redefining

[Staygold]

Hello,

I'm trying to get my .cpp files to interact with eachother using .h files. But everytime I try to compile it it says the variables are already defined in the header file.

I get these errors:

c:\users\gebruiker\desktop\grid control\grid control\grid control\loadimages.cpp(37): error C2086: 'SDL_Surface *horStripeImage' : redefinition
1>          c:\users\gebruiker\desktop\grid control\grid control\grid control\loadimages.h(6) : see declaration of 'horStripeImage'
1>c:\users\gebruiker\desktop\grid control\grid control\grid control\loadimages.cpp(38): error C2086: 'SDL_Surface *verStripeImage' : redefinition
1>          c:\users\gebruiker\desktop\grid control\grid control\grid control\loadimages.h(7) : see declaration of 'verStripeImage'
1>c:\users\gebruiker\desktop\grid control\grid control\grid control\loadimages.cpp(39): error C2086: 'SDL_Surface *playfieldImage' : redefinition
1>          c:\users\gebruiker\desktop\grid control\grid control\grid control\loadimages.h(8) : see declaration of 'playfieldImage'

This is what my files look like:

loadImages.h

#ifndef LOADIMAGES_H_GUARD
#define LOADIMAGES_H_GUARD

#include "SDL.h"

SDL_Surface* horStripeImage;
SDL_Surface* verStripeImage;
SDL_Surface* playfieldImage;

#endif 

loadImages.cpp

#include "loadImages.h"

//Load stripe_horizontal.bmp
SDL_Surface* loadHorStripe()
{
	SDL_Surface* loadedImage = SDL_LoadBMP( "stripe_horizontal.bmp" );
	SDL_Surface* stripe_horizontal = SDL_DisplayFormat( loadedImage );

	SDL_FreeSurface( loadedImage );

	return stripe_horizontal;
}

//Load stripe_vertical.bmp
SDL_Surface* loadVerStripe()
{
	SDL_Surface* loadedImage = SDL_LoadBMP( "stripe_vertical.bmp" );
	SDL_Surface* stripe_vertical = SDL_DisplayFormat( loadedImage );

	SDL_FreeSurface( loadedImage );

	return stripe_vertical;
}

//Load playfield.bmp
SDL_Surface* loadPlayfield()
{
	SDL_Surface* loadedImage = SDL_LoadBMP( "playfield.bmp" );
	SDL_Surface* playfield = SDL_DisplayFormat( loadedImage );

	SDL_FreeSurface( loadedImage );

	return playfield;
}

//Assign variables
SDL_Surface* horStripeImage = loadHorStripe();
SDL_Surface* verStripeImage = loadVerStripe();
SDL_Surface* playfieldImage = loadPlayfield();

I think it has something to do with this

//Assign variables
SDL_Surface* horStripeImage = loadHorStripe();
SDL_Surface* verStripeImage = loadVerStripe();
SDL_Surface* playfieldImage = loadPlayfield();

but I don't know what to do. Please help me.

Thanks in advance.

[Catfish4]

In the header file you should declare variables, not define them. (Hence the redefinition error.)

You declare by using the extern keyword.

#ifndef LOADIMAGES_H_GUARD
#define LOADIMAGES_H_GUARD

#include "SDL.h"

extern SDL_Surface* horStripeImage;
extern SDL_Surface* verStripeImage;
extern SDL_Surface* playfieldImage;

#endif  

[TheGrayWolf]

You can declare a variable/function many times, but there has to be only one definition. You have re-defined the variables in loadImages.cpp. Although i would use extern just as Catfish proposed, it's considered redundant in C++.

[Catfish4]

Although i would use extern just as Catfish proposed, it's considered redundant in C++.

How so? If you want to access a global variable defined in a source file, you will need to declare it as extern somewhere - best bet is the header file of that source file.

[cire]

Although i would use extern just as Catfish proposed, it's considered redundant in C++.

I'll bite. For what reason is it considered redundant in C++?

[vlad from moscow]

@cire
I'll bite. For what reason is it considered redundant in C++?

For the same error as in the original post.:)

[TheGrayWolf]

@cire, this is linked to the OP's example (again), but the following

 
extern int x = 3; 
int x = 3; 

are essentially equivalent, so had Staygold defined the images the first way, he could have just dropped the extern. Do note that it is not my personal opinion, but rather what i've read in "The C++ Programming Language" not too long ago. Apart from the
cases in which you have to use extern as Catfish mentioned, it also conveys your intents better and makes them explicit.

[cire]

@cire, this is linked to the OP's example (again), but the following

extern int x = 3; 
int x = 3; 

re essentially equivalent, so had Staygold defined the images the first way, he could have just dropped the extern.

You responded to and referenced Catfish's post, where extern was required and far from redundant. If the OP had moved the initialization and definition of the variables to the header file as you seem to be suggesting, he would've had redefinitions of the variables in multiple source files. I don't see your point.

[vlad from moscow]

According to the C++ Standard

2 A declaration is a definition unless ... it contains the extern specifier (7.1.1) or a linkage-specification25 (7.5) and neither an initializer nor a function body,

[Staygold]

Extern solved the problem. Thanks alot.