多包含一个.h文件

Question

In an .h file, I am declaring a global variable as:

#pragma data_seg(".shared")
#ifndef DEF_VARX
#define DEF_VARX
int VARX=0;
#endif /*DEF_VARX*/
#pragma data_seg()
#pragma comment(linker, "/SECTION:.shared,RWS")

However if I include this file in multiple cpp files, when I try to compile, I get " error LNK2005: "int VARX" (?VARX@@3HA) already defined in Dll.obj" error. If I include in only one cpp file, no problem is encountered.

Isn't #IFNDEF.... check enough for preventing this? Am I missin something?

Answer 1

The reason of this behavior is, that you compile the line

int VARX=0;

into each .obj file. Thats OK for compiling, but upon linking the symbol becomes multiply defined, which is illegal. Using

extern int VARX;

in the header file, and

int VARX=0;

in one (and only one) source file resolves this problem.

Answer 2

I think you're supposed to forward declare the variable in the .h and later define it in its shared section in a .cpp, something like:

// in a header file
#pragma once
extern int VARX;

// in a .cpp
#pragma data_seg(".shared")
int VARX=0;
#pragma data_seg()
#pragma comment(linker, "/SECTION:.shared,RWS")

Answer 3

The problem is that is that you prevent multiple inclusion of the file for a given translation unit. (for a given say cpp file)

But if several of your cpp include this VARX.H, then you'll have more than one definition for this variable.

Instead, you should only declare the variable in the .H file, but initialize it to 0 in only one location.

Answer 4

Yes, you're missing the extern keyword.

In your header file, use:

extern int VARX;

In a source file, actually declare space for the variable:

int VARX = 0;

Answer 5

ifdef prevents it for a separe object file. When the header is include in several source (cpp) files, VARX will be dedfined in all of them. Consider declaring it as extern in header file, and initialize in one cpp file.

Answer 6

The problem is you must be including the file in multiple compilation units. Let's say you have a.cpp and b.cpp. Both include your header file. So the compiler will compile (and pre-process) separately, so for both files, DEF_VARX is not yet defined. When you go to link the to object files together, the linker notices that there is a name collision.

As others have suggested, the solution would be to declare it 'extern', then place the actual value in a cpp file, so it only is compiled once, and linked to everything without name collisions.