Fedor Pikus wrote:

I was trying to come up with a solution for the following problem: say, I want to declare this class template 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 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::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