const volatile, register volatile, static volatile

Question

I am wondering about the different uses of the volatile keyword in combination with register, const and static keywords. I am not sure what are the effects, so I think:

register volatile int T=10;

Suggest the compiler to store T in a register and the value of T can be modified from somewhere outside (OS, hardware, another thread)

const volatile int T=10;

The program itself can not modify T, but T can be modified frow somewhere outside the code.

static volatile int T=10;

If T is a data member of a class it means that all the objects of the class have the same value for T and T can be modified from somewhere outside. If T is a global variable in a file, the source code in other files (that are part of the project) cannot access T, but T can be accessed from somewhere outside. If T is a local variable in a function,once it has been initialized remains in the memory until the end of the program and can be modified from somewhere outside.

Are my thoughts correct and can any experienced C++ developer give an example where the above maybe used in real-world applications or it is very rare?

Answer 1

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.

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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.

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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.