C++: Avoiding repetition by calling pure virtual function

Question

For the following code,

class A
{
    private:
        virtual void f() = 0;
        virtual void g() = 0;
        ...

    public:
        A()
        {
        }

        ...
}

class B : public A
{
    private:
        void f()
        {
            ...
        }
        void g()
        {
            ...
        }
        void h()
        {
            if (...)
            {
                f();
            }
            else
            {
                g();
            }
        }

        ...

    public:
        B()
        {
            h();
        }

        ...

}


class C : public A
{
    private:
        void f()
        {
            ...
        }
        void g()
        {
            f();
        }
        void h()
        {
            if (...)
            {
                f();
            }
            else
            {
                g();
            }
        }

        ...

    public:
        C()
        {
            h();
        }

        ...

}

is there a way to avoid repetition of h() (the context of function h() in both classes B and C is the same) or we cannot avoid it simply because we cannot call pure virtual functions in constructors?

it is an example from textbook, does it matter in the context of my question? — Kudayar Pirimbaev, Apr 25 '14 at 00:45
This is a very vague question, and your constructor setup isn't quite right. Also if you create a private pure virtual function then it cannot be instantiated NOR can any of its child classes. — Ahmed Masud, Apr 25 '14 at 00:45
To your question: Yes, that's one of the main uses of `virtual`. — Deduplicator, Apr 25 '14 at 00:48
It works, just give it a spin in the compiler of your choice. — Deduplicator, Apr 25 '14 at 00:48
For the private virtual, see: http://stackoverflow.com/a/3978552/2077394 For the question on h, just put it in the base class if it is common? — Joky, Apr 25 '14 at 00:49
won't it cause "pure virtual function call" error if i put it in the base class since i call it in constructors of derived classes? — Kudayar Pirimbaev, Apr 25 '14 at 00:51
The order of construction is base first and derived after. Moreover h does not need to be virtual, does it? — Joky, Apr 25 '14 at 00:55

score 2 · Accepted Answer · answered Apr 25 '14 at 01:09

You can implement your function h() as a member of A.

The pure virtual function call would only be an issue if h() gets executed during the body of A::A() (or other A constructors or A::~A()). Inside the body of B::B() or C::C(), the polymorphic type of *this is B or C respectively, so your virtual overrides declared in class B or C will have effect.

score 0 · Answer 2 · answered Apr 25 '14 at 00:52

0

Sure, add an intermediate class D which only implements that function and inherits from A. Then B and C can inherit from D.

answered Apr 25 '14 at 00:52

Cramer

1,785
1
12
20

won't it cause "pure virtual function call" error since i call it in constructors of B and C? – Kudayar Pirimbaev Apr 25 '14 at 00:55

C++: Avoiding repetition by calling pure virtual function

2 Answers2