volatile member function vs. constant member function in C++

Question

A lot of people are saying "volatile member function is completely analogous to how const works."

They are quite similar in the sense of if a pointer is marked as const/volatile, it can only access member functions marked as const/volatile.

But actually defining a member function as const has an additional effect, which makes the function read-only. Any modifications of the object inside the function will cause a compiler error. Is there such analogs in volatile member function?

Answer 1

Well, a volatile member function will make the object members volatile, that is, this will be as if it were defined volatile T * const this. And as a consequence, any reference to a member variable is also volatile.

Remember that volatile read/writes are operations that cannot be elided/reordered by the compiler. They are usually used to implement memory-mapped hardware devices or things like that.

Frankly speaking I've never been a use of this feature, other than doing smart tricks to filter the access to the function, not to make use of the volatile-ness of the object. If your code is low level enough to need volatile you probably will want to go putting the volatile just in the variables you need.