C++20's constexpr dynamic memory allocation allows pointers assigned using new or std::allocator::allocate to compare against one another only if they come from a single allocation. For example, constant evaluation of the following is fine:

int* p = new int[2];
assert(&p[0] < &p[1]);
delete [] p;

So too, using std::allocator and its allocate and deallocate member functions will work out nicely. However, comparing two pointers derived from separate allocations will fail, and produce a compilation error.

int* p1 = new int, * p2 = new int;
assert(p1 <= p2 || p1 > p2); // error
delete p1;
delete p2;

Included here in constexpr-pool-allocator.hpp is the template class pool_alloc. Thanks to Corentin Jabot and David Ledger's C++ proposal P2738, void pointers can now be cast to a pointer of the type of the pointed to object. This allows a simple pool allocator class to de defined; initialising its void pointer member by the typed pointer argument given to the constructor. Using two of these as the allocator arguments for corresponding containers, allows the element addresses of two corresponding containers to be compared, with no compilation error.