Call C++ member function in C

Compiler & OS used:

gcc (Ubuntu 7.3.0-27ubuntu1~18.04) 7.3.0

g++ (Ubuntu 7.3.0-27ubuntu1~18.04) 7.3.0

Linux(Ubuntu-18.04) 

There are three steps involved to achieve this goal.

1. Create C++ class with member function.
2. Create a wrapper in C++ which can be used by C code to call C++ member function.
3. Write C code, use the wrapper to call C++ member function.

Step1:

    i) cppdemo.h

#ifndef CPP_DEMO_H
#define CPP_DEMO_H

class CppDemo {

public:

CppDemo();

void Print(const char * str);

};
#endif 

ii) cppdemo.cpp

#include <iostream>

#include "cppdemo.h"

CppDemo::CppDemo() {

    std::cout << "CPP Demo Constructor Called!!!" << std::endl;
}

void CppDemo::Print(const char *str) {

    std::cout << "I am in C++ code, printing your message: " << str << std::endl;
}

Compile: 

g++ -fpic -shared cppdemo.cpp -o libcppdemo.so

Step2:

i) c_wrapper.h

#ifndef C_WRAPPER_H 
#define C_WRAPPER_H 
 

#ifdef __cplusplus
extern "C" {
#endif

void print(const char *name);

#ifdef __cplusplus
}
#endif

#endif

     

ii)  c_wrapper.cpp

// NULL can be replaced by nullptr(C++11/14/17 support of compiler is needed)


#include <iostream>


#include "c_wrapper.h"
#include "cppdemo.h"

#ifdef __cplusplus
extern "C" {
#endif

static CppDemo* obj = NULL;

void CreateObject() {

	if (obj == NULL) {
		obj = new CppDemo();
	}
}

void DeleteObj() {


  if(obj) {

    obj->Print("Delete called\n");
    delete obj;
    obj = NULL;
  }
}

void print(const char* str) {

    CreateObject();
    obj->Print(str);
    DeleteObj();
}

#ifdef __cplusplus
}
#endif

Compile: 

g++ -fpic -shared c_wrapper.cpp -L. -lcppdemo -o libc_wrapper.so 

Step3:

main.c

#include "c_wrapper.h"

int main() {
  print("Subrat");
  print("Suraj");
  return 0;
}

Setting library path:

export LD_LIBRARY_PATH=$PWD

Compile:

 gcc main.c -L. -lc_wrapper -o output

Run:

./outpt

Output:

CPP Demo Constructor Called!!!
I am in C++ code, printing your message: Subrat
I am in C++ code, printing your message: Delete called

CPP Demo Constructor Called!!!
I am in C++ code, printing your message: Suraj
I am in C++ code, printing your message: Delete called

Conclusion:

This article gives you an idea about how to migrate existing C source code to C++ code.

Here we have created the class "CppDemo" having the member function "Print". We created a shared library of it and it is being used by wrapper class "c_wrapper.cpp". The wrapper class is also built to a shared library which is being used by C souce code.

Note:

You may get a linking error. Something like below

./libc_wrapper.so: undefined reference to `CppDemo::Print(char const*)'
./libc_wrapper.so: undefined reference to `CppDemo::CppDemo()'

You can resolve that by setting the library path by below command (this is the path of your .so files).

export LD_LIBRARY_PATH=$PWD