Multiple definition linker error

This is a discussion on Multiple definition linker error within the C++ Programming forums, part of the General Programming Boards category; Why does code like this Code: #ifndef _foo_h #define _foo_h class Foo { public: static int sInt; }; int Foo::sInt ...

  1. #1
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    Multiple definition linker error

    Why does code like this
    Code:
    #ifndef _foo_h
    #define _foo_h
    
    class Foo
    {
      public:
      static int sInt;
    };
    
    int Foo::sInt = 1;
    
    #endif
    cause linker error "multiple definition of 'Foo::sInt' when foo.h is included in several source files? I was under the impression that the purpose of header guards was precisely to prevent such an error.

  2. #2
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    Nope, you need to put the line:
    Code:
    int Foo::sInt = 1;
    in a .cpp file.

  3. #3
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    Quote Originally Posted by bithub View Post
    Nope, you need to put the line:
    Code:
    int Foo::sInt = 1;
    in a .cpp file.
    Yes, I already figured that out. My question is why? (If the guards prevent multiple definition of Foo, why don't they prevent multiple definition of Foo::sInt?)

  4. #4
    Captain Crash brewbuck's Avatar
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    Quote Originally Posted by R.Stiltskin View Post
    Yes, I already figured that out. My question is why? (If the guards prevent multiple definition of Foo, why don't they prevent multiple definition of Foo::sInt?)
    It prevents multiple inclusion in any one source module. It doesn't prevent anything at all across different modules.
    Code:
    //try
    //{
    	if (a) do { f( b); } while(1);
    	else   do { f(!b); } while(1);
    //}

  5. #5
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    Quote Originally Posted by brewbuck View Post
    It prevents multiple inclusion in any one source module. It doesn't prevent anything at all across different modules.
    Good to know. Thanks.

  6. #6
    Registered User Tonto's Avatar
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    It prevents multiple inclusion in any one source module. It doesn't prevent anything at all across different modules.
    Damn really? That's crazy

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  7. #7
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    Quote Originally Posted by brewbuck View Post
    It prevents multiple inclusion in any one source module. It doesn't prevent anything at all across different modules.
    Wait a minute. How can that be? Of course I can have
    Code:
    #ifndef _foo_h
    #define _foo_h
    
    class Foo
    {
      public:
      int bar;
    };
    
    #endif
    and #include "foo.h" in several different source files with no problem. It's only the initialization of the static member that causes a problem. Or does "different modules" mean something else?

  8. #8
    Captain Crash brewbuck's Avatar
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    Quote Originally Posted by R.Stiltskin View Post
    Wait a minute. How can that be? Of course I can have
    Code:
    #ifndef _foo_h
    #define _foo_h
    
    class Foo
    {
      public:
      int bar;
    };
    
    #endif
    and #include "foo.h" in several different source files with no problem. It's only the initialization of the static member that causes a problem. Or does "different modules" mean something else?
    Uh... Yeah, because in this case you're declaring something, not creating an object/variable...
    Code:
    //try
    //{
    	if (a) do { f( b); } while(1);
    	else   do { f(!b); } while(1);
    //}

  9. #9
    30 Helens Agree neandrake's Avatar
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    You can think of the headers as basically being copy/pasted into the source files where they are included. So if you put global declarations inside a header, the linker will find them in each module which had it included. This is why you separate .h and .c files so the .c is compiled once, and any other modules which would need something from it include the header files to get references. I'm not sure about C++ static vars, but you might be able to declare this in the .cpp file and then declare it extern in the .h file. Anyone know this off hand? I'd experiment but have to go.
    Environment: OS X, GCC / G++
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  10. #10
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    Quote Originally Posted by brewbuck View Post
    Uh... Yeah, because in this case you're declaring something, not creating an object/variable...
    [smacks self in head...] Yeah, I lost sight of the declaration/definition distinction.

  11. #11
    C++まいる!Cをこわせ!
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    Quote Originally Posted by neandrake View Post
    I'm not sure about C++ static vars, but you might be able to declare this in the .cpp file and then declare it extern in the .h file. Anyone know this off hand? I'd experiment but have to go.
    It's the same as any global var. Define in one source file, declare extern in a header.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

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