如何声明头文件中的变量在两个.cpp中使用？

Question

My target module is an executable to be built from X.cpp and Y.cpp , both these two files need a common .h file:

extern HANDLE hPipe;
extern IMediaSample *pSave = NULL;

But when I build the module, I got an error saying :

Y.obj : error LNK2005: "struct IMediaSample * pSave" (?pSave@@3PAUIMediaSample@@A) already defined in X.obj

How to solve this issue?

Answer 1

extern IMediaSample *pSave = NULL;

This is not just a declaration. This will define pSave to NULL . Since both .cpp include the .h , this variable will be defined in 2 translation units, which causes the conflict.

You should just rewrite it as

extern IMediaSample *pSave;

in the .h , then add IMediaSample *pSave = NULL; in exactly one of the .cpp s.

Answer 2

try using ifndef statement. define a variable unique to each header file you create then while including use something like:

#ifndef commonh
include common.h
#endif