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 &lt;typename F, typename Arg&gt;
    typename std::result_of&lt;F(Arg)&gt;::type
    invoke(F f, Arg a)
    {
        return f(a);
    }

I don&#39;t really see the difference with the following code:

    template &lt;typename F, typename Arg&gt;
    auto invoke(F f, Arg a) -&gt; decltype(f(a)) //uses the f parameter
    {
        return f(a);
    }

or

    template &lt;typename F, typename Arg&gt;
    auto invoke(F f, Arg a) -&gt; decltype(F()(a)); //&quot;constructs&quot; 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?

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Difference between std::result_of and decltype