How can I modify my own C header files so it's easier to #include them in C++ code?, C++ FAQ

C++ FAQ / Section 32 / FAQ 32.4

Section 32:

32.1	What do I need to know when mixing C and C++ code?
32.2	How can I include a standard C header file in my C++ code?
32.3	How can I include a non-system C header file in my C++ code?
32.4	How can I modify my own C header files so it's easier to `#include` them in C++ code?
32.5	How can I call a non-system C function `f(int,char,float)` from my C++ code?
32.6	How can I create a C++ function `f(int,char,float)` that is callable by my C code?
32.7	Why is the linker giving errors for C/C++ functions being called from C++/C functions?
32.8	How can I pass an object of a C++ `class` to/from a C function?
32.9	Can my C function directly access data in an object of a C++ `class`?
32.10	Why do I feel like I'm "further from the machine" in C++ as opposed to C?

[32.4] How can I modify my own C header files so it's easier to #include them in C++ code?

If you are including a C header file that isn't provided by the system, and if you are able to change the C header, you should strongly consider adding the extern "C" {...} logic inside the header to make it easier for C++ users to #include it into their C++ code. Since a C compiler won't understand the extern "C" construct, you must wrap the extern "C" { and } lines in an #ifdef so they won't be seen by normal C compilers.

Step #1: Put the following lines at the very top of your C header file (note: the symbol __cplusplus is #defined if/only-if the compiler is a C++ compiler):

#ifdef __cplusplus
extern "C" {
#endif

Step #2: Put the following lines at the very bottom of your C header file:

#ifdef __cplusplus
}
#endif

Now you can #include your C header without any extern "C" nonsense in your C++ code:

// This is C++ code

// Get declaration for f(int i, char c, float x)
#include "my-C-code.h"   // Note: nothing unusual in #include line

int main()
{
  f(7, 'x', 3.14);       // Note: nothing unusual in the call
  ...
}

Note: Somewhat different guidelines apply for C headers provided by the system (such as <cstdio>) and for C headers that you can't change.

Note: #define macros are evil in 4 different ways: evil#1, evil#2, evil#3, and evil#4. But they're still useful sometimes. Just wash your hands after using them.