volatile vs. mutable in C++

Question

I have a question about the difference between volatile and mutable. I noticed that both of the two means that it could be changed. What else? Are they the same thing? What's the difference? Where are they applicable? Why the two ideas are proposed? How to use them in different way?

Thanks a lot.

Answer 1

A mutable field can be changed even in an object accessed through a const pointer or reference, or in a const object, so the compiler knows not to stash it in R/O memory. A volatile location is one that can be changed by code the compiler doesn't know about (e.g. some kernel-level driver), so the compiler knows not to optimize e.g. register assignment of that value under the invalid assumption that the value "cannot possibly have changed" since it was last loaded in that register. Very different kind of info being given to the compiler to stop very different kinds of invalid optimizations.