使用基类指针访问派生类成员

Question

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?

Answer 1

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 (ie 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.

Answer 2

I would consider doing something like this:

public:
void setId(int id) 
{

    Id = id;

}

void virtual setName( string name ) = 0; // Virtual Function 
string virtual getName() = 0; // Virtual Function

int getId() { return Id; }

protected:
    int Id;

};

class entPlayer : public Entity {

    string Name;

public:
    entPlayer() {

        Name = "";
        Id = 0;

    }

    void entPlayer::setName(string name) {  // Must define function

        Name = name;
}

string entPlayer::getName() { return Name; } // again must define function here

};

Answer 3

C++ makes what you are trying to do really awkward, because this is probably not what you should be doing, and it is trying to lead you to good object-oriented design. In fact, by default, compilers often disable run-time type information (RTTI), which is needed to make dynamic_cast work.

Without knowing your broader context, it's hard to say what you should do instead. What I can say is that if you wanted a more specific pointer, you should have put a ring on it you almost certainly shouldn't use a function that returns an Entity* , and there is probably a better approach.

Answer 4

You can do a dynamic cast:

entPlayer * pPlayer = dynamic_cast<entPlayer *>(pointer_to_base);

This will (if successful) result in a derived pointer.

Otherwise NULL is returned.