using friend function template in class template

Discussion in 'C++' started by Yueh-Wei Hu, May 23, 2004.

  1. Yueh-Wei Hu

    Yueh-Wei Hu Guest

    Victor Bazarov <> wrote in message news:
    ==============================================================
    > > Question 1:
    > > ==============================================================


    > Your code refuses to compile. In function 'foo<a>' 't' is a pointer to
    > const. Friendship has nothing to do with it. When I remove the attempt
    > to change 't->c' in foo<a>, it compiles and runs and displays
    >
    > T<a, 3>
    >


    This is my fault, sorry.

    The 't->c' assignment is added by me after I copied and pasted the
    source codes to the news.

    The reason why I did this is to emphasize the need of 'friend'
    declarations.

    > Fall back on gcc-3.2


    I fall back to gcc-3.2, and I can _not_ compile it !

    The error message from gcc-3.2 is as following:

    ==============================================================

    eee.cpp: In member function `void T<a, b>::ttt() const [with int a =
    2, int b =
    3]':
    eee.cpp:52: instantiated from here
    eee.cpp:44: call of overloaded `func(const T<2, 3>* const)' is
    ambiguous
    eee.cpp:8: candidates are: void func(const T<a, b>*) [with int a = 2,
    int b =
    3]
    eee.cpp:29: void func(const T<a, 3>*) [with int a = 2]
    eee.cpp:8: void func(const T<a, b>*) [with int a = 2,
    int b =
    3]
    eee.cpp:29: void func(const T<a, 3>*) [with int a = 2]

    ==============================================================

    The source code is as following:

    ==============================================================

    #include <iostream>

    template<int a, int b>
    class T;

    template<int a, int b>
    void func(T<a, b> const * const t)
    {
    std::cerr << "T<a, b>" << std::endl;
    }

    template<int a, int b>
    class T
    {
    private:

    friend void func<a, b>(T<a, b> const * const t);
    friend void func<a>(T<a, 3> const * const t);

    public:

    void ttt() const;
    };

    template<int a>
    void
    func(T<a, 3> const * const t)
    {
    std::cerr << "T<a, 3>" << std::endl;
    }


    template<int a, int b>
    void
    T<a, b>::ttt() const
    {
    func(this);
    }

    int
    main()
    {
    T<2, 3> t;

    t.ttt();

    return 0;
    }

    ==============================================================

    However, when I use gcc-3.2, gcc-3.3, gcc-3.4 to compile my code, the
    result is strange.

    Here is my result:

    1) gcc-3.2: can _not_ compile, the error message is copied and pasted
    above.

    2) gcc-3.3: can compile, but the result is

    T<a, b>

    3) gcc-3.4: can compile, but the result is

    T<a, b>

    So....
    What behavior conforms to the C++ standard?
    And which version of gcc is correct?

    =============================================================
    > > Question 2:
    > > =============================================================


    >
    > Neither can I. The code compiles file once the offending lines that
    > attemp to change 't->c' are commented out and the resulting program outputs
    >
    > T<a, 3>
    >
    >
    > Victor


    I can _not_ compile the code using gcc-3.2, and the error message is
    like the one I posted above:

    ==============================================================

    eee.cpp: In instantiation of `T<2, 3>':
    eee.cpp:50: instantiated from here
    eee.cpp:19: ambiguous template specialization `func<3>' for `void
    func(const
    T<3, 3>*)'
    eee.cpp: In member function `void T<a, b>::ttt() const [with int a =
    2, int b =
    3]':
    eee.cpp:52: instantiated from here
    eee.cpp:44: call of overloaded `func(const T<2, 3>* const)' is
    ambiguous
    eee.cpp:8: candidates are: void func(const T<a, b>*) [with int a = 2,
    int b =
    3]
    eee.cpp:29: void func(const T<a, 3>*) [with int a = 2]
    eee.cpp:8: void func(const T<a, b>*) [with int a = 2,
    int b =
    3]
    eee.cpp:29: void func(const T<a, 3>*) [with int a = 2]

    ==============================================================

    The source code I use is as following:

    ==============================================================

    #include <iostream>

    template<int a, int b>
    class T;

    template<int a, int b>
    void func(T<a, b> const * const t)
    {
    std::cerr << "T<a, b>" << std::endl;
    }

    template<int a, int b>
    class T
    {
    private:

    friend void func<a, b>(T<a, b> const * const t);
    friend void func<a>(T<a, 3> const * const t);
    friend void func<b>(T<3, b> const * const t); <-- add this -->

    public:

    void ttt() const;
    };

    template<int a>
    void
    func(T<a, 3> const * const t)
    {
    std::cerr << "T<a, 3>" << std::endl;
    }

    template<int b> <-- add this definition -->
    void
    func(T<3, b> const * const t)
    {
    std::cerr << "T<3, b>" << std::endl;
    }

    template<int a, int b>
    void
    T<a, b>::ttt() const
    {
    func(this);
    }

    int
    main()
    {
    T<2, 3> t;

    t.ttt();

    return 0;
    }

    ==============================================================

    Again, the following is the result when I compile it using gcc-3.2,
    gcc-3.3 and gcc-3.4:

    1) gcc-3.2: can _not_ compile, the error message posted above.
    2) gcc-3.3: can _not_ compile, the error message just like the one
    posted for gcc-3.2.
    3) gcc-3.4: can compile, but the result is

    T<a, b>

    ==============================================================

    I can not figure it out why overload doesn't appear.
    Could somebody explain this?

    Thank you for you help.
     
    Yueh-Wei Hu, May 23, 2004
    #1
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