Include and compile C++ header file from C

Question

I have a C++ file and its header file. I need to include this header file in a C code and use the functions in it.

When the cpp.h file is compiled through main.c, compilation fails because of the C++ linkage.

On using the macro __cplusplus stream and string are not resolved, is there some way to compile the cpp.h file through and execute?

I have given a outline of my code only.

C++ header file cpp.h:

struct s1
{
string a;
string b;
};
typedef struct s1 s2;
class c1
{
public:
void fun1(s2 &s3);
private: 
fun2(std::string &x,const char *y);
};

C++ file cpp.cpp:

c1::fun1(s2 &s3)
{
 fstream file;

}

c1::fun2(std::string &x,const char *y)
{

}

C file main.c:

#include "cpp.h"
void main()
{
 c1 c2;
 s1 structobj;
 c2.fun1(&structobj);
 printf("\n value of a in struct %s",structobj.a);

}

No... there is no possibility of compiling main.c using c++ compiler because of dependency — Nerdy
– Nerdy, Commented Apr 23, 2016 at 12:08
Does call back solves this problem ? By introducing another cpp file to handle call back — Nerdy
– Nerdy, Commented Apr 23, 2016 at 12:08

jdarthenay · Accepted Answer · 2016-04-23 11:11:46Z

9

Basically, you can't. You need to put only C functions in your header file. You put them in a extern "C" block this way:

#ifdef __cplusplus
extern "C"
{
#endif

extern void myCppFunction(int n);

#ifdef __cplusplus
}
#endif

The extern "C" block is not recognized by a C compiler, but the C++ compiler need it to understand he have to consider functions inside as C functions.

In your cpp file you can define myCppFunction() so that she uses any C++ code, you will get a function C code can use.

Edit: I add a full example of how to link a program with a C main() using some C++ functions in a module.

stackoverflow.c:

#include "outputFromCpp.h"

int main()
{
    myCppFunction(2000);

    return 0;
}

outputFromCpp.h:

#ifndef OUTPUT_FROM_CPP_H
#define OUTPUT_FROM_CPP_H

#ifdef __cplusplus
extern "C"
{
#endif

extern void myCppFunction(int n);

#ifdef __cplusplus
}
#endif

#endif

outputFromCpp.cpp:

#include "outputFromCpp.h"

#include <iostream>

using namespace std;

void myCppFunction(int n)
{
    cout << n << endl;
}

Compiling and linking:

gcc -Wall -Wextra -Werror -std=gnu99 -c stackoverflow.c
g++ -Wall -Wextra -Werror -std=c++98 -c outputFromCpp.cpp
g++ -o stackoverflow.exe stackoverflow.o outputFromCpp.o -static

You cannot link such a program with gcc. If you want to link with gcc you need to put all the C++ code in a shared library, I don't put an example as it would be a bit platform dependent.

edited Apr 23, 2016 at 11:11

answered Apr 23, 2016 at 4:43

jdarthenay

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4 Comments

Jonathan Leffler Over a year ago

If your program includes any C++ code or libraries, link with the C++ compiler. It's also sensible to make the main() program into C++ code, even if it is written as: int main(int argc, char **argv) { return real_main(argc, argv); } (or, if the code ignores command line arguments, then int main() { return real_main(); }) where, in each case, real_main() is the C function that would be main() if you weren't using a minimal C++ main() instead.

Ben Voigt Over a year ago

@JonathanLeffler: The toolchain is providing a wrapper main for you anyway, that knows about C++ globals initialization.

Nerdy Over a year ago

Is that possible to include a callback by introducing another c++ file. where the new cpp file uses the function to extract value and using call back from c file to the new c++ file.

jdarthenay Over a year ago

@user4402433 I am not sure I understand your question. In my example I can put any C++ code in myCppFunction(), including creating C++ objects with new and deleting them with delete, or calling code from another C++ module. Does that answer your question?

Nerdy · Accepted Answer · 2016-06-04 03:58:54Z

3

This can be done by introducing a wrapper to c++ function. The C function calls the wrapper function which inturn calls the desired C++ function (including member functions). More details are available here

answered Jun 4, 2016 at 3:58

Nerdy

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