CODEWITHSUNDEEP
c++c++11
Seong
Seong

Reputation: 616

using declaration in class definition

I found that 'using' keyword can be used in class definition. according to https://en.cppreference.com/w/cpp/language/using_declaration, its used for member function is as follows:

If the name is the name of an overloaded member function of the base class, all base class member functions with that name are introduced. If the derived class already has a member with the same name, parameter list, and qualifications, the derived class member hides or overrides (doesn't conflict with) the member that is introduced from the base class.

an example code is as follows:

#include <iostream>
struct B {
    virtual void f(int) { std::cout << "B::f\n"; }
    void g(char)        { std::cout << "B::g\n"; }
    void h(int)         { std::cout << "B::h\n"; }
 protected:
    int m; // B::m is protected
    typedef int value_type;
};

struct D : B {
    using B::m; // D::m is public
    using B::value_type; // D::value_type is public

    using B::f;
    void f(int) { std::cout << "D::f\n"; } // D::f(int) overrides     B::f(int)
    using B::g;
    void g(int) { std::cout << "D::g\n"; } // both g(int) and g(char) are visible
                                       // as members of D
    using B::h;
    void h(int) { std::cout << "D::h\n"; } // D::h(int) hides B::h(int)
};

int main()
{
    D d;
    B& b = d;

//    b.m = 2; // error, B::m is protected
    d.m = 1; // protected B::m is accessible as public D::m
    b.f(1); // calls derived f()
    d.f(1); // calls derived f()
    d.g(1); // calls derived g(int)
    d.g('a'); // calls base g(char)
    b.h(1); // calls base h()
    d.h(1); // calls derived h()
}

From the above code, I am not sure what is the difference, for instance 

using B::f;
void f(int)

and

virtual void f(int)

Is there definite difference for using 'using' keyword in order to override class member function?

Upvotes: 3

Views: 645

Answers (1)

Lightness Races in Orbit
Lightness Races in Orbit

Reputation: 385098

The wording from that article is unfortunate. They tried to fix it with the parenthetical, but…

This has nothing to do with virtual function overriding.

The two examples you give are different and unrelated.

One brings the base class f into scope, then chucks that away and introduces a new D::f function. (This will not be virtual unless B::f already was, per the normal rules).

The other declares a virtual D::f.

Upvotes: 2

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