static mutable member variables in C++?

Question

Hello,

why or for what reason is it not possible to declare a class member variable in C++ as "static mutable"? Something like

static mutable int t; //This won't compile

For me, there is no reason to ban such declarations. E.g. for reasons like maintaining a global class-wide statistics, it may be convenient to have static variable that can be altered by (logically) const methods. So either I this is sort of a misdesign in C++ and unnecessarily complicated, or there is a practical or theoretical reason which I cannot see as of now.

I could imagine it makes it easier to compile and optimize a C++-code which does not feature something like this, but I am not a compiler expert. Does anybody has advice on this topic?

Thanks!

Answer 1

Non-const static members of the class can already be modified by any (const and non-const) methods of the class. There's no need and no point in declaring it with mutable. That would achieve absolutely nothing.

Answer 2

The mutable keyword allows a "const" and therefore non-static member function to change non-static member variables marked as such (i.e., mutable). Static functions cannot be const and const member functions can change non-const static members. I know this is somewhat confusing, but it is because of this that there is no need to allow a mutable static member variable.