Question

I’ve defined a struct in a header file global.h, that i try to use it in a another class, but i get this error : Error 6 error LNK2001: unresolved external symbol "struct tag_KG_Data g_GlobalVar" (?g_GlobalVar@@3Utag_KG_Data@@A) KGComThread.obj


#ifndef GLOBAL_H_
#define GLOBAL_H_

#include <stdio.h>

typedef struct tag_KG_Data
{
      int nKGStationID;
      int nKGComPort;

}GLOBAL_VAR;

#endif

and in KGComThread.cpp file i use it like this:

#include "global.h"

extern GLOBAL_VAR g_GlobalVar;

I think the compiler can’t find the global.h file so it defines a meaningless tag_KG_Data struct, but i can’t understand why.

Answer 1

This

extern GLOBAL_VAR g_GlobalVar;

is only a declaration. The variable is not yet defined:

GLOBAL_VAR g_GlobalVar;

You need the previous line in a single implementation file.

Also, since this is C++, you don’t need a tag for the struct, you can just write

struct GLOBAL_VAR
{
      int nKGStationID;
      int nKGComPort;
};