`boost::container::deque` stores a redundant copy of the allocator, increasing size

[glebov-andrey]

Hi,

With the 1.90.0 release, deque stores two copies of effectively the same allocator, just for two different value_types - T * and T:

container/include/boost/container/deque.hpp

Lines 716 to 718 in f818ee9

	struct members_holder
	: public ptr_alloc_t
	, public allocator_type

Because of this, the size of the object increases, especially if the allocator is stateful, e.g. std::pmr::polymorphic_allocator.

Even with an empty allocator, it still increases the size for some ABIs, e.g. the Microsoft x86_64 ABI (MSVC and Clang). This is presumably due to different empty-base-optimization rules in different ABIs.

It should be possible to always store a single allocator, e.g. allocator_type, and copy-construct ptr_alloc_t from it only when needed. For any reasonable allocator (i.e. small and trivially copyable) this should be a zero-overhead operation for optimized builds.

sizeof results for reference:
Windows, x86_64, MSVC or Clang:

sizeof(boost::container::deque<int>)                                       = 40
sizeof(boost::container::deque<int, std::allocator<int>>)                  = 40
sizeof(boost::container::deque<int, std::pmr::polymorphic_allocator<int>>) = 48

Linux, x86_64, GCC or Clang:

sizeof(boost::container::deque<int>)                                       = 32
sizeof(boost::container::deque<int, std::allocator<int>>)                  = 32
sizeof(boost::container::deque<int, std::pmr::polymorphic_allocator<int>>) = 48