Question

I have one file example1.cpp with the main function. This file must have #include mylib.h and #include lib.h. File mylib.h also has #include lib.h. When I try to compile this program, the error redefinition xyz function ocurs.

example1.cpp

#include mylib.h
#include lib.h

int main(){
//code
}

mylib.h

#include lib.h

//rest code

Answer 1

You have to wrap the .h files in #defines to avoid the redifinitions. For example:

#if !defined(_MY_LIB_H_)
#define _MY_LIB_H_

// Add your function definitions here...

#endif  // _MY_LIB_H_

You can now include it anywhere and the function definition will be read once. Also note that you can use #ifndef depending on the compiler. VC++ for example, allows “#pragma once” if it’s version 10 or higher:

#if _MSC_VER > 1000
#pragma once
#endif

In this case, you don’t need to use the #defines explained above.