A simple sample code involving templates, friends and lookup

[Dragan Milenkovic]

Hello,

I'm aware about these issues being around for quite a while now, but
there is still a chance this sample code can be helpful. I apologize
if someone gets annoyed :-P


template 
struct Foo
{
template 
friend void func(const Foo &);
};

void check(const Foo & x)
{
// Foo weird;  // uncomment this line and all works

func(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.


Best regards,

Dragan Milenkovic

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

[Dragan Milenkovic]

Richard Guenther wrote:
[snip]

template 
struct Foo
{
 template 
 friend void func(const Foo &);
};

void check(const Foo & x)
{
 // Foo weird;  // uncomment this line and all works

 func(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

Dragan

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

[Dragan Milenkovic]

Jonathan Wakely wrote:

On 17/01/2008, Richard Guenther wrote:

Well, a language lawyer can probably clear things up.  From a look
at the std it looks like w/o a previous declaration the above should
be invalid.  And at a different point it suggests the decl becomes
available.


Yes, at the point of instantiation of Foo the friend is declared,
and can then be found by ADL because Foo is an associated type.
The reference parameter 'x' doesn't cause an instantiation, only 'weird' does.

However, see the discussion in section 9.2.2 of Vandevoorde and
Josuttis' C++ Templates book and the footnote saying the standard
isn't exactly clear.

[snip]

Thanks for clarification and info. I believe issue #34 addresses exactly
what we're talking about.

http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_closed.html#34

Hey... the current number of issues is kinda scary... :-O

Dragan