. 

  16. <p>
    17. 

  17. This example illustrates the high level form of C++ class wrapping performed
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  18. by SWIG.  In this case, a C++ class has a proxy Java class, which 
    19. 

  19. provides access to C++ class members.
    20. 

  20. 

  21. <h2>The C++ Code</h2>
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  22. 

  23. Suppose you have some C++ classes described by the following (and admittedly lame) 
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  24. header file:
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  25. 

  26. <blockquote>
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  27. <pre>
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  28. /* File : example.h */
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  29. 

  30. class Shape {
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  31. public:
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  32.   Shape() {
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  33.     nshapes++;
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  34.   }
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  35.   virtual ~Shape() {
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  36.     nshapes--;
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  37.   };
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  38.   double  x, y;   
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  39.   void    move(double dx, double dy);
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  40.   virtual double area() = 0;
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  41.   virtual double perimeter() = 0;
    42. 

  42.   static  int nshapes;
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  43. };
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  44. 

  45. class Circle : public Shape {
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  46. private:
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  47.   double radius;
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  48. public:
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  49.   Circle(double r) : radius(r) { };
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  50.   virtual double area();
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  51.   virtual double perimeter();
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  52. };
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  53. 

  54. class Square : public Shape {
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  55. private:
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  56.   double width;
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  57. public:
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  58.   Square(double w) : width(w) { };
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  59.   virtual double area();
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  60.   virtual double perimeter();
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  61. };
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  62. </pre>
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  63. </blockquote>
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  64. 

  65. <h2>The SWIG interface</h2>
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  66. 

  67. A simple SWIG interface for this can be built by simply grabbing the header file
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  68. like this:
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  69. 

  70. <blockquote>
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  71. <pre>
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  72. /* File : example.i */
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  73. %module example
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  74. 

  75. %{
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  76. #include "example.h"
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  77. %}
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  78. 

  79. /* Let's just grab the original header file here */
    80. 

  80. %include "example.h"
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  81. </pre>
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  82. </blockquote>
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  83. 

  84. Note: when creating a C++ extension, you must run SWIG with the <tt>-c++</tt> option like this:
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  85. <blockquote>
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  86. <pre>
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  87. % swig -c++ -java example.i
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  88. </pre>
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  89. </blockquote>
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  90. 

  91. <h2>A sample Java program</h2>
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  92. 

  93. Click <a href="main.java">here</a> to see a Java program that calls the C++ functions from Java.
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  94. 

  95. <h2>Key points</h2>
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  96. 

  97. <ul>
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  98. <li>To create a new object, you call a constructor like this:
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  99. 

  100. <blockquote>
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  101. <pre>
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  102. Circle c = new Circle(10);
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  103. </pre>
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  104. </blockquote>
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  105. 

  106. <p>
    107. 

  107. <li>To access member data, a pair of accessor functions are used.
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  108. For example:
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  109. 

  110. <blockquote>
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  111. <pre>
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  112. c.setX(15);        // Set member data
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  113. x = c.getX();      // Get member data
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  114. </pre>
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  115. </blockquote>
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  116. 

  117. <p>
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  118. <li>To invoke a member function, you simply do this
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  119. 

  120. <blockquote>
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  121. <pre>
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  122. System.out.println( "The area is " + c.area() );
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  123. </pre>
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  124. </blockquote>
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  125. 

  126. <p>
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  127. <li>To invoke a destructor, simply do this
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  128. 

  129. <blockquote>
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  130. <pre>
    131. 

  131. c.delete();     // Deletes a shape
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  132. </pre>
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  133. </blockquote>
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  134. 

  135. <p>
    136. 

  136. <li>Static member variables are wrapped with java static get and set access functions.  For example:
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  137. 

  138. <blockquote>
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  139. <pre>
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  140. n = Shape.getNshapes();     // Get a static data member
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  141. Shape.setNshapes(13);       // Set a static data member
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  142. </pre>
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  143. </blockquote>
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  144. 

  145. </ul>
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  146. 

  147. <h2>General Comments</h2>
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  148. 

  149. <ul>
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  150. <li>This high-level interface using shadow classes is not the only way to handle C++ code. 
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  151. A low level interface using c functions to access member variables and member functions is the alternative SWIG
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  152. 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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  153. The abstraction of the underlying pointer by the java shadow classes far better fits the java programming paradigm.
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  154. 

  155. <p>
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  156. <li>SWIG *does* know how to properly perform upcasting of objects in an inheritance
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  157. hierarchy (including multiple inheritance).  However Java classes can only derive from one base class so multiple inheritance
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  158. is not implemented. Java classes can implement more than one interface so there is scope for improvement in the future. 
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  159. 

  160. <p>
    161. 

  161. <li>A wide variety of C++ features are not currently supported by SWIG.  Here is the
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  162. short and incomplete list:
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  163. 

  164. <p>
    165. 

  165. <ul>
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  166. <li>Overloaded methods and functions.  SWIG wrappers don't know how to resolve name
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  167. conflicts so you must give an alternative name to any overloaded method name using the
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  168. %name directive like this:
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  169. 

  170. <blockquote>
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  171. <pre>
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  172. void foo(int a);  
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  173. %name(foo2) void foo(double a, double b);
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  174. </pre>
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  175. </blockquote>
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  176. 

  177. <p>
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  178. <li>Overloaded operators.  Not supported at all. The only workaround for this is
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  179. to write a helper function. For example:
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  180. 

  181. <blockquote>
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  182. <pre>
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  183. %inline %{
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  184.     Vector *vector_add(Vector *a, Vector *b) {
    185. 

  185.           ... whatever ...
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  186.     }
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  187. %}
    188. 

  188. </pre>
    189. 

  189. </blockquote>
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  190. 

  191. <p>
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  192. <li>Namespaces.  Not supported at all. Won't be supported until SWIG2.0 (if at all).
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    194. 

  194. </ul>
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  195. </ul>
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  196. 

  197. <hr>
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  198. </body>
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