UntypedAllocator encapsulates an untyped memory allocation service, similar to Allocator, defined by the standard, but untyped like std::malloc. UntypedAllocator supports over-alignment and alignment offset. UntypedAllocator is also similar to the class std::pmr::memory_resource (introduced in C++17), but is not polymorphic.

UntypedAllocator Requirements
Requirement	Semantic
Non-static member function: void * allocate( size_t i_size, size_t i_alignment, size_t i_alignment_offset = 0);	Allocates a memory block large at least i_size bytes. The address at the offset i_alignment_offset from the beginning of the block is aligned at least to i_alignment. On failure this function should throw a std::bad_alloc. The return value is a pointer to the first byte of the memory block. The content of the block is undefined. If i_alignment_offset is not zero, in general the allocator has no requirements on the alignment of the first byte of the block. Anyway, since the address at i_alignment_offset will have n low-order zero bits (where i_alignment = 1 << n), if i_alignment_offset has m low-order zero bits (that is aligned to 1 << m), then the address of the first byte of the block is aligned at least to 1 << min(n,m). The user is responsible to ensure that: i_alignment must be greater than zero and an integer power of 2 i_alignment_offset must be less than or equal to i_size otherwise the behavior may be undefined.
Non-static member function: void deallocate( void * i_block, size_t i_size, size_t i_alignment, size_t i_alignment_offset = 0) noexcept;	Deallocates a memory block. The user is responsible to ensure that: i_block is a block allocated by the same allocator, or by an allocator that compares equal to it. i_size, i_alignment and i_alignment_offset are the same used when the block was allocated. otherwise the behavior may be undefined.
Operators == and !=	Checks for equality\inequality.
Default constructor and non-throwing destructor	A default constructed allocator is usable to allocate and deallocate memory block. The destructor must be no-except.
Copy constructor and copy assignment	A copy-constructed allocator compares equal to the source of the copy. As a consequence, a block allocated by an allocator can be deallocated by an allocator that was copy-constructed from it. Same for assignment.
Non-throwing move constructor and non-throwing move assignment	A move-constructed allocator may not compare equal to the source of the move. After a move construction or assignment, the source of the move cannot be used to allocate or deallocate any block. As a consequence, after an allocator A is used as source for a move construction or assignment to an allocator B, any block allocated by A must be deallocated by B.

basic_default_allocator satisfies the requirements of UntypedAllocator.