如何强制子虚拟函数首先调用其父虚拟函数

How to force child same virtual function call its parent virtual function first(如何强制子虚拟函数首先调用其父虚拟函数)

问题描述

伙计们，我有一个案例，需要子类需要先调用其父虚函数，然后再调用其覆盖虚函数.

guys, I have a case that needs the child class needs to call its parent virtual function at first before call its override virtual function.

BaseClass::Draw()
{

}

ChildClass::Draw()
{
    BaseClass::Draw(); // BaseClass Draw must be called first.
}

GrandChildClass::Draw()
{
    ChildClass::Draw(); // ChildClass Draw must be called first.
}

我想对客户隐藏这种行为.这个有图案吗?

I want to hide this behavior from clients. Is there pattern on this?

谢谢.

推荐答案

对于简单的情况，您可以使用第二个私有成员函数来实现可重写行为:

For simple cases you can use a second, private member function for the overrideable behavior:

class Base {
public:
    void Draw() { 
        // Base-class functionality here
        OverrideableDraw();
    }

private:
    virtual void OverrideableDraw() { }
};

class Derived : public Base {
private:
    virtual void OverrideableDraw() {
        // Derived-class functionality here
    }
};

<小时>

对于更复杂的层次结构(例如，您有多个继承级别)，这是不可能的，因为任何派生类都可以覆盖任何虚拟成员函数(C++ 中没有 final).通常可以安全地假设每个派生类都在做正确的事情.虽然我可以想到有几次我因为派生类搞砸了覆盖而遇到问题，但这些情况通常很容易调试.

---

For more complex hierarchies (e.g. where you have multiple levels of inheritance), this isn't possible since any derived class can override any virtual member function (there is no final in C++). Usually it is safe to assume that each derived class is doing the right thing. While I can think of a few times that I've run into issues because a derived class screwed up overriding, those cases were usually pretty straightforward to debug.

如果您真的很担心并且真的想保证首先执行基类覆盖，您可以使用类似这样的方法，尽管这非常昂贵(至少这种幼稚的实现非常昂贵):

If you are really worried about it and really want to guarantee that base-class overrides are executed first, you could use something like this, though this is quite expensive (at least this naive implementation is quite expensive):

#include <functional>
#include <iostream>
#include <vector>

class Base {
public:

    Base() {
        RegisterDrawCallback(std::bind(&Base::DrawCallback, this));
    }

    void Draw() {
        for (auto it(drawCallbacks_.begin()); it != drawCallbacks_.end(); ++it)
            (*it)();
    }

protected:

    typedef std::function<void(void)> DrawCallbackType;
    typedef std::vector<DrawCallbackType> DrawSequence;

    void RegisterDrawCallback(DrawCallbackType f) {
        drawCallbacks_.push_back(f);
    }

private:

    void DrawCallback() { std::cout << "Base" << std::endl; }

    DrawSequence drawCallbacks_;
};

class Derived : public Base {
public:

    Derived() {
        RegisterDrawCallback(std::bind(&Derived::DrawCallback, this));
    }

private:

    void DrawCallback() { std::cout << "Derived" << std::endl; }
};

class DerivedDerived : public Derived {
public:

    DerivedDerived() {
        RegisterDrawCallback(std::bind(&DerivedDerived::DrawCallback, this));
    }

private:

    void DrawCallback() { std::cout << "DerivedDerived" << std::endl; }
};

[这只是一种选择；其他人可能会想出一个更优雅的解决方案.就我个人而言，我只是要确保虚拟成员函数的文档齐全，然后就可以了.]

[This is just one option; someone else can probably come up with a far more elegant solution. Personally, I'd just make sure the virtual member functions are well-documented and leave it at that.]

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