C ++ When can I expand the `std` namespace?

Question

The thread on SO says the extension stdis UB (well, if you are, of course, not standard authors). But from time to time stdhappily expands. When is everything alright?

Answer 1

The only time you can add a definition to a namespace stdis to specialize a template that already exists in the namespace and explicitly creates an instance of the template. However, only if they depend on a user-defined type.

[namespace.std] (standard draft):

• ++ undefined, std std, . std , , .

• ++ undefined,

  (2.1) -

  (2.2) -

  (2.3) .

  , , , .

---

, : std::hash std::iterator_traits.

Answer 2

.

: std::hash std::unordered_map