Question

Suppose I have a base class foo

class foo {
    virtual int get() const = 0;
}

and maybe 20 sub-classes foo_1, foo_2 … inherited from foo and in the form of:

class foo_1 : public foo {
    int get() const { return 1; }
}
...
class foo_20 : public foo {
    int get() const { return 20; }
}

Suddenly life is not so easy! I have a class foo_21, which has to do this:

class foo_21 : public foo {
    int get() { member_ = false; return 0; }
    bool member_;
}

The problem is get is declared as const in the base class, and I have to change
something in the sub-class foo_21. How could I find a way to go around it?

Answer 1

Your base function isn’t virtual, which makes all this highly speculative. Your code should already be working as you posted it (though maybe not as you expect).

You could use mutable members:

class foo_21 : public foo
{
    int get() const { member_ = false; return 0; }
    mutable bool member_;
};

It’s important that the mutable variable doesn’t affect the logical constness of your class. If it does, you should rework your design.