[Boost-users] Re: result_of example?

From: David Abrahams (dave_at_[hidden])
Date: 2004-12-14 07:47:58

Fedor Pikus wrote:
> I was trying to come up with a solution for the following problem:
> say, I want to declare this class
> template <typename T1, typename T2> class A
> {
>     public:
>     A( T1 x, T2 y ) : xy( x + y ) {}
>     private:
>     The_Right_Type xy;
> };
> 
> Or I want to declare a function template:
> template <typename T1, typename T2> The_Right_Type f( T1 x, T2 y ) {
> return x + y; }
> 
> The problem is, of course, what is The_Right_Type?
> I could not get a solution better than something which would require
> me to "register" all types in some way (then I could get types of all
> expressions involving registered types). Then Scott Meyers suggested I
> look at boost::result_of.

It doesn't work for this purpose.  result_of<x(a, b)>::type is only
useful when you can find a type for x.   But what is the type of
operator+?  It begs the original question: which operator+ do you mean?

What you really need is called decltype, a language feature that
unfortunately doesn't exist yet.  The next closest thing would be the
typeof library described in
http://aspn.activestate.com/ASPN/Mail/Message/boost/2239061
It innovates over some earlier efforts in several ways, one significant
one being that in most cases it can detect when the expression has an
rvalue result.

-- 
Dave Abrahams
Boost Consulting
http://www.boost-consulting.com