Difference between std::result_of and decltype
- by Luc Touraille
I have some trouble understanding the need for std::result_of in C++0x. If I understood correctly, result_of is used to obtain the resulting type of invoking a function object with certain types of parameters. For example:
template <typename F, typename Arg>
typename std::result_of<F(Arg)>
invoke(F f, Arg a)
{
return f(a);
}
I don't really see the difference with the following code:
template <typename F, typename Arg>
auto invoke(F f, Arg a) -> decltype(f(a)) //uses the f parameter
{
return f(a);
}
or
template <typename F, typename Arg>
auto invoke(F f, Arg a) -> decltype(F()(a)); //"constructs" an F
{
return f(a);
}
The only problem I can see with these two solutions is that we need to either:
have an instance of the functor to use it in the expression passed to decltype.
know a defined constructor for the functor.
Am I right in thinking that the only difference between decltype and result_of is that the first one needs an expression whereas the second does not?
