Programming Forums > Application Development > C++

some1 · Apr 27th, 2005, 10:14 AM

How to prevent header files from being included twice?
My code is messed up and I need to include lots of .h files in other files so I get errors from the linker that X is already defined in x.obj file.

I tried using inclusion guards like this:

#ifndef INC_HEADER1
#define INC_HEADER1

<file content goes here>

#endif

and also tried

#pragma once

but i still get the same errors
But even if i include e.g windows.h or string.h a hundred times i never get linker errors about them.

help?

iignotus · Apr 27th, 2005, 2:45 PM

I'm not sure but I think only one compiled version of any library you include goes into the final product. I may be confusing this with a feature of another C-type language, though.

Eggbert · Apr 27th, 2005, 3:47 PM

A header should only have declarations. It sounds like you are trying to define something multiple times outside of the headers. Definitions of shared names should be placed in a separate source file so as to ensure that they are only defined once:

(Toggle Plain Text)

/* defs.h */
#ifndef HEADER
#define HEADER

/* Declarations */

#endif

(Toggle Plain Text)

/* defs.c */
#include "defs.h"

/* Definitions */

Compile and link the source file along with your project, and all should be well.

some1 · Apr 27th, 2005, 5:16 PM

actually its a global HBRUSH that is used by the functions in the file i`m trying to include. I`m not getting errors for the functions but the global HBRUSH only (which is the only global in that file). Should I not put variables in header files?

What I`m making is an application divided into 7 parts. they`re big parts so I`m dividing each into a seperate .cpp & .h file plus a main .cpp & .h file. Dunno if thats ok. I`m a noob afterall

.

Any tips?

Eggbert · Apr 27th, 2005, 5:26 PM

>Should I not put variables in header files?
Variable declarations, yes. Not variable definitions. To adjust my previous example:

(Toggle Plain Text)

/* defs.h */
#ifndef HEADER
#define HEADER

extern HBRUSH brush;

#endif

(Toggle Plain Text)

/* defs.c */
#include "defs.h"

HBRUSH brush;

some1 · Apr 27th, 2005, 6:11 PM

thanks for helping

i have 2 questions:

1. What does the keyword extern mean?
2. What is the difference between variable definition and declaration?

Eggbert · Apr 27th, 2005, 9:05 PM

>1. What does the keyword extern mean?
As used, it means that the name is defined elsewhere.

>2. What is the difference between variable definition and declaration?
A declaration tells the compiler that a name exists. A definition allocates storage for that name and optionally initializes the storage with a value.

mackenga · Apr 28th, 2005, 8:35 AM

Just a little something to add; when you're making sure your header doesn't get included twice using ifndef and define, sometimes defining a constant without a value isn't a hot idea (depends on the compiler and any other software you have parsing the program):

(Toggle Plain Text)

#ifndef _HEADER_FILE_H_
#define _HEADER_FILE_H_ 1

/* ... declarations here ... */

#endif /* _HEADER_FILE_H_ */

Obviously, don't use _HEADER_FILE_H_ as the name of your constant, but something more directly related to what you're working on and what the header file contains/is for. The _HEADER_FILE_H_ in the comment beside #endif is not strictly necessary but aids readability.

This means even if you do declare and define variables within the header file, as long as it's between the #ifndef and the #endif that matches it, that code will only be processed beyond the preprocessor once so it'll work.

If you have global variables, though, declaring them extern in the header and putting them in a file called globals.c or whatever is often a good idea. Global variables can be hard enough to keep track of after all.

Eggbert · Apr 28th, 2005, 9:59 AM

>sometimes defining a constant without a value isn't a hot idea
Only when working with legacy code and very old compilers. Any compiler that correctly supports ISO C89 should have no trouble with it.

>Obviously, don't use _HEADER_FILE_H_ as the name of your constant, but
>something more directly related to what you're working on
Don't use a name with a leading underscore for this use either, as that invades the implementation's name space.

Apr 27th, 2005, 10:14 AM	#1
some1 Programmer Join Date: Dec 2004 Posts: 50 Rep Power: 4	Including header files How to prevent header files from being included twice? My code is messed up and I need to include lots of .h files in other files so I get errors from the linker that X is already defined in x.obj file. I tried using inclusion guards like this: #ifndef INC_HEADER1 #define INC_HEADER1 <file content goes here> #endif and also tried #pragma once but i still get the same errors But even if i include e.g windows.h or string.h a hundred times i never get linker errors about them. help?

Apr 27th, 2005, 2:45 PM	#2
iignotus Professional Programmer Join Date: Apr 2005 Location: Nowhere Special Posts: 466 Rep Power: 4	I'm not sure but I think only one compiled version of any library you include goes into the final product. I may be confusing this with a feature of another C-type language, though. __________________ (Toggle Plain Text) % rc4 hexkey < input > output #define S ,t=s[i],s[i]=s[j],s[j]=t /* rc4 hexkey <file / unsigned char k[256],s[256],i,j,t;main(c,v,e)charv;{++v;while(++i)s[ i]=i;for(c=0;(v)++;k[c++]=e)sscanf((v)++-1,"%2x",&e);while(j+=s[i] +k[i%c]S,++i);for(j=0;c=~getchar();putchar(~c^s[t+=s[i]]))j+=s[++i]S;}

Apr 27th, 2005, 3:47 PM	#3
Eggbert Professional Programmer Join Date: Nov 2004 Posts: 250 Rep Power: 5	A header should only have declarations. It sounds like you are trying to define something multiple times outside of the headers. Definitions of shared names should be placed in a separate source file so as to ensure that they are only defined once: (Toggle Plain Text) /* defs.h / #ifndef HEADER #define HEADER / Declarations / #endif / defs.h / #ifndef HEADER #define HEADER / Declarations / #endif (Toggle Plain Text) / defs.c / #include "defs.h" / Definitions / / defs.c / #include "defs.h" / Definitions */ Compile and link the source file along with your project, and all should be well.

Apr 27th, 2005, 5:26 PM	#5
Eggbert Professional Programmer Join Date: Nov 2004 Posts: 250 Rep Power: 5	>Should I not put variables in header files? Variable declarations, yes. Not variable definitions. To adjust my previous example: (Toggle Plain Text) /* defs.h / #ifndef HEADER #define HEADER extern HBRUSH brush; #endif / defs.h / #ifndef HEADER #define HEADER extern HBRUSH brush; #endif (Toggle Plain Text) / defs.c / #include "defs.h" HBRUSH brush; / defs.c */ #include "defs.h" HBRUSH brush;

Apr 28th, 2005, 8:35 AM	#8
mackenga Professional Programmer Join Date: Mar 2005 Location: Glasgow, Scotland Posts: 328 Rep Power: 4	Just a little something to add; when you're making sure your header doesn't get included twice using ifndef and define, sometimes defining a constant without a value isn't a hot idea (depends on the compiler and any other software you have parsing the program): (Toggle Plain Text) #ifndef _HEADER_FILE_H_ #define _HEADER_FILE_H_ 1 /* ... declarations here ... / #endif / _HEADER_FILE_H_ / #ifndef _HEADER_FILE_H_ #define _HEADER_FILE_H_ 1 / ... declarations here ... / #endif / _HEADER_FILE_H_ */ Obviously, don't use _HEADER_FILE_H_ as the name of your constant, but something more directly related to what you're working on and what the header file contains/is for. The _HEADER_FILE_H_ in the comment beside #endif is not strictly necessary but aids readability. This means even if you do declare and define variables within the header file, as long as it's between the #ifndef and the #endif that matches it, that code will only be processed beyond the preprocessor once so it'll work. If you have global variables, though, declaring them extern in the header and putting them in a file called globals.c or whatever is often a good idea. Global variables can be hard enough to keep track of after all.

Apr 27th, 2005, 5:16 PM	#4
some1 Programmer Join Date: Dec 2004 Posts: 50 Rep Power: 4	actually its a global HBRUSH that is used by the functions in the file i`m trying to include. I`m not getting errors for the functions but the global HBRUSH only (which is the only global in that file). Should I not put variables in header files? What I`m making is an application divided into 7 parts. they`re big parts so I`m dividing each into a seperate .cpp & .h file plus a main .cpp & .h file. Dunno if thats ok. I`m a noob afterall . Any tips?

Apr 27th, 2005, 6:11 PM	#6
some1 Programmer Join Date: Dec 2004 Posts: 50 Rep Power: 4	thanks for helping i have 2 questions: 1. What does the keyword extern mean? 2. What is the difference between variable definition and declaration?

Apr 27th, 2005, 9:05 PM	#7
Eggbert Professional Programmer Join Date: Nov 2004 Posts: 250 Rep Power: 5	>1. What does the keyword extern mean? As used, it means that the name is defined elsewhere. >2. What is the difference between variable definition and declaration? A declaration tells the compiler that a name exists. A definition allocates storage for that name and optionally initializes the storage with a value.

Apr 28th, 2005, 9:59 AM	#9
Eggbert Professional Programmer Join Date: Nov 2004 Posts: 250 Rep Power: 5	>sometimes defining a constant without a value isn't a hot idea Only when working with legacy code and very old compilers. Any compiler that correctly supports ISO C89 should have no trouble with it. >Obviously, don't use _HEADER_FILE_H_ as the name of your constant, but >something more directly related to what you're working on Don't use a name with a leading underscore for this use either, as that invades the implementation's name space.

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Programming Forums > Application Development > C++

Including header files