Question

To be more specific, suppose I am writing template<class Pointer> class Foo and I want to declare a typedef inside the class for the type that *p would have if p were of type Pointer.

In C++03, as far as I am aware, the only way to do this is with something like

typename std::iterator_traits<Pointer>::reference

The disadvantage of this method is that it won’t work if Pointer is some custom iterator type and the author forgot to extend std::iterator or otherwise define a std::iterator_traits specialization.

In C++11, my colleague suggested

decltype(*Pointer())

But this won’t work if Pointer is not default-constructible, so he amended this to

decltype(**(Pointer*)0)

I tried this, and it worked, but then I thought that it looked a bit iffy because it involves the dereference of a null pointer, and thus might not be standards-compliant.

Can we do better?

Answer 1

You’re right to be cautious about dereferencing a null pointer, but the fact is that it’s ok here! decltype does not evaluate its operand, so dereferencing a null pointer inside is perfectly valid.

The proper solution, however, is std::declval, introduced inside <utility> in C++11:

decltype(*std::declval<Pointer>())