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2.11.3 A Simple Example

The following example, provided as part of the GNAT examples, shows how to achieve procedural interfacing between Ada and C++ in both directions. The C++ class A has two methods. The first method is exported to Ada by the means of an extern C wrapper function. The second method calls an Ada subprogram. On the Ada side, The C++ calls are modelled by a limited record with a layout comparable to the C++ class. The Ada subprogram, in turn, calls the C++ method. So, starting from the C++ main program, the process passes back and forth between the two languages.

Here are the compilation commands:

     $ gnatmake -c simple_cpp_interface
     $ c++ -c cpp_main.C
     $ c++ -c ex7.C
     $ gnatbind -n simple_cpp_interface
     $ gnatlink simple_cpp_interface -o cpp_main --LINK=$(CPLUSPLUS)
           -lstdc++ ex7.o cpp_main.o

Here are the corresponding sources:

     #include "ex7.h"
     extern "C" {
       void adainit (void);
       void adafinal (void);
       void method1 (A *t);
     void method1 (A *t)
       t->method1 ();
     int main ()
       A obj;
       adainit ();
       obj.method2 (3030);
       adafinal ();
     class Origin {
       int o_value;
     class A : public Origin {
       void method1 (void);
       void method2 (int v);
       int   a_value;
     #include "ex7.h"
     #include <stdio.h>
     extern "C" { void ada_method2 (A *t, int v);}
     void A::method1 (void)
       a_value = 2020;
       printf ("in A::method1, a_value = %d \n",a_value);
     void A::method2 (int v)
        ada_method2 (this, v);
        printf ("in A::method2, a_value = %d \n",a_value);
        a_value = 1010;
       printf ("in A::A, a_value = %d \n",a_value);
     -- Ada sources
     package body Simple_Cpp_Interface is
        procedure Ada_Method2 (This : in out A; V : Integer) is
           Method1 (This);
           This.A_Value := V;
        end Ada_Method2;
     end Simple_Cpp_Interface;
     with System;
     package Simple_Cpp_Interface is
        type A is limited
              Vptr    : System.Address;
              O_Value : Integer;
              A_Value : Integer;
           end record;
        pragma Convention (C, A);
        procedure Method1 (This : in out A);
        pragma Import (C, Method1);
        procedure Ada_Method2 (This : in out A; V : Integer);
        pragma Export (C, Ada_Method2);
     end Simple_Cpp_Interface;