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using friend function template in class template

P: n/a
Victor Bazarov <v.********@comAcast.net> 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.

Jul 22 '05 #1
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