Re: A simple sample code involving templates, friends and lookup

["Richard Guenther" <richard.guenther@gmail.com>]

On 17 Jan 2008 09:09:38 -0800, Ian Lance Taylor <iant@google.com> wrote:
>
> "Richard Guenther" <richard.guenther@gmail.com> writes:
>
> > On Jan 17, 2008 2:12 PM, Dragan Milenkovic <dragan@plusplus.co.yu> wrote:
> > > Richard Guenther wrote:
> > > [snip]
> > > >> template <typename T>
> > > >> struct Foo
> > > >> {
> > > >>      template <typename Z>
> > > >>      friend void func(const Foo &);
> > > >> };
> > > >>
> > > >> void check(const Foo<int> & x)
> > > >> {
> > > >>      // Foo<int> weird;  // uncomment this line and all works
> > > >>
> > > >>      func<int>(x);    // <-- ERROR
> > > >> }
> > > >>
> > > >>
> > > >> Tested with gcc 4.0 - 4.3, and all behave the same:
> > > >>
> > > >> "error: 'func' was not declared in this scope"
> > > >>
> > > >> but it works if you uncomment the weird line.
> > > >
> > > > Actually even with the weird line the program is invalid.  What are
> > > > you trying to do? ;)
> > > >
> > > > Richard.
> > >
> > > Ok... afaik, that func should be defined on that very place where it is
> > > declared as friend. But could you please elaborate why it is invalid,
> > > since you made me start questioning my C++ knowledge... :-D
> >
> > How should name-lookup find func?
>
> It should use argument dependent lookup.  This is like the common case
> of
>
> class x
> {
>   friend x operator+(const& x, const& x) { return x.var + x.var; }
> };
>
> in which x::operator+ is found without taking special action.

Well, first I think you'd need friend-injection or otherwise a global
decl of the function.  Second I thought argument dependent
name-lookup only applies to namespaces, not classes.

EDG rejects this consistently btw.

Richard.