reinterpret_cast

register

The register storage class modifier requests that access to a variable be optimized for speed. Traditionally, register applied only to integer, character, or pointer variables, causing them to be stored in a register of the CPU instead of being placed in memory. However, the meaning of register has since been broadened to include all types of data. However, data other than integers, characters, and pointers cannot usually be stored in a CPU register. For other types of data, either cache memory (or some other sort of optimizing scheme) is used, or the register request is ignored.

register can only be used on local variables. In C, you cannot take the address of a register variable, but in C++ you can (although doing so may prevent the variable from being optimized).