Jump to content
  • Advertisement
Sign in to follow this  
oatz01

multiple definitions linker error

This topic is 4270 days old which is more than the 365 day threshold we allow for new replies. Please post a new topic.

If you intended to correct an error in the post then please contact us.

Recommended Posts

Heh, this is driving me nuts. The linker is saying I have multiple definitions of certain variables:
// Resource Variables
StateMachine *state;
BITMAP *ship_img;
BITMAP *bg;
BITMAP *buffer;
BITMAP *alien_img[2];
CShipSprite *ship;
CAlienSprite *aliens[8];
Those are all defined in a game.h file. The rest of it is a class definition and that doesn't get caught in the error. The header file is included once in game.cpp, where the variables are initialized in a function, and once in main.cpp. It also has #pragma once. I can't imagine how any of those variables are being defined multiple times. Any ideas?

Share this post


Link to post
Share on other sites
Advertisement
I'm not that familiar with the #pragma once directive (I tend to avoid nonstandard directives, even if they are widely implemented), but my guess is that it includes the header file once per translation unit. If you haven't done so read this article - try putting extern before the variable declarations in the header file and move the definitions to a .cpp file.

Share this post


Link to post
Share on other sites
Every time a file is #included, its contents are simply copied as if the contents are part of the file that includes it. Now imagine what would happen if instead of including game.h, you simply paste its contents into every file that includes it. You would get multiple definition errors, right?

Your header file currently contains variable "definitions". A variable definition creates a variable. The solution is to put "extern" at the front of each of those lines in the header file. "extern" makes the line a "declaration". A declaration tells the compiler that the variable exists somewhere, but doesn't "define" it. Now that you have declarations everywhere, you also need to define the variables somewhere (once).

Your code should look something like this:
game.h

extern StateMachine *state; // Declaration. This line tells the compiler
// that a variable with this name and type
// exists somewhere.


game.cpp

#include "game.h" // This is always a good idea for many reasons
...
StateMachine *state; // Definition. Here is where the variable named
// 'state' resides.


main.cpp

#include "game.h" // Include this so that the declarations tell the
// compiler about your variables in game.cpp
Finally, read this article: Organizing Code Files in C and C++

Share this post


Link to post
Share on other sites
Ah I should have known that! Its works pefectly now. Thank you both for the help and the quick response.

Share this post


Link to post
Share on other sites
Sign in to follow this  

  • Advertisement
×

Important Information

By using GameDev.net, you agree to our community Guidelines, Terms of Use, and Privacy Policy.

We are the game development community.

Whether you are an indie, hobbyist, AAA developer, or just trying to learn, GameDev.net is the place for you to learn, share, and connect with the games industry. Learn more About Us or sign up!

Sign me up!