Defined in header
std::pmr::memory_resource* null_memory_resource() noexcept;
Returns a pointer to a
memory_resource that doesn't perform any allocation.
 Return value
Returns a pointer
p to a static storage duration object of a type derived from std::pmr::memory_resource, with the following properties:
allocate()function always throws std::bad_alloc;
deallocate()function has no effect;
- for any
&r == p.
The same value is returned every time this function is called.