Invoke C++ code within C

README.md

#Invoking C++ code within a C file

I don't know how useful this is in practice, but today I learned that you can invoke C++ code within a C library if needed. The basic process is to forward-declare C++ structs, then declare the function(s) to be invoked from the C library within an extern "C" block. The file containing these declarations will need to be included in the C source file and in the C++ header and source files. The C file can then be compiled normally with gcc and the C++ source with g++. Finally, the executable will need to be linked with g++ so that C++ operators and keywords will be defined.

This trivial example seems silly, but one can imagine a scenario where a C-library does useful work while taking a function pointer as an argument. The function declared in the extern "C" block here might be accepted as such an argument, and as long as the final result is linked with a C++ standard library, everything will work.

build.sh

#!/bin/bash

gcc -o extern_new_main.o -c extern_new_main.c # This works!
g++ -o extern_new.o -c extern_new.cpp
g++ -o extern_new extern_new_main.o extern_new.o # This also works!
./extern_new # => DerivedClass object with address: 00464680

extern_new.cpp

#include "extern_new.h"

extern "C" {
  DerivedClass *create_derived_class(char a, char b) {
    return new DerivedClass(a, b);
  }
}

extern_new.h

#ifndef EXTERN_NEW_H
#define EXTERN_NEW_H

#include "extern_new_function.h"

struct BaseClass {
public:
  BaseClass(char a) : m_a(a) {}
  void SetA(char a) { m_a = a; }
  char GetA() { return m_a; }
private:
  char m_a;
};

struct DerivedClass : public BaseClass {
public:
  DerivedClass(char a, char b) : BaseClass(a), m_b(b) {}
  void SetB(char b) { m_b = b; }
  char GetB() { return m_b; }
private:
  char m_b;
};

#endif

extern_new_function.h

#ifndef EXTERN_NEW_FUNCTION_H
#define EXTERN_NEW_FUNCTION_H

#ifdef __cplusplus
extern "C" {
#endif

  typedef struct DerivedClass DerivedClass;
  DerivedClass *create_derived_class(char a, char b);

#ifdef __cplusplus
}
#endif

#endif

extern_new_main.c

#include "extern_new_function.h"
#include <stdio.h>

int main() {
  DerivedClass *derived = create_derived_class('a', 'b');
  printf("DerivedClass object with address: %p\n", derived);
  return 0;
}