Multiple Definition of "..." c

Question

I have a header file "USpecs.h":

#ifndef USPECS_H
#define USPECS_H
#include "Specs.h"


#include <iostream>
#include <vector>

std::vector<Specs*> UcakSpecs;


#endif

I am using this header both in main function and another class named Ucak.

But when i build it the following error occurs:

Ucak.cpp|6|multiple definition of `UcakSpecs'|

As i searched before, it should be okay with #ifndef but it is not.

Answer 1

The include guards only prevent multiple definitions within a single translation unit (i.e. a single source file with its included headers). They do not prevent multiple definitions when you include the header from multiple source files.

Instead, you should have a declaration in the header:

extern std::vector<Specs*> UcakSpecs;

and a definition in one (and only one) source file:

std::vector<Specs*> UcakSpecs;