Are public, in-class typedefs necessary in C++1y?

Question

I've noticed std:: containers tend to have public, in-class type aliases (typedef/using). E.g., see Member Types at http://en.cppreference.com/w/cpp/container/vector.

How are they useful? Aren't they just a relic of times when C++ didn't have things like auto and decltype?

When implementing a custom container, should it have such typedefs? What do I lose if I fail to provide them?

Answer 1

If you want a standard-library-compatible container, you do have to supply the typedefs.

If you look at the documentation, e.g. at cppreference, you will see passages like this:

std::vector meets the requirements of Container, AllocatorAwareContainer, SequenceContainer, ContiguousContainer (for T other than bool) (since C++17) and ReversibleContainer.

If you look up Container or SequenceContainer or any other thing listed there, you will find a list of requirements, and the typedefs (or rather types—they don't have to be typedefs, though they often are) are among them.

So if you are building a Container in the standard sense of the term, you need to provide the typedefs (and of course satisfy all other requirements too).

C++11 could in theory loosen the requirements, but it didn't. Perhaps because

std::vector<int>::iterator

is a heck of a lot more readable than

decltype(std::declval<std::vector<int>>().begin())

Or perhaps for some other reason.