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 templateoptunqualified-id

nested-name-specifier:
    [...]
    decltype-specifier ::

decltype-specifier:
    decltype ( expression )

And for completeness's sake:

The original core issue

Proposal for wording

Answer 2

It's because of the way that the language is parsed.

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

You want it to become

(decltype(obj)::iterator) it;

But in actual fact, it becomes

decltype(obj) (::iterator) it;

I have to admit, I was also surprised to see that this was the case, as I'm certain that I've done this before. However, in this case, you could just use auto, or even decltype(obj.begin()), but in addition, you can do

typedef decltype(obj) objtype;
objtype::iterator it;

Answer 3

Yet another workaround until VC++'s parser is fixed to reflect the FDIS is to use the std::identity<> metafunction:

std::identity<decltype(obj)>::type::iterator it = obj.begin();