Wrapping a C++ class in C

I didn’t think this could deserve a post, but a friend of mine asked me about how to wrap a C++ class in Fortran, which means wrapping it in C.

I thought of one obvious way, but there is also a typesafe version that should be used.

The simple way

The simple way consists of a simple void* interface:

 
#ifndef INTERFACE_H
#define INTERFACE_H
 
#ifdef __cplusplus
extern "C"
{
#endif
 
void* create_manager();
 
void destroy_manager(void* manager);
 
void print_manager(void* manager);
 
#ifdef __cplusplus
}
#endif
 
#endif

The associated implementation is thus:

#include "manager.h"
#include "interface.h"
 
void* create_manager()
{
  return reinterpret_cast<void*>(new Manager);
}
 
void destroy_manager(void* manager)
{
  delete reinterpret_cast<Manager*>(manager);
}
 
void print_manager(void* manager)
{
  reinterpret_cast<Manager*>(manager)->print();
}

Simple, easy and not typesafe.

The correct way

In fact, it is possible to keep the type information, without havong to use reinterpret_casts.

#ifndef INTERFACE_H
#define INTERFACE_H
 
#ifdef __cplusplus
extern "C"
{
#endif
 
typedef struct Manager Manager;
 
Manager* create_manager();
 
void destroy_manager(Manager* manager);
 
void print_manager(Manager* manager);
 
#ifdef __cplusplus
}
#endif
 
#endif

The tric is to declare the type Manager, but without having to expose it to C. As only pointer is needed, a forward declaration is possible, and as we are in C, a typedef is the only needed instruction.

 
#include "manager.h"
#include "interface.h"
 
Manager* create_manager()
{
  return new Manager;
}
 
void destroy_manager(Manager* manager)
{
  delete manager;
}
 
void print_manager(Manager* manager)
{
  manager->print();
}

Of course it is not exception safe, I expect my friend to use Fortran 2003’s class to achieve it.

1 thought on “Wrapping a C++ class in C”

Thank you so much. Your example is finally working after I found so many code fragments (even high rated content on Stack Overflow) that always threw up errors. Thanks a lot!

german says:

March 30, 2016 at 2:19 PM

Thank you so much. Your example is finally working after I found so many code fragments (even high rated content on Stack Overflow) that always threw up errors. Thanks a lot!