如何强制孩子使用相同的虚函数首先调用其父虚函数 [英] How to force child same virtual function call its parent virtual function first

问题描述

guys，
我有一个case需要子类需要调用其父虚拟函数之前调用其重写虚函数。

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.
}

我想从客户端隐藏此行为。是否有模式？

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

感谢。

推荐答案

您可以使用第二个私有成员函数作为可覆盖的行为：

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
    }
};

---

对于更复杂的层次结构这是不可能的，因为任何派生类都可以覆盖任何虚拟成员函数（在C ++中没有 final ）。通常可以安全地假设每个派生类正在做正确的事情。虽然我可以想到几次，我遇到了问题，因为派生类拧紧了重写，这些情况通常是很容易调试。

---

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.

如果你是真的担心它，真的想保证基类覆盖首先执行，你可以使用像这样，虽然这是相当昂贵的（至少这个天真的实现是相当昂贵的）：

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; }
};

[这只是一个选项;别人可能想出一个更优雅的解决方案。就个人而言，我只是确保虚拟成员函数有详细的文档，并保留它。]

[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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