Question

I have a C++ struct with methods inside:

struct S
{
   int a;
   int b;

   void foo(void)
   {
       ... 
   };
}

I have a userprogram, written in C. Is it possible to get a pointer to a S-struct and access the member aand b?

Answer 1

You can access the members of a struct written in C++ from a C-program given you ensure that the C++ additions to the struct syntax are removed:

// header
struct S {
  int a, b;

#ifdef __cplusplus
  void foo(); 
#endif
};

// c-file:
#include "header.h"

void something(struct S* s)
{
  printf("%d, %d", s->a, s->b);
}

The memory layout of structs and classes in C++ is compatible with C for the C-parts of the language. As soon as you add a vtable (by adding virtual functions) to your struct it will no longer be compatible and you must use some other technique.