No global using statements in headers ever

[cjbacchus]

No description provided.

[PetrilloAtWork]

Suggested change

	Instead, importing specific symbols at local scope should be preferred
	You may import specific symbols at local scope (i.e. within the body of your `class`) if absolutely necessary for readability
	You may import specific symbols at local scope (i.e. within the body of your `class`)
	if absolutely necessary for readability (`using Iterator_t = std::vector<std::vector<std::pair<int, int>>>::const_iterator;`) or for functionality

(the using std::begin; example makes the difference in template code like:

template <typename Coll>
auto const& firstElement(Coll const& coll) {
  using std::empty, std::begin;
  return empty(coll)? typename Coll::value_type{}: *(begin(coll));
}

[cjbacchus]

I'm not sure I understand the example. Does return std::empty(coll) ? typename Coll::value_type{} : *std::begin(coll); not work?

[PetrilloAtWork]

If you are using a Coll that does not have a begin() method (e.g. because it's a C data structure or as it was in the past TCollection from ROOT, or maybe it's like std::filesystem::directory_iterator), the portal to STL is to have a free function begin(Coll const&) (and friends), ideally in the same namespace as Coll (alternatively, at global scope). So, we may implement begin(linked_list*) and begin(TCollection const&) ourselves and welcome there in STL world.
std::begin() would not find that function, but it covers the cases where that function is not available.