Add a default constructor to intrusive unordered_set and unordered_multiset

[anonymous]

All the constructors of intrusive unordered_set and unordered_multiset require a bucket_traits. It is currently not possible to instantiate an unordered_set without having allocated some buckets.

It would be useful to add a default constructor to unordered_set and unordered_multiset. That would allow to create hash containers that allocate buckets lazily at the first element insertion.

That would also allow this kind of pattern:

unordered_set<MyType, ...> v;
if (some_condition) {
  v = some_collection();
} else {
  v = some_other_collection();
}

Bonus if the const methods of a default-constructed unordered_set/unordered_multiset can be called and behave as an empty collection (in particular empty, size, begin/cbegin, end/cend, count, find and equal_range).

[Ion Gaztañaga]

Allowing a intrusive container without buckets, apart from pessimizing functions that must check for this special condition, makes all other insertion functions invalid (the intrusive container can't allocate memory so all functions should have a precondition saying that a bucket must have been introduced). This precondition-free behaviour was a design decision.

You can always use a single bucket for the "nearly default constructed" type, in case you want to minimize memory waste (a single bucket requires just the storage of a pointer).