default argument in friend function of template class

[xuatla]

The following is just a sample code to demostrate my question:

-----------
template <typename T>
class C
{
public:
friend void f1(double i=2) { std::cout << i; } ;
};

-----------
compile result:
test1.cpp: In instantiation of `void f1(double)':
test1.cpp:95: instantiated from here
test1.cpp:88: error: default argument given for parameter 1 of `void
f1(double)'
test1.cpp:88: error: after previous specification in `void f1(double)'
------------

If I don't use template or I remove the default argument in f1, then no
problem occurs. But with both of them it won't work.

Anyone know the reason for this problem?

Thanks a lot in advance.

X

[Victor Bazarov]

xuatla wrote:

The following is just a sample code to demostrate my question:

-----------
template <typename T>
class C
{
public:
friend void f1(double i=2) { std::cout << i; } ;
This defines a [global] function 'f1', and as soon as you try to
instantiate the template more than once, you get multiple definition
of 'f1'.
};

-----------
compile result:
test1.cpp: In instantiation of `void f1(double)':
test1.cpp:95: instantiated from here
test1.cpp:88: error: default argument given for parameter 1 of `void
f1(double)'
test1.cpp:88: error: after previous specification in `void f1(double)'
------------

If I don't use template or I remove the default argument in f1, then no
problem occurs. But with both of them it won't work.
Of course it won't. Did you mean to define a template function 'f1'?
Anyone know the reason for this problem?

Yes, see above. Of course, it's unclear what you're trying to achieve,
but here is a potential solution:

#include <iostream>

template<class T> void foo(double = 2) { std::cout << i; }

template<class T>
class C
{
public:
friend void foo<T>(double i);
};

int main() {
C<char> cc;
C<int> ci;
}
V

[xuatla]

Thank you, Victor.

Victor Bazarov wrote:

xuatla wrote:

The following is just a sample code to demostrate my question:

-----------
template <typename T>
class C
{
public:
friend void f1(double i=2) { std::cout << i; } ;

This defines a [global] function 'f1', and as soon as you try to
instantiate the template more than once, you get multiple definition
of 'f1'.

see below.

};

-----------
compile result:
test1.cpp: In instantiation of `void f1(double)':
test1.cpp:95: instantiated from here
test1.cpp:88: error: default argument given for parameter 1 of `void
f1(double)'
test1.cpp:88: error: after previous specification in `void f1(double)'
------------

If I don't use template or I remove the default argument in f1, then
no problem occurs. But with both of them it won't work.

Of course it won't. Did you mean to define a template function 'f1'?

I mean function 'f1' is a friend of the template class 'C'.
It was not clear in my former post. But what I want to do is
friend void f1(const C& c, double i=2);

Here 'f1' has default argument 'i' and it calls some private members of
class 'c'.

For you above multiple definition of 'f1': if I add c as an argument of
'f1', then for different instantiate of class C, 'f1' has different
arguments so it should not have the confliction. Am I correct?

Anyone know the reason for this problem?

Yes, see above. Of course, it's unclear what you're trying to achieve,
but here is a potential solution:

#include <iostream>

template<class T> void foo(double = 2) { std::cout << i; }

template<class T>
class C
{
public:
friend void foo<T>(double i);
};

int main() {
C<char> cc;
C<int> ci;
}
V

This way works. Thank you. 'f1' can get access to the private members.
Thanks a lot!

X