Re: [boost] Boost Overload Documentation

From: Marco Costalba (mcostalba_at_[hidden])
Date: 2007-10-13 02:20:32

On 10/12/07, Marco <mrcekets_at_[hidden]> wrote:
>    - if the intersection between "rs" and "ss" is not empty then the new
>      target (i.e. "f") overwrites all the targets related to the
>      signatures included in the intersection.
>
> I hope to have answered to your question.
>

Yes thanks. I would have read the previous posted policy better.

My concern has more to do with a language corner than with overload 'per se'.

The whole point is that the 'comparison' used when adding a function
to the overload set is different from the 'comparison' used when
choosing among the function given a set of arguments, i.e. at call
time.

In particular at registering time we (but also C++ compiler) uses
perfect signature match to test a function signature against the
others.

As example

class A
{
   void foo(char);
   void foo(long);
};

compiles because void(char) and void(long) are different function types. Also

void foo1(char);
void foo2(long);

boost::overload<void(char), void(long)> ov;

ov.set(&foo1);
ov.set(&foo2);

compiles the same.

But at call time matching between passed in arguments and function to
call is done (in C++) not with "perfect type match" comparison but
with "type match + implicit conversions" comparison.

That's the reason that instruction;

A a;
a.foo(7); // compile error!

fails with message "ambiguous call, 2 candidates...", not with message
'I don't find a suitable foo()'

This ambiguity is, of course, intrinsically present also in 'overload'
because registering is done on perfect signature matching, while at
runtime class member overloading is used (i.e. implicit conversions
are taken in account).

Please note that this is good! is what C++ does, the only difference
is that with

a.foo(7);

a compile error is produced and user is warned about this ambiguity
that he must resolve explicitly in some way.

I didn't know what, in the above case

ov(7);

should produce.

Thanks
Marco

P.S: In case you are wondering "why didn't he tried?" is that I had
some compiler error (probably SFINAE related) and didn't had the time
to dig it more.