Geek asks and answers

Are C ++ polymorphic classes possible without any functions?

I am trying to create several classes that contain only data elements (no functions), but I would like them to be polymorphic - I mean that I will go around the objects with a pointer to the base class, and I need the ability dynamic_castto a specific derived type ( and have the resulting value be NULLif the instance is not of this type.)

As an example, I have an element:

struct Item {
    int x, y;
}

I also have an element that moves and the other is text:

struct MovingItem: virtual public Item {
    int speedX, speedY;
}

struct TextItem: virtual public Item {
    std::string text;
}

Presumably, I should use the virtual inheritance above, because I also need an element that moves and has text, but I only need one set of coordinates from the top level Item:

struct MovingTextItem: virtual public MovingItem, virtual public TextItem {
}

, dynamic_cast a Item * , , , .

void example(Item *i) {
    MovingTextItem *mti = dynamic_cast<MovingTextItem *>(i);  // error!
}

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enum ItemType {HasSpeed = 1, HasText = 2};
struct Item {
    int type;
    int x, y;

    int speedX, speedY;
    std::string text;
}

Then I just set type = HasSpeed | HasTextif both speed and text fields are working. Maybe a little old school, but it's a lot easier!

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