shunsuke wrote:
Eric Niebler wrote:
I think that's right so far. Now imagine a simple identity function object, that just returns its argument unmodified:
struct identity { template<typename Sig> struct result;
template<typename This, typename Arg> struct result<This(Arg)> { typedef Arg type; };
template<typename Arg> Arg const &operator()(Arg const &arg) { return arg; }
// and a non-const overload, too };
Let me clarify. This identity implementation is simply wrong (if you pass an rvalue to identity.) because of inconsistency between decltype and result_of.
Yes, it's wrong, result<> and operator() are out of sync. result_of<identity(X&)>::type returns X&, whereas identity()(x), where x is an lvalue of type X, returns X const&. Arg& operator()( Arg& ) const; Arg operator()( Arg const& ) const; maybe? I still don't remember how these overloads are resolved for a const lvalue. Either way, this seems the best we can do in C++03.