C++：通过调用纯虚函数避免重复

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?

Answer 1

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.

Answer 2

当然，添加一个仅实现该功能并从 A 继承的中间类 D。然后 B 和 C 可以从 D 继承。