C++ mutex and const correctness

You can mark data members with the keyword mutable to allow them to be modified in a constant member function, e.g.:

struct foo 
{
    mutable mutex foo_mutex;
    // ....
    void bar() const
    {
        auto_locker lock(foo_mutex);
        // ...
    }
};

Try to do this as little as possible because abusing mutable is evil.