Does the 'mutable' keyword have any purpose other than allowing the variable to be modified by a const function?

Question

A while ago I came across some code that marked a member variable of a class with the `mutable` keyword. As far as I can see it simply allows you to modify a variable in a `const` method: class Foo { private: mutable bool done_; public: void doSomething() const { ...; done_ = true; } }; Is this the only use of this keyword or is there more to it than meets the eye? I have since used this technique in a class, marking a `boost::mutex` as mutable allowing `const` functions to lock it for thread-safety reasons, but, to be honest, it feels like a bit of a hack.

Answer 1

Hello,

Actually, the hack would be to const-cast the variable you want to be mutated, e.g. within a const method. "mutable" is useful not only to allow such mutation, but to tell the user of the class that the field (and, presumably, *only* that field) can change 'behind their back'.