[boost] Re: Explicit Conversion Operators (N1592)

From: David Abrahams (dave_at_[hidden])
Date: 2004-03-07 08:35:22

Aleksey Gurtovoy <agurtovoy_at_[hidden]> writes:

> To: C++ extensions mailing list
> Message c++std-ext-6834
>
> David Abrahams writes:
>> "Powell, Gary" <powellg_at_[hidden]> writes:
>> >>From my reading of this paper, it would appear that this change does
>> > that. I also like that the change reuses an existing keyword in what
>> > would appear to be a natural extension of its use. i.e. explicit,
>> > still means explicit, you have to specify not rely on the default.
>> 
>> I haven't read the paper yet, but IMO a bigger problem is the fact
>> that we don't have a single syntax for requesting an explicit
>> conversion that will always be "safe".  For example:
>> 
>>   T(x)              // if T is a builtin type this might be a
>>                     // reinterpret_cast
>> 
>>   static_cast<T>(x) // might be a downcast
>
> That's solvable, though:
>
> template< typename T, typename S >
> T explicit_cast(S const& s, typename enable_if< is_class<T> >::type* = 0 )
> {
>     return T(s);
> }
>
> template< typename T, typename S >
> T explicit_cast(S const& s, typename disable_if< is_class<T> >::type* = 0)
> {
>     return static_cast<T>(s);
> }

This formulation requires T (and S, if you want to pass rvalues) to
have an accessible copy ctor.  I'm not sure it's better than the
formulation in boost/implicit_cast.hpp, which only puts a restriction
on T.

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com