c ++全局变量

Question

i have a header file global.h where i declare a few variables that i intend to use in other files.

#ifndef GLOBAL_H_
#define GLOBAL_H_

#include <stdio.h>

typedef struct tag_KG_Data
{
   int nKGStationID;
   int nKGComPort;

 }GLOBAL_VAR;

 GLOBAL_VAR g_GlobalVar;

 BOOL b_newDataReady;
 BOOL b_startedSocketClient;



#endif

At first i declared only GLOBAL_VAR g_GlobalVar in file test1.cpp with extern GLOBAL_VAR g_GlobalVar; , and worked just fine. Then i declared the 2 BOOL s and used them in test2.cpp , but i get an error LNK2005: "struct tag_KG_Data g_GlobalVar" (?g_GlobalVar@@3Utag_KG_Data@@A) already defined in test1.obj and for every global variable i have i get a similar error. The thing is that i don't use GLOBAL_VAR g_GlobalVar in test2.cpp or any of the BOOL s in test1.cpp .

Answer 1

This is because you are defining the globals in the header, while you should be only declaring them.

Add extern in front of your global definitions, and create a definition in a single cpp file.

In the header:

// Declarations
extern GLOBAL_VAR g_GlobalVar;
extern BOOL b_newDataReady;
extern BOOL b_startedSocketClient;

In a cpp file:

// Definitions
GLOBAL_VAR g_GlobalVar;
BOOL b_newDataReady;
BOOL b_startedSocketClient;