Why do standard containers require allocator_type::value_type to be the element type?

问题

Related: Deprecation of std::allocator<void>.

The following description about template parameter Allocator is found for both std::vector and std::list (emphasis mine):

An allocator that is used to acquire/release memory and to construct/destroy the elements in that memory. The type must meet the requirements of Allocator. The behavior is undefined if Allocator::value_type is not the same as T.

The last sentence does not make sense to me. If a specific value_type is required, couldn't it just do an allocator rebind?

回答1:

The reason is mostly historical—rebinding was more complicated before C++11 added allocator_traits. The Networking TS defines a “proto-allocator” concept ([async.reqmts.proto.allocator]) where rebinding is always applied before any use, so it seems likely that the requirement will be relaxed someday.

来源：https://stackoverflow.com/questions/50245277/why-do-standard-containers-require-allocator-typevalue-type-to-be-the-element