When would I use const volatile, register volatile, static volatile in C++?

register volatile int T=10;

volatile qualifier means that the compiler cannot apply optimizations or reorder access to T, While register is a hint to the compiler that T will be heavily used. If address of T is taken, the hint is simply ignored by the compiler. Note that register is deprecated but still used.

Practical Usage:

I have never used it never felt the need for it and can’t really think of any right now.

const volatile int T=10;

const qualifier means that the T cannot be modified through code. If you attempt to do so the compiler will provide a diagnostic. volatile still means the same as in case 1. The compiler cannot optimize or reorder access to T.

Practical Usage:

• Accessing shared memory in read-only mode.
• Accessing hardware registers in read-only mode.

static volatile int T=10;

static storage qualifier gives T static storage duration (C++11 §3.7) and internal linkage, while volatile still governs the optimization and reordering.

Practical Usage:

• Same as volatile except that you need the object to have static storage duration and to be inaccessible from other translation units.