c++ - Behavior of decltype

Question

Say I have an object of some of stl container classes obj. I can define other object of same type this way:

decltype(obj) obj2;

But I can't declare iterator for the container this way:

decltype(obj)::iterator it = obj.begin();

Why? Am I doing something wrong?

Answer 1

Your code is well-formed according to the final C++0x draft (FDIS). This was a late change that's not yet been implemented by the Visual Studio compiler.

In the meantime, a workaround is to use a typedef:

typedef decltype(obj) obj_type;
obj_type::iterator it = obj.begin();

EDIT: The relevant chapter and verse is 5.1.1/8:

qualified-id:
    [...]
    nested-name-specifier template

_opt unqualified-id nested-name-specifier: [...] decltype-specifier :: decltype-specifier: decltype ( expression )

And for completeness's sake:

The original core issue

Proposal for wording