Multiple Definition problems

This is a discussion on Multiple Definition problems within the C++ Programming forums, part of the General Programming Boards category; Hi guys, I wonder if you can help me out and point me to the area of my brain that ...

  1. #1
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    Question Multiple Definition problems

    Hi guys,

    I wonder if you can help me out and point me to the area of my brain that is defective.

    Basically what I have is the following:

    Code:
    //main.h
    #ifndef _MAIN_H_
    #define _MAIN_H_
    
    #include "extra.h"
    
    #endif
    Code:
    //main.cpp
    #include "main.h"
    
    int main() {
        ext_implement = new Extra();
        return 0;
    }
    Code:
    //extra.h
    #ifndef _EXTRA_H_
    #define _EXTRA_H_
    
    class Extra {
        public:
            Extra();
            void DoSomething();
        private:
    };
    
    Extra *ext_implement;
    #endif
    Code:
    //extra.cpp
    #include "extra.h"
    
    Extra::Extra() {
    }
    
    void Extra::DoSomething() {
    }
    Code:
    //another.h
    #ifndef _ANOTHER_H_
    #define _ANOTHER_H_
    
    #include "extra.h"
    
    class Another {
        public:
            Another();
    };
    
    #endif
    Code:
    //another.cpp
    #include "another.h"
    
    Another::Another() {
        ext_implement->DoSomething();
    }
    And here is the errors I receive:

    obj\Debug\main.o: In function `main':
    C:/Personal/Code/Test/main.cpp:3: multiple definition of `ext_implement'
    obj\Debug\extra.o:C:/Personal/Code/Test/extra.cpp:3: first defined here
    obj\Debug\another.o: In function `ZN7AnotherC2Ev':
    C:/Personal/Code/Test/another.cpp:3: multiple definition of `ext_implement'
    obj\Debug\extra.o:C:/Personal/Code/Test/extra.cpp:3: first defined here



    I realise that I am defining ext_implement multiple times due to me having the line
    #include "extra.h" (where ext_implement is defined) in a few different places, but I am
    not sure of the right way to do it.

    I need to have access to that variable in a few different places, so I don't mind what the solution is really. As long as there is one

    If you need more detail please let me know.
    Any pointers would be greatly appreciated.

    Thanks in advance...
    Shiver

  2. #2
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    The problem isn't the #include "extra.h" all over the place (although that might not be the best thing anyway). The problem is that you have a global variable that is defined multiple times. If you want ext_implement to be available to any file that includes extra.h, then you can declare it as extern in that file and define it in a single cpp file like extra.cpp.

  3. #3
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    Ok well a simple solution is to include "extra.h" before the others, and therefor the ext_implement is global then you can use it at any point in your program

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    Raigne, I did try that but without any success.
    The errors it gives are at linking time and not compile, so I assume that the order of #includes doesn't make a difference? Correct me if I'm wrong...

    Daved, awesome-ness. That works perfectly
    Never used extern before. Are there any immediate down sides to doing it this way?
    I know that the multiple #includes of the same file all over the place looks bad, just too much of a beginner to know a better way to go about it

  5. #5
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    It's fine if you are using things from that file. In the code above, main.h doesn't use anything from extra.h, so it shouldn't include it. On the other hand, main.cpp does use something from there, so it should include extra.h directly. Same thing with another.h and another.cpp.

  6. #6
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    shiver, my solution would have worked, but for it to work you have to get rid of all those includes everywhere. You should always try to only include 1 of each file.

  7. #7
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    Raigne, your "solution" only works in very limited cases, and is not a general solution. Specifically, it only works if it is possible to restructure code so that header files are only #include'd once within a complete project. It is not always possible to ensure that main.h or extra.h is only #include'd once, particularly in scenarios where multiple source files need to #include a particular header.

    Daved's solution (making the variable extern in the header file, and define it in exactly one source file) is a more general solution.

  8. #8
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    >> You should always try to only include 1 of each file.
    You should always include each file as many times as you need it. If a header file declares a class, and you need that class in multiple source files, then include the header in multiple source files. Most projects with more than a few files will require this. In this case, all three source files require the inclusion of extra.h.

  9. #9
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    If you were forced to have only one #include of each of your .h files, there wouldn't even be a reason to make .h files; you could throw all the stuff at the top of the .CPP file.

    The very fact you're making a .h file at all should be a sign you plan to have it included in more than one source file.
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  10. #10
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    Thanks a lot guys for all the comments.
    I think I'll hang around these boards more often, so many intelligent people, perhaps I'll catch the disease

    Just to clarify, the .h files I'm using are pretty much only for defining classes, so I definately need them to be included in multiple places. That extern was exactly what I was looking for.

    Thanks again everyone.

  12. #12
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    One other thing: identifiers starting with an underscore followed by another underscore or an uppercase letter are reserved by the implementation. You might want to use inclusion guards that are called HEADER_H_ or HEADER_H or some other variation.
    dwk

    Seek and ye shall find. quaere et invenies.

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    thanks dwks

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