Multiple definition error

This is a discussion on Multiple definition error within the C++ Programming forums, part of the General Programming Boards category; Hi everyone! I have a .h file in which I declare a global variable. Many .cpp files include my .h, ...

  1. #1
    Disturbed Boy gustavosserra's Avatar
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    Multiple definition error

    Hi everyone!

    I have a .h file in which I declare a global variable.
    Many .cpp files include my .h, and this generates an error in my linker.

    ////// my.h /////
    int variable;
    ////// 1.cpp //////
    #include my.h
    ////// 2.cpp //////
    #include my.h

    How can I solve this problem? Thanks any help!
    Nothing more to tell about me...
    Happy day =)

  2. #2
    Code Goddess Prelude's Avatar
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    Only declarations should be in headers, but you can avoid multiple definition errors by enclosing the header in inclusion guards:
    Code:
    #ifndef MY_H
    #define MY_H
    
    int variable;
    
    #endif
    My best code is written with the delete key.

  3. #3
    pronounced 'fib' FillYourBrain's Avatar
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    but even then the linker will complain if the include is used in more than one module.

    You may however declare the global in a cpp file and use the extern keyword in the .h file. For instance:

    // in the cpp module
    int variable;


    //in the .h file
    extern int variable;

    This will give every module that includes the .h file the ability to use it.
    "You are stupid! You are stupid! Oh, and don't forget, you are STUPID!" - Dexter

  4. #4
    Disturbed Boy gustavosserra's Avatar
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    Prelude: sorry for not have told, but I already tryied to do this.
    FillYourBrain: it worked, I tryied to do this too, but in every .cpp I put extern, now, with the definition in my main .cpp file and just extern in .h worked.

    Thanks to both of you!!!!
    Nothing more to tell about me...
    Happy day =)

  5. #5
    mov.w #$1337,D0 Jeremy G's Avatar
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    This also works
    Code:
    // main.cpp
    #define NEED_VARS
    #include "local.h"
    
    // local.h
    #ifdef NEED_VARS
         int var;
         float other var;
    #else
         extern int var;
         extern float var;
    #endif
    c++->visualc++->directx->opengl->c++;
    (it should be realized my posts are all in a light hearted manner. And should not be taken offense to.)

  6. #6
    Disturbed Boy gustavosserra's Avatar
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    This is a bit strange dbgt goten, never saw nothing like this.
    Nothing more to tell about me...
    Happy day =)

  7. #7
    pronounced 'fib' FillYourBrain's Avatar
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    I don't suggest you do it that way.
    "You are stupid! You are stupid! Oh, and don't forget, you are STUPID!" - Dexter

  8. #8
    Code Goddess Prelude's Avatar
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    >Prelude: sorry for not have told, but I already tryied to do this.
    That's okay, it was a bogus answer written by a sleepy programmer.
    My best code is written with the delete key.

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