Why doesn't std types provide conversion constructor / assignment from sources differing in allocator

Question

For example, why doesn't template< typename Elem, typename Traits, typename Alloc > basic_string { ... } provide:

template< typename OtherAlloc >
basic_string( const basic_string< Elem, Traits, OtherAlloc >& a_Other ) { ... }

It would seem fairly trivial to implement such a conversion constructor which respects both allocators. The current state of affair makes it very cumbersome to interface between types just differing in allocators.

Answer 1

The Standard hash also does not permit allocator fun- it can hash std::string and std::wstring but not std::basic_string<char, std::char_traits<char>, custom_alloc>. In addition, you cannot create an unordered_map or unordered_set with just an allocator- you must also provide a bucket number, which defaults to an implementation-defined constant you can't access, so you effectively must just make something up. The support, generally, is not very good.

It seems to me that, relatively simply, nobody proposed such a function or explored this use space.