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From: Hyman Rosen <hyrosen@mail.com>
Newsgroups: comp.std.c++
Subject: Re: Core Language Active Issue #329
Date: Fri, 10 May 2002 03:03:04 GMT
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Garry Lancaster wrote:
> Hyman Rosen:
>>The declaration itself can involve the template
>>parameters in such a way that it is legal for some
>>types and illegal for others.
> 
> Could you explain what you mean by that?

Simple enough -

template<typename T>
struct X
{
	friend void f(typename T::t) { }
};

Whenever you instantiate an X<T>, you have to inject
void f(T::t) into the enclosing namespace. Whether
you inject the declaration or the definition, it's
stiil going to be illegal if the T in question doesn't
have a member type t.

> Perhaps you could give some reasons why the behaviour
> should remain as currently specified?

There's the issue of what happens when two instantiations
cause an ODR violation:

template <int N> struct X { friend int f() { return N; } };
X<1> x1; X<2> x2;

Would this be an error? (It is now.)

You could also break existing programs:

file1.cpp:
template <typename T> struct X { friend void f(T) { }; };
X<int> x;

file2.cpp:
extern void f(int);
int main() { f(3); }

This would fail to link, whereas it would work currently.

Finally, 14.5.3/5 is crystal clear in its description of what
happens when templates define friend functions. I understood
that the issues list was to resolve defects and ambiguities
in the standard, not to change the language.

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