Class Templates Again

This is a discussion on Class Templates Again within the C++ Programming forums, part of the General Programming Boards category; Hey Everybody. I have a question. I have a code like this in my .cpp file of the class: Code: ...

  1. #1
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    Class Templates Again

    Hey Everybody. I have a question. I have a code like this in my .cpp file of the class:
    Code:
    template <typename T>
    int myClass<T>::getNumber()
    {
    return 1;
    }
    In my .h file I have everything set up correctly for the template class. However, when I compile it I get this error:

    myClass.cpp: error: no ‚int myClass<T>::getNumber()‚ member function declared in class ‚myClass<T>‚
    myClass.cpp: error: template definition of non-template ‚int myClass<T>::getNumber()‚

    I do know however, that I DO have the getNumber in my .h file. Any ideas?

    Thank You.

  2. #2
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    Did you declare a getNumber() member function in myClass?

    Generally, with template classes everything goes into the header, your function definitions would just go at the bottom after the class declaration.

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    Never use implementation files (.cpp) when dealing with templates. Put the whole code (which deals with templates) into a .h file.

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    I declared the getNumber in myClass. Also, I include a .cpp in my client program. Any ideas?

  5. #5
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    Quote Originally Posted by Desolation View Post
    Never use implementation files (.cpp) when dealing with templates. Put the whole code (which deals with templates) into a .h file.
    *coughs*

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    Huh! Once I did as you guys said everything compiles now. Just one thing: I thought that it was always a bad practice to put code in the .h file. Is this an exception, and are there alternatives (ie before I was including a .cpp file). I am not challenging your correctness, just wondering.

    Thank you guys for helping me.

  7. #7
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    >> I thought that it was always a bad practice to put code in the .h file.
    It is generally bad practice, but not always. Templates are different, they almost have to be included in the code that calls them so that the compiler can generate code for the type you are using for the template type.

    Some people leave the code in a cpp file and then #include that cpp file at the bottom of their header file. If you do that you can make it a .inl file instead of .cpp to indicate that it isn't a normal source file.

    More information is here: http://www.parashift.com/c++-faq-lit...html#faq-35.12

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    I'm adding this as a FYI only because it is widely criticized and not widely implemented by compilers but the export keyword is used to counter that issue. Using either Daved's suggestion or mine is a better alternative, however.

  9. #9
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    Quote Originally Posted by Desolation View Post
    I'm adding this as a FYI only because it is widely criticized and not widely implemented by compilers but the export keyword is used to counter that issue. Using either Daved's suggestion or mine is a better alternative, however.
    Thank You!

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