C++ const method on non const pointer member

Question

I was wondering how protect a non const pointer member from an object throught a const method. For example:

class B{
    public:
        B(){
            thi         


        

Answer 1

The problem you have is that a const method makes all the member variables const. In this case however, it makes the pointer const. Specifically, it's as if all you have is B * const b, which means a constant pointer to a (still) mutable B. If you do not declare your member variable as const B * b, (that is, a mutable pointer to a constant B), then there is no way to protect from this behavior.

If all you need is a const B, then by all means, define A like this:

class A {
public:
    A() : b(new B) {}

    // This WILL give an error now.
    void changeMemberFromConstMethod() const { b->setVal(); }
private:
    const B* b;
}

However, if other methods of A mutate B, then all you can do is make sure that B does not get mutated in your const methods of A.