Using C++ class inside C

Question

I am new to C++. What I am doing is creating a callback library file in C. That file will get use in C++ application. C file will have only three functions and FYI I don't want to make another cpp file/class for that because of class overhead and also classes should be in cpp file only. please go through code snippet. C file XYZ.c is using class of ABC.hpp in its function.

Allheader.hpp

#include<iostream>
 .
 .

ABC.hpp

#include "Allheader.hpp"
  .
  .

XYZ.h

func1();
func2();
func3();

XYZ.c

include "XYZ.h"
include "ABC.hpp"

func1()
{
   using class of ABC here
}

func2() {}
func3() {}

MyApplication.cpp

extern "C" { #include "XYZ.h"} 
   .
   .

So This file XYZ.c / XYZ.h will be use in My CPP application as callback.

Compiling with g++

What I have done.

1. Compiled : It's Showing error, No file iostream.

2. As I was compiling C file and using C++ inside it started using __BEGIN_DECLS

   and code in XYZ.c file was in

   __BEGIN_DECLS __END_DECLS

   block suggested by a colleague. But still error was the same.

How can I use C++ inside C function? Please Suggest any solution.

Answer 1

You can't compile C++ code as C code. This C code is invalid:

#include "XYZ.h"
#include "ABC.hpp"  // this is wrong

func1()
{
   using class of ABC here  // this is also wrong (and incomplete...)
}

func2() {}
func3() {}

Again - you simply can not do that. C and C++ are different languages. You might as well try asking "Why can't I compile Java or Python with a C compiler" and the answer would be the same, "Because it's not C."

You can call C++ functions with extern "C" linkage from C.

From a C++ file:

extern "C" void calledFromC()
{
     // C++ code
}

And then, in a C++ header that can be #include'd by C:

#ifdef __cplusplus
extern "C" {
#endif

void calledFromC();

#ifdef __cplusplus
}
#endif

Answer 2

After few days of searching in parallel to my work. I am able to do it. As I said in my requirement that I need a Callback C file that contains some functions and We need to use it because We don't want a cpp file. (Because in our application cpp file will contain Class and We don't want a class for the callbacks).

Now What I did :

Application uses cmake to build. So I added

set_source_files_properties(${BASE_DIR}/path/to/XYZ.c PROPERTIES LANGUAGE CXX)

in CMakeLists.txt

It makes compiler know that It should compile XYZ.c file like a cpp file.

Basically If you "man g++" then it's written that

C++ source files conventionally use one of the suffixes .C, .cc, .cpp, .CPP, .c++, .cp, or .cxx; C++ header files often use .hh, .hpp, .H, or (for shared template code) .tcc; and preprocessed C++ files use the suffix .ii. GCC recognizes files with these names and compiles them as C++ programs even if you call the compiler the same way as for compiling C programs (usually with the name gcc).

And This is what I need. Just to tell the compiler that Compile like cpp.

Conclusion :

This is working and Yes this is not a way to call C++ class from C. As @Andrew told C and C++ are different languages. It's kind of the requirement that I need to follow and I am new to it so took some time in searching all these.

Thanks @Andrew & @Galik for your time.