__gnu_cxx::_ExtPtr_allocator(3cxx) on Linux
The text 2c0238f6e39e, as Debian 10 (Buster) shipped it.
An example allocator which uses a non-standard pointer type. This allocator specifies that containers use a 'relative pointer' as it's pointer type. (See ext/pointer.h) Memory allocation in this example is still performed using std::allocator.