如何在调用派生类构造函数之前设置基类成员? [英] How to set base class members before derived class constructor is called?

问题描述

There is a base class for components of my application which provides some members and the functions Init() and Update() which must be overwritten.

class Component
{
public:
    void Set(type* Member1, type* Member2, type* Member3)
    {
        this->Member1 = Member1;
        this->Member2 = Member2;
        this->Member3 = Member3;
    }
    virtual void Init() = 0;
    virtual void Update() = 0;
    virtual ~Component() {}
protected:
    type* Member1;
    type* Member2;
    type* Member3;
};

For handeling the components, there is a manager. It first sets the members of a component and then calls Init() on it. Later on it can be used to update all assigned components but that isn't related to this question.

class Manager
{
public:
    void Add(string Name, Component* Component)
    {
        list[Name] = Component;
    }
    void Init(type* Member1, type* Member2, type* Member3)
    {
        for (auto i = list.begin(); i != list.end(); i++)
        {
            i->second->Set(Member1, Member2, Member3);
            i->second->Init();
        }
    }
    void Update()
    {
        for (auto i = list.begin(); i != list.end(); i++)
            i->second->Update();
    }
private:
    unordered_map<string, Component*> list;
};

I am not really happy with my implementation since I would like components to use their constructor for initialization instead overwriting the Init() function. But at construction time the base class member aren't available yet.

I know that I could pass the members through the derived class constructor, but I do not want specific components to care about their base class' members. That would look like the following but anyway I do not want that.

class Component
{
public:
    Component(type* Member1, type* Member2, type* Member3)
    {
        this->Member1 = Member1;
        this->Member2 = Member2;
        this->Member3 = Member3;
    }
    virtual void Update() = 0;
    virtual ~Component();
protected:
    type* Member1;
    type* Member2;
    type* Member3;
};

class Specific : public Component
{
public:
    Specific(type* Member1, type* Member2, type* Member3) : Component(Member1, Member2, Member3)
    {

    }
    void Update()
    {

    }
};

So how can I insure that the base class members are initialized before calling the constructor of the derived class?

解决方案

You don't really have a choice. Either the derived class knows about the initialisation needs of the base class (because the base class needs that information in its constructor/initialisation function), or you have to move the derive-class' initialisation out of its constructor (to be called by the client after it initialised the base class).

If the list of members that need to be set on the base class is long, you could package them all in a structure and pass that, via the derived class, to the base class.

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