施工解决方案期间的虚拟功能 [英] Virtual Function During Construction Workaround

问题描述

我有一个具有虚函数的基类。我想在构造过程中调用该类，因为我希望为每个派生类调用该函数。我知道我在构造过程中不能调用虚拟函数，但是我想不出一种优雅的（即避免重复代码）解决方案。

I've got a base class that has a virtual function. I want to call that class during the construction because I want the function called for each of the derived classes. I know I can't call a virtual function during construction, but I can't think of an elegant (i.e., avoid repeating code) solution.

在构造过程中调用虚拟函数有哪些解决方法？

我想避免这种情况的原因是因为我不想创建仅调用基类的构造函数。

The reason I want to avoid this is because I don't want to have to create constructors that just call the base class.

class A {
    public:
        A() {
            read();
        }

        // This never needs to be called
        virtual void read() = 0;
}

class B:A {
    public:
        B():A() {   };
        read() { /*Do something special for B here.*/ }

}

class C:A {
    public:
        C():A() {   };
        read() { /*Do something special for C here.*/ }

}

PS：执行此操作的Python方法只是在 A :: read（） NotImplementedError c $ c>。我将返回C ++，并且比我想像的还要生锈。

PS: The Python way of doing this is simply to raise NotImplementedError in A::read(). I'm returning to C++ and I'm more rusty than I thought.

推荐答案

这是工厂方法，将工厂进入基类：

This is the factory method approach, putting the factory into the base class:

class A {
public:
    virtual void read() = 0;
    template<class X> static X* create() {X* r = new X;X->read();return X;}
    virtual A* clone() const = 0;
};

class B : public A {
    B():A() {   };
    friend class A;
public:
    void read() { /*Do something special for B here.*/ }
    B* clone() const {return new B(*this);}
};

class C : public A {
    C():A() {   };
    friend class A;
public:
    void read() { /*Do something special for C here.*/ }
    C* clone() const {return new C(*this);}
};

添加了 clone 方法并具有协变量收益

Added a clone-method with covariant return type as a bonus.

使用 CRTP ：

class A {
public:
    // This never needs to be called
    virtual void read() = 0;
    virtual A* clone() const = 0;
};
template<class D, class B> struct CRTP : B {
    D* clone() {return new D(*this);}
    static D* create() {return new D();}
};

class B : public CRTP<B, A> {
    B() {   };
public:
    void read() { /*Do something special for B here.*/ }
};

class C : public CRTP<C, A> {
    C() {   };
public:
    void read() { /*Do something special for C here.*/ }
};

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