为什么标准容器需要allocator_type :: value_type作为元素类型？

Question

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?

Answer 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.