multiple definition in C++ including header

Question

I have a project with multiple file as below:

//header.h
class example {...}

//variable.h
#include "header.h"
example ex;

//main.cpp
#include "variable.h"
....

//src1.cpp
#include "variable.h"

when compiling the compiler error as: multiple definition of "ex" I dont understand why, I want to use "ex" in main.cpp and src1.cpp, how should I do. Thanks,

duplicate ? https://stackoverflow.com/questions/11072244/c-multiple-definitions-of-a-variable — user30701, Jun 27 '18 at 10:11
Possible duplicate of [c++ multiple definitions of a variable](https://stackoverflow.com/questions/11072244/c-multiple-definitions-of-a-variable) — Søren D. Ptæus, Jun 27 '18 at 10:35
Just a clarification: you'd have exactly the same problem without "header.h" if "variable.h" had `int ex;` in it. — Pete Becker, Jun 27 '18 at 12:02

score 1 · Accepted Answer · answered Jun 27 '18 at 10:41

By #including variable.h in both main.cpp and src1.cpp, you have defined variable ex twice. The linker (not the compiler) won't like this.

Instead, change variable.h to look like this:

extern example ex;

And put:

example ex;

in (say) src1.cpp.

And yes, use include guards also, but that's not the problem here.

score 0 · Answer 2 · answered Jun 27 '18 at 10:25

0

Use include guards in header.h and variable.h:

#ifndef HEADER_H
#define HEADER_H

class header {...}

#endif

answered Jun 27 '18 at 10:25

rcs

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multiple definition in C++ including header

2 Answers2