template class ..

This is a discussion on template class .. within the C++ Programming forums, part of the General Programming Boards category; Hello All, Can someone pls explain where i'm going wrong?: here's a snippet of what i've written : Matrix.h *********************************** ...

  1. #1
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    template class ..

    Hello All,

    Can someone pls explain where i'm going wrong?:
    here's a snippet of what i've written :

    Matrix.h ***********************************
    Code:
    template <typename T>
    class MTX{
          public:
                 /* Constructors */
                 MTX(int, int);  
          .............
    ******************************************
    Matrix.cpp**********************************
    Code:
    template <typename T> MTX<T>::MTX(int n, int m){
             A = new std::vector<std::vector<T> >(n, m);             
    }
    ******************************************
    main.cpp***********************************
    Code:
    main(){
          MTX<int> mtx(50,50); 
           
    }
    ******************************************


    i've put my includes in both main.cpp and Matrix.cpp (#include "Matrix.h")
    and when i compile DevCpp gives ma the following error:

    "[Linker error] undefined ref. to MTX<int>::MTX(int, int)"

    I really don't see what's going wrong(i know what linker error means btw) because i'm
    copying straight from an example in a book.

    Any help would be great as its my first time trying to make a template class.

    Thanks

  2. #2
    C++まいる!Cをこわせ!
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    Templates are tricky creatures. Try putting your definition (the .cpp file contents) into the header file and it should compile.

  3. #3
    Super Moderator VirtualAce's Avatar
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    I would highly advise against using a vector of vector<T>'s since it could end up being quite confusing. It's confusing just to look at it let alone use it.

  4. #4
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    Thanks for the suggestions


    1 - I tried putting the .ccp stuff in the .h file and i get the same error.

    2- What would you suggest other than using vector<T> ? I'm using it to try and make the
    class a generic as possible so i can do double and float math.


    Thanks again.

    Any more suggestions?

  5. #5
    C++まいる!Cをこわせ!
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    Notes:
    - Main should return int.
    - Main should have "return 0;" at the end.
    You can also inline your templates:
    Code:
    template<typename T> class foo()
    {
    	foo(int i, int j)
    	{
    		i = j; // Or whatever here
    	}
    };

  6. #6
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    See this FAQ for more information, especially starting with 35.12: http://www.parashift.com/c++-faq-lite/templates.html

    I don't see the problem with std::vector<std::vector<T> >. However, I wouldn't make A a pointer allocated with new. There's no point to doing that. Just make it a regular object.

  7. #7
    and the hat of sweating
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    Does your book actually show the template split up into a .h & .cpp file? That's bad. Most compilers barf when you do that, so everything should be in the .h file.

  8. #8
    C++まいる!Cをこわせ!
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    It's still possible to put both declaration and definition in the header file, too.
    Your can make the usual declaration of the class and after that you can also define the class like you would in a .cpp file, but in the header. It makes the declaration much cleaner, so I do recommend it.

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