Question

Can’t understand what is a problem here:
I have got main.cpp file where I am including:

#include "lexan.h"
...

The lexan.h file:

#ifndef _LEXAN_
#define _LEXAN_
enum Statements ...
//some function prototypes
...
struct TokensList {
    Statements statement;
    std::string value;
    struct TokensList *next;
};
struct TokensList *tokens = NULL;
#endif _LEXAN_

In lexan2.h:

#include "lexan.h"
// and some function prototypes

The problem is that I get the link error 2005:

1>lexan2.obj : error LNK2005: "struct TokensList * tokens" (?tokens@@3PAUTokensList@@A) already defined in lexan.obj
1>main.obj : error LNK2005: "struct TokensList * tokens" (?tokens@@3PAUTokensList@@A) already defined in lexan.obj

Where is my mistake? I thought the

#ifndef _LEXAN_
#define _LEXAN_

in the lexan.h file would protect me from such linking problems.

Answer 1

You are creating struct TokensList * tokens in the header file lexan.h and then including the header in both lexan.cpp & lexan2.cpp which violates the One Definition Rule(ODR).

Note that, header guards prevent including the same header in the same Translation Unit. When you create a variable in the header file, a copy of the variable with the same name gets created in every translation unit where you include the header. This leads to multiple same named variables in your project which the linker complains about.

Solution:
If you need to share it across files, you need to use extern.

How to use extern?
Have a look at:
What are extern variables in C?
How to correctly use the extern keyword in c?