In ref : Linking C compiled static library to C++ Program

Question

Stackoverflow won't let me comment because I am a newbie.

Borrowing the title from the question posted here, the accepted solution says that you have to modify the C header file using #ifdef __cplusplus and extern "C". I have the exact same question but I have been provided a C library containing over 100 .h C header files and .c C program files and now I have to use a C++ library so my main() will be in C++.

So do I now have to add those modifications to each one of those C header files or is there some other approach? My main() is in a .cpp file which will use functions from this C library.

EDIT This question has been resolved. I wish for this question to be closed or removed.

score 3 · Accepted Answer · answered Jun 27 '18 at 18:19

You don't have to modify any headers. What is necessary is to wrap all C declarations with extern C {}. You can do this just as well in you cpp file:

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

int main() {
    std::cout << "This is C++ code\n";
}

It might be useful to create a header file which simply includes C headers with C linkage:

// c_header.hpp
#pragma once

#ifdef __cplusplus
extern "C" {
#endif 

#include "c_header_1.h"
#include "c_header_2.h"

#ifdef __cplusplus
}
#endif

// main.cpp
#include "c_header.hpp" // extern "C" {} already handled for you

score 1 · Answer 2 · answered Jun 27 '18 at 18:20

Only those declarations of C functions and objects that are accessed by C++ components need to declared with C linkage, and only to the C++ components. Presumably, your new C++ library accesses very little, if anything, from the existing C code, so the primary concern would be whatever is accessed (directly) by main(). Those functions must be declared (to main) with C linkage. If that turns out to be a significant number of things then you could consider refactoring to make it fewer.

Moreover, no, you do not need to modify existing header files. The C++ files that reference your C library, and that therefore include some of its headers, can wrap the relevant #include statements an an extern "C" block to take care of the issue.

In ref : Linking C compiled static library to C++ Program

2 Answers2