Re: [Boost-users] Using boost::enable_if with template copy constructors

From: Tim Robertson (timr_at_[hidden])
Date: 2006-09-06 21:03:36

On Sep 6, 2006, at 4:15 PM, David Abrahams wrote:

> The problem is that is_convertible<Foo,Foo>::value is not yet known,
> and (aside from the fact that there's an implicitly generated copy
> ctor that's a perfect match which the compiler seems to ignore at this
> stage, of course)

Consistently so, for both g++3.x and g++4.x.

(Am I missing something, or is this not sufficiently weird to register 
as a problem?)

> without that ::value the compiler can't decide
> whether either of those constructors is a match for a Foo argument.
> In fact, we're only checking for that match because we want to know
> that very ::value!

Got it.  The compiler can't instantiate is_convertible<Foo,Foo> if the 
constructor(s) that enable the conversion (from Foo to Foo) themselves 
have signatures that depend on is_convertible<Foo,Foo>.

>> Right.  So again, it seems as though it isn't possible to do what I'm
>> trying to do.
>
> Sure it is.  You could use
>
>   enable_if< mpl::or_<is_same<T,Foo>, is_convertible<T,Foo> >, ... >
>
> Type sameness can be determined without looking at constructors or
> even knowing anything about the definition of the type.

I see...skip the is_convertible<> check when the types are actually the 
same, thus avoiding the instantiation of is_convertible<Foo,Foo>.  This 
might work for my real problem.  I'll give it a shot.

Thanks,
Tim