Make mutexes mutable to preserve function constness

Something I have recently seen is people removing const from read only functions due to adding a mutex to control access and of course mutex changes state when used.

Example:

// rvo will use the vector move constructor...
std::vector<widget> get() const
{
    std::unique_lock<std::mutex> lk(cs_);
    std::vector<widget> copy(widgets_);
    return copy;
}

// in class definition
mutable std::mutex cs_;
std::vector<widget>widgets_;