sgi-stl | SGI implement | Graphics library

This allocator uses an 8-byte header in front of every allocation to record its size. It is common to store metadata like this next to the actual allocation. Even though the header contains a 4-byte int, 8 bytes ensure that the returned pointer is aligned to 8 bytes, a common requirement for larger types..

As you can read from the code, allocate asks for n+8 bytes (call the resulting allocation p) , stores the size at the the beginning and returns p+8.

Deallocate and reallocate are given the pointer p+8,so have to first subtract to get back to the real allocation.