使用基类指针访问派生类成员 [英] Accessing derived class members with a base class pointer

问题描述

我在C ++中创建一个简单的控制台游戏

我想知道如果我可以从'entPlayer'类访问成员，到基类（'Entity'）：

  class Entity {
 public：
 void setId int id）{Id = id; } 
 int getId（）{return Id; } 
 protected：
 int Id; 
}; 
 
 class entPlayer：public Entity {
 string Name; 
 public：
 void setName（string name）{Name = name; } 
 string getName（）{return Name; } 
}; 
 
 Entity * createEntity（string Type）{
 Entity * Ent = NULL; 
 if（Type ==player）{
 Ent = new entPlayer; 
} 
 return Ent; 
} 
 
 void main（）{
 Entity * ply = createEntity（player）; 
 ply-> setName（Test）; 
 ply-> setId（1）; 
 
 cout<< ply-> getName（）<< endl 
 cout<< ply-> getId（）<< endl 
 
 delete ply; 
} 
  

如何调用ply-> setName等？ >

或

如果这样不可能，更好的方法是什么？

解决方案

可以使用转换。如果你知道一个事实，基类指针指向一个派生类的对象，你可以使用 static_cast ：

  Entity * e = / *指向entPlayer对象的指针* /; 
 entPlayer * p = static_cast< entPlayer *>（e）; 
 p-> setName（Test）; 
  

如果你不确定，那么你需要使用 dynamic_cast 并测试结果，以确定它不为null。请注意，如果基类至少有一个虚函数，则只能使用 dynamic_cast 。例如：

 实体* e = / *指向某个实体的指针* /; 
 entPlayer * p = dynamic_cast< entPlayer *>（e）; 
 if（p）
 {
 p-> setName（Test）; 
} 
  

也就是说，使用多态

说到虚函数，你的类层次结构作为实现具有未定义的行为：你只能通过指向一个派生类型的指针来删除一个派生类型的对象。其基类如果基类为虚拟析构函数。因此，您需要向基类添加一个虚拟析构函数。

I am making a simple console game in C++

I would like to know if I can access members from the 'entPlayer' class while using a pointer that is pointing to the base class ( 'Entity' ):

class Entity {
public:
    void setId(int id) { Id = id; }
    int getId() { return Id; }
protected:
    int Id;
};

class entPlayer : public Entity {
    string Name;
public:
    void setName(string name) { Name = name; }
    string getName() { return Name; }
};

Entity *createEntity(string Type) {
    Entity *Ent = NULL;
    if (Type == "player") {
        Ent = new entPlayer;
    }
    return Ent;
}

void main() {
    Entity *ply = createEntity("player");
    ply->setName("Test");
    ply->setId(1);

    cout << ply->getName() << endl;
    cout << ply->getId() << endl;

    delete ply;
}

How would I be able to call ply->setName etc?

OR

If it's not possible that way, what would be a better way?

解决方案

It is possible by using a cast. If you know for a fact that the base class pointer points to an object of the derived class, you can use static_cast:

Entity* e = /* a pointer to an entPlayer object */;
entPlayer* p = static_cast<entPlayer*>(e);
p->setName("Test");

If you don't know for sure, then you need to use dynamic_cast and test the result to see that it is not null. Note that you can only use dynamic_cast if the base class has at least one virtual function. An example:

Entity* e = /* a pointer to some entity */;
entPlayer* p = dynamic_cast<entPlayer*>(e);
if (p)
{
    p->setName("Test");
}

That said, it would be far better to encapsulate your class's functionality using polymorphism (i.e. virtual functions).

Speaking of virtual functions, your class hierarchy as implement has undefined behavior: you can only delete an object of a derived type through a pointer to one of its base classes if the base class as a virtual destructor. So, you need to add a virtual destructor to the base class.

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