Aleksey Gurtovoy <agurtovoy@meta-comm.com> writes:

To: C++ extensions mailing list Message c++std-ext-6834

David Abrahams writes:

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"Powell, Gary" <powellg@amazon.com> writes:

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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