Re: [boost] boost::result_of and C++03

From: Anthony Williams (anthony_w.geo_at_[hidden])
Date: 2008-04-07 12:58:53

"Peter Dimov" <pdimov_at_[hidden]> writes:

> Eric Niebler:
>
>> Can you write what the identity function object would look like in C++0x?
>
> struct identity
> {
>     template<class A> A operator()( A && a ) const
>     {
>         return std::forward<A>( a );
>     }
> };
>
> I guess.

No. That will convert lvalues to rvalues. You also need the lvalue overload in
order to preserve the reference-ness of lvalues. IIRC, in Andrei
Alexandrescu's talk at ACCU 2008 he also claimed you need a const-reference
overload:

struct identity
{
    template<class A> A operator()( A && a ) const
    {
        return std::move( a );
    }
    template<class A> A& operator()( A & a ) const
    {
        return a;
    }
    template<class A> A const& operator()( A const & a ) const
    {
        return a;
    }
};

I'm not sure we need the third overload, as the second will deduce A to be
"const A". Unfortunately, the only compiler I have which handles rvalue
references says that the first two are ambiguous when given an lvalue.

Anthony
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