Thread: Have an error that i can't solve

  1. #1
    Registered User
    Join Date
    Apr 2011

    Have an error that i can't solve

    redefinition of 'class Data'
    previous definition of 'class Data'

    ---note--- Data is a class that i declare in a header file

  2. #2
    Registered User
    Join Date
    Oct 2006
    you probably don't have include guards. you need to either say

    #pragma once
    #ifndef MY_HEADER_FILE
    #define MY_HEADER_FILE
    class Data
    #endif // MY_HEADER_FILE

  3. #3
    Registered User
    Join Date
    Apr 2011
    thanks it worked, but what exactly are include guards and what instance do you need to use them

  4. #4
    Registered User C_ntua's Avatar
    Join Date
    Jun 2008
    Usually they are used on the top and bottom of a file. Think for example a common header file, like <stdlib.h>. You might included it in a lot of files. If you use guards, it will just use it once, which is your intention, the compiler just doesn't do this automatically for you.

    To use it to skip a piece of code can be tricky and I won't recommend it. You might have two Data class, but different. In this case you should use namespaces for example or rename one to solve your problem. If you have to have the exact same class, then you should write it on a separate file and include it when needed.

    Technically speaking, what happens is that the pre-compiler will check if MY_HEADER_FILE variable is defined. If it is it will scroll down to #endif. So it will skip the whole Data class. If it is not defined it will #define it. Now, if you have another header file with the same Data class, it will skip it, giving no errors. So when used on a whole file, you simply skip the whole file.

  5. #5
    [](){}(); manasij7479's Avatar
    Join Date
    Feb 2011
    How does
    #pragma once
    work ?

  6. #6
    Registered User
    Join Date
    Feb 2003
    #pragma once does basically the same thing as include guards, except that it is non-standard, i.e. is only supported by specific compilers.

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