How to avoid multiple definition of a variable by multiple inclusion of a header file

Discussion in 'C++' started by lars.uffmann, Aug 1, 2006.

  1. lars.uffmann

    lars.uffmann Guest

    Easily described problem:

    Using g++ version 3.3.5 under suse 9.3,

    bla.h:
    -----------
    #ifndef myTEST
    #define myTEST
    ZFSInt test;
    #endif
    ------------

    leads to an error at link time...
    application.o(.bss+0x0):/path_here/application.cpp:19: multiple
    definition of `test'
    app.o(.bss+0x0):/path_here/app.cpp:17: first defined here
    ....for EVERY call of #include "bla.h"
    :(

    Apparently the #ifndef only works at compile time, but somehow causes
    an error at linktime. Any solution? I want the variable "test"
    available in all modules that #include "bla.h"

    Regards,

    Lars Uffmann
     
    lars.uffmann, Aug 1, 2006
    #1
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  2. lars.uffmann

    Howard Guest

    First, copy the definition of test above into an implementation file you'll
    always link with (one which includes the header file where the definition of
    ZFSInt resides). Then, preface the above declaration in blah.h with the
    keyword "extern" (followed by at least one space). That tells the compiler
    to look elsewhere for the actual declaration of test, allows other units to
    see the variable through this header, and prevents the linker from seeing
    multiple definitions.

    -Howard
     
    Howard, Aug 1, 2006
    #2
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  3. lars.uffmann

    lars.uffmann Guest

    Issue solved - thank you! I put the actual declaration in the cpp-file
    belonging
    to the blah.h class definition header file.

    Any link to a website explaining this behaviour of #ifdef / #ifndef ?

    Best Regards,

    Lars
     
    lars.uffmann, Aug 1, 2006
    #3
  4. lars.uffmann

    red floyd Guest

    That's not a preprocessor issue, it's the ODR (one definition rule).
     
    red floyd, Aug 2, 2006
    #4
  5. lars.uffmann

    lars.uffmann Guest

    To me, that doesn't explain why
    #ifndef FOO
    #define FOO
    class foo {
    foo(){};
    ~foo(){};
    };

    int test;
    #endif

    successfully avoids multiple class definition but NOT multiple
    declaration of the variable "test" as an integer...

    I used to think #ifdef and the likes were preprocessor commands that
    would completely SKIP sections of code if the condition was not met -
    all the way to the next #endif.
    Apparently this does not work for variable declarations - That's the
    part I do not understand.

    Regards,

    Lars
     
    lars.uffmann, Aug 2, 2006
    #5
  6. lars.uffmann

    red floyd Guest

    Because you're defining a class... which is a type. No storage is
    allocated. When you're declaring a variable, storage is allocated.

    The other thing you need to know is that preprocessor commands don't go
    across translation units.
     
    red floyd, Aug 2, 2006
    #6
  7. lars.uffmann

    Marco Costa Guest

    Let's say that you include this file in two source files. What happens?
    a) The class foo gets its declaration. Good, now you can create foo objects.
    b) The integer test gets allocated. Once in one file and once in the second file.
    Everything is fine until the link stage. The linker will try to make sense of all and will see *two* instances of int test. Which one to use? Nobody will ever be able to answer that.

    The solution,

    extern int test;

    Goes in the header. That will tell to the compiler: There is a variable called test that is to be used, but it is only declared here. Its definition will be done somewhere else.

    Then, in one of your source files, you define this variable on the normal way.
    When you compile, you can refer to this variable in any source code that uses the include file, but at link time, only one instance will exist. At this point, no link error. Hooray!

    I hope I was clear.

    Regards,

    Marco
     
    Marco Costa, Aug 2, 2006
    #7
  8. You can imagine the preprocessor is a bunch of text editor macros. Do the
    substitutions by hand, or use the commands or options of your compiler that
    shows the preprocessor output, and look the generated code. This result is
    what the "real" compiler see.
     
    =?ISO-8859-15?Q?Juli=E1n?= Albo, Aug 2, 2006
    #8
  9. Yes. You want the variable available. That means you'll need a
    declaration,
    not the definition. The typical variable declaration is "extern <type>
    <name>;"
    You put the definition in a .cpp, just like you would with function
    definitions.

    This has nothing to do with #ifdef, which is a preprocessor command. It
    works only at compile time, and skips every second defintion of 'test'
    *in
    a single .cpp*, not across .cpp's. As the compiler works on single
    ..cpp's
    it won't notice that.

    HTH,
    Michiel Salters
     
    Michiel.Salters, Aug 2, 2006
    #9
  10. lars.uffmann

    lars.uffmann Guest

    Woohoo - I see light in the dark! Finally the explanation I've been
    waiting for :) To be honest, I suspected something like this, because
    the compiler creates libraries for each cpp file separately, so the
    linker would spot multiple declarations of the variable, but in that
    case the sense of the preprocessor commands did not occur to me - I
    never realized it was only to skip further definitions within the same
    cpp-File.

    Thanks for clearing this up! :)
    And to all the others that helped solving the problem!

    Regards,

    Lars
     
    lars.uffmann, Aug 2, 2006
    #10
  11. lars.uffmann

    chinthanep Guest

    Hi,

    In my implementation, it is like

    #ifndef TEST
    #define TEST
    const char* strDescriptions[] = {"Desc1", "Desc2", "Desc3"};
    #endif

    How to split the definition and definition of the above.

    Regds,
    Chinthan EP
     
    chinthanep, Aug 5, 2006
    #11
  12. lars.uffmann

    Ian Collins Guest

    First, please don't top-post on Usenet, your reply belongs below or
    interleaved with the post you are replying to.

    to split, declare the variable in a header and define in once and once
    only in a source module.

    In test.h:

    extern const char* strDescriptions[];

    in somefile.cc:

    const char* strDescriptions[] = {"Desc1", "Desc2", "Desc3"};
     
    Ian Collins, Aug 5, 2006
    #12
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