Exempting Individual Members

Sometimes we need to change the access level of a name that a derived class inherits. We can do so by providing a using declaration (§3.1, p. 82):

class Base {
public:
    std::size_t size() const { return n; }
protected:
    std::size_t n;
};
class Derived : private Base {    //  note: private inheritance
public:
    // maintain access levels for members related to the size of the object
    using Base::size;
protected:
    using Base::n;
};

Because Derived uses private inheritance, the inherited members, size and n, are (by default) private members of Derived. The using declarations adjust the accessibility of these members. Users of Derived can access the size member, and classes subsequently derived from Derived can access n.

A using declaration inside a class can name any accessible (e.g., not private) member of a direct or indirect base class. Access to a name specified in a using declaration depends on the access specifier preceding the using declaration. That is, if a using declaration appears in a private part of the class, that name is accessible to members and friends only. If the declaration is in a public section, the name is available to all users of the class. If the declaration is in a protected section, the name is accessible to the members, friends, and derived classes.