Programming Forums > Application Development > C++

some1 · Nov 13th, 2005, 9:22 AM

Hello everyone!

I've been having this annoying problem for a while and I cant sort it out.

I have file "globals.h" where I store all my global vars in. I also have 2 other files main.cpp and code2.cpp where I write my code.
The problem is, when I #include "Globals.h" in both main.cpp and code2.cpp it gives me this error:
fatal error LNK1169: one or more multiply defined symbols found

So I tried using inclusion guards like this:
#ifndef _INC_GLOBALS
#define _INC_GLOBALS
....variable definitions here.....
#endif

but it keeps giving me the same error.
if I remove the #include "Globals.h" from one of the files it just says that variable x is not defined.

I used #pragma message to see how many times the linker uses the file and it outputs my msg twice. So here is a sample application I built just to test this:

Main.cpp

(Toggle Plain Text)

#include <iostream.h>
#include "globals.h"

int main(int argc, char *argv[])
{
	x = 123;

	return 0;
}

code2.php

(Toggle Plain Text)

#include "globals.h"

void DoSomething();

void DoSomething()
{
	x = 456;
}

Globals.h

(Toggle Plain Text)

#ifndef _INC_GLOBALS
#define _INC_GLOBALS
#pragma message("Including globals.h")

int x = 0;

#endif

I also tried using "#pragma once" but nothing changes.

Here is the build output

Quote:

Originally Posted by Build output

Compiling...
code2.cpp
Including globals.h
main.cpp
c:\Program Files\Microsoft Visual Studio .NET\Vc7\include\useoldio.h(29) : warning C4995: '_OLD_IOSTREAMS_ARE_DEPRECATED': name was marked as #pragma deprecated
Including globals.h
Generating Code...
Linking...
code2.obj : error LNK2005: "int x" (?x@@3HA) already defined in main.obj
Debug/InclusionGuards.exe : fatal error LNK1169: one or more multiply defined symbols found

Note that the "Including Globals.h" message appears twice.

I also dont understand why, when even if I include e.g. windows.h or iostream.h 100 times in the same file I never get such errors while they use the same inclusion guard method...

In desperate need of help,
Some1

DaWei · Nov 13th, 2005, 10:39 AM

This generates code. Don't do that in your header files or you'll (obviously) get multiple definitions.

(Toggle Plain Text)

int x = 0;

There are infrequent exceptions to that rule, but you haven't hit them. Go with a declaration and initialize it elsewhere.

Globals aren't evil. They are VERY, VERY OFTEN misused, and a source of headaches and bugs. Without even looking, I'll guess you have too many and can do it more robustly.

Header guards are to prevent the inclusion of a header file more than once in the SAME source file. They don't do diddly squat in regard to multiple source files.

some1 · Nov 13th, 2005, 11:24 AM

DaWei thanks for your reply.

I replaced the "int x = 0;" with just "int x;" but I still get the same linking error .

Ras Qulec · Nov 13th, 2005, 11:34 AM

Try making the header file have extern int x; and put int x=0; in one of the source files as a global.

OpenLoop · Nov 13th, 2005, 11:35 AM

The #ifndef will only work at compile time in the same file or in other included files, but linked files are a different issue. You compiled main which included Globals.h. Then you seperatly compiled code2.cpp which also included Globals because as far as the compiler's concerned, main and code2 are two seperate files that do not include each other. However, at linkage time, you have two object files with the definition of int x in each one thus generating the error of duplicate definition.

DaWei · Nov 13th, 2005, 11:35 AM

Declare it external in the header and put the Real Thang in one source file. Why are you doing this? Practice? One rarely needs a global variable, particularly one named, "x".

lectricpharaoh · Nov 13th, 2005, 11:44 AM

Rather than defining your variables in your header, define them in a C or C++ source file, outside of all functions. You can initialize them in this file. Do not declare them with the static keyword, as this will limit their visibility to this single source file. Then, in your header file, simply declare them, using the extern keyword. This tells the compiler that the variables are defined elsewhere, but allows the declaration to be in scope so you don't get an undefined symbol error.

To paraphrase, use this in your source file:

(Toggle Plain Text)

long count = 0;
int errorCode = 0;

And use this in your header file:

(Toggle Plain Text)

extern long count; 
extern int errorCode;

The reason you do it like this is to a) avoid multiple definitions of the same symbol within the same scope, and b) allow you to compile your source file that defines these variables into an object file (possibly adding it to a library), and use it without receiving 'undefined symbol' errors.

The trick here is to realize the difference between a 'definition' and a 'declaration'. A declaration merely declares something, so the compiler knows of its existence and what type it is. This applies to functions as well as variables. For example, a prototype is a declaration. The sole purpose of this is to allow the compiler to perform proper error checking (such as issuing a warning or error when you assign a long to a short, or call a function with the incorrect number or type of arguments) and possibly some optimizations (the more information the compiler has, the better it can do its job). A definition, on the other hand, actually creates an instance and allocates storage (for variables) or contains the code body (for functions). Note that a definition is essentially a stronger form of a declaration; in other words, every definition is a declaration as well, but not every declaration is a definition.

some1 · Nov 14th, 2005, 1:01 PM

Quote:

Originally Posted by DaWei

Declare it external in the header and put the Real Thang in one source file. Why are you doing this? Practice? One rarely needs a global variable, particularly one named, "x".

This is just a test app. I am building an application with many windows and I have an array of HWND vars to hold their handles and I want the array to be global so I can access it anywhere from the application (each window is like a small application so I have different .cpp and .h files for each).

I also have a char array which holds the application name so I can use it in MessageBox() calls throughout my app.

I dont really know which is the right way to do it.

DaWei · Nov 14th, 2005, 3:15 PM

This shows where you will place the global declarations/definitions. Then proceed according to lectric's explanation.

(Toggle Plain Text)

#include <myheaderfiles.>


This area, where you might find other preprocessing commands,
is where you declare "global" variables.

int myGlobal int;
char myGlobalString [] = "My Global String";


int main (int argc, char *argv [])
{
   // Once you're in here, you're in 
   // "local" territory, with declarations.
   // However, you may INITIALIZE globals here.

...

some1 · Nov 27th, 2005, 9:19 AM

Ok, so I've done that and everything works, thx everyone.

Now I have another problem, I'm trying to create a global pointer to a custom class in the global.h like this

extern MYCLASS *pMyClass;

but I get yet another linking error:
"error LNK2001: unresolved external symbol "class MYCLASS * pMyClass""

Nov 13th, 2005, 10:39 AM	#2
DaWei Resident Grouch Join Date: Jun 2005 Posts: 6,453 Rep Power: 10	This generates code. Don't do that in your header files or you'll (obviously) get multiple definitions. (Toggle Plain Text) int x = 0; int x = 0; There are infrequent exceptions to that rule, but you haven't hit them. Go with a declaration and initialize it elsewhere. Globals aren't evil. They are VERY, VERY OFTEN misused, and a source of headaches and bugs. Without even looking, I'll guess you have too many and can do it more robustly. Header guards are to prevent the inclusion of a header file more than once in the SAME source file. They don't do diddly squat in regard to multiple source files. __________________ Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code. Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers

Nov 13th, 2005, 11:35 AM	#6
DaWei Resident Grouch Join Date: Jun 2005 Posts: 6,453 Rep Power: 10	Declare it external in the header and put the Real Thang in one source file. Why are you doing this? Practice? One rarely needs a global variable, particularly one named, "x". __________________ Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code. Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers

Nov 13th, 2005, 11:44 AM	#7
lectricpharaoh Caffeinated Neural Net Join Date: Jun 2005 Location: Dry west coast of Canada Posts: 927 Rep Power: 4	Rather than defining your variables in your header, define them in a C or C++ source file, outside of all functions. You can initialize them in this file. Do not declare them with the static keyword, as this will limit their visibility to this single source file. Then, in your header file, simply declare them, using the extern keyword. This tells the compiler that the variables are defined elsewhere, but allows the declaration to be in scope so you don't get an undefined symbol error. To paraphrase, use this in your source file: (Toggle Plain Text) long count = 0; int errorCode = 0; long count = 0; int errorCode = 0; And use this in your header file: (Toggle Plain Text) extern long count; extern int errorCode; extern long count; extern int errorCode; The reason you do it like this is to a) avoid multiple definitions of the same symbol within the same scope, and b) allow you to compile your source file that defines these variables into an object file (possibly adding it to a library), and use it without receiving 'undefined symbol' errors. The trick here is to realize the difference between a 'definition' and a 'declaration'. A declaration merely declares something, so the compiler knows of its existence and what type it is. This applies to functions as well as variables. For example, a prototype is a declaration. The sole purpose of this is to allow the compiler to perform proper error checking (such as issuing a warning or error when you assign a long to a short, or call a function with the incorrect number or type of arguments) and possibly some optimizations (the more information the compiler has, the better it can do its job). A definition, on the other hand, actually creates an instance and allocates storage (for variables) or contains the code body (for functions). Note that a definition is essentially a stronger form of a declaration; in other words, every definition is a declaration as well, but not every declaration is a definition. __________________ A man's knowledge is like an expanding sphere, the surface corresponding to the boundary between the known and the unknown. As the sphere grows, so does its surface; the more a man learns, the more he realizes how much he does not know. Hence, the most ignorant man thinks he knows it all. - L. Sprague de Camp

Nov 14th, 2005, 3:15 PM	#9
DaWei Resident Grouch Join Date: Jun 2005 Posts: 6,453 Rep Power: 10	This shows where you will place the global declarations/definitions. Then proceed according to lectric's explanation. (Toggle Plain Text) #include <myheaderfiles.> This area, where you might find other preprocessing commands, is where you declare "global" variables. int myGlobal int; char myGlobalString [] = "My Global String"; int main (int argc, char argv []) { // Once you're in here, you're in // "local" territory, with declarations. // However, you may INITIALIZE globals here. ... #include <myheaderfiles.> This area, where you might find other preprocessing commands, is where you declare "global" variables. int myGlobal int; char myGlobalString [] = "My Global String"; int main (int argc, char argv []) { // Once you're in here, you're in // "local" territory, with declarations. // However, you may INITIALIZE globals here. ... __________________ Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code. Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers

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Nov 13th, 2005, 11:24 AM	#3
some1 Programmer Join Date: Dec 2004 Posts: 50 Rep Power: 4	DaWei thanks for your reply. I replaced the "int x = 0;" with just "int x;" but I still get the same linking error .

Nov 13th, 2005, 11:34 AM	#4
Ras Qulec Newbie Join Date: Sep 2005 Posts: 10 Rep Power: 0	Try making the header file have extern int x; and put int x=0; in one of the source files as a global.

Nov 13th, 2005, 11:35 AM	#5
OpenLoop Expert Programmer Join Date: May 2005 Location: East Lansing, MI Posts: 663 Rep Power: 4	The #ifndef will only work at compile time in the same file or in other included files, but linked files are a different issue. You compiled main which included Globals.h. Then you seperatly compiled code2.cpp which also included Globals because as far as the compiler's concerned, main and code2 are two seperate files that do not include each other. However, at linkage time, you have two object files with the definition of int x in each one thus generating the error of duplicate definition.

Nov 27th, 2005, 9:19 AM	#10
some1 Programmer Join Date: Dec 2004 Posts: 50 Rep Power: 4	Ok, so I've done that and everything works, thx everyone. Now I have another problem, I'm trying to create a global pointer to a custom class in the global.h like this extern MYCLASS pMyClass; but I get yet another linking error: "error LNK2001: unresolved external symbol "class MYCLASS pMyClass""

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Programming Forums > Application Development > C++

Including files and inclusion guards