CODEWITHSUNDEEP
c++decorator
Domenico Paolicelli
Domenico Paolicelli

Reputation: 97

How Decorator pattern works in c++

I hope this is not a duplicate, and sorry for my not perfect english.

I'm trying to understand how exactly the decorator pattern works in c++.

I'm using this implementation found online and the design is clear to me.

What i can't understand is how technically it's working.

When i'm creating a new object and decorating it, for example:

AbstractNPC *goblin1= new Elite(new Shaman(new NPC("Goblin")));

The type of goblin1 will be Elite, Shaman, NPC or what?

And how the function render() "takes" the implementation of the other classes and use them together?

I would expect that when i call render(), i'm calling recursively the functions from the other decorator classes using this instruction:

NPCDecorator::render();

Still this doesn't make sense to me.

How and why this works?

Upvotes: 0

Views: 374

Answers (1)

Caleth
Caleth

Reputation: 62864

The relevant type of goblin1 is right there in the declaration, AbstractNPC *.

NPCDecorator implements this abstract class by holding on to another instance of that abstract class. In the expression new Elite(new Shaman(new NPC("Goblin"))) there are three AbstractNPC instances, two decorators and a "normal" instance.

When the virtual call render comes into the first decorator, it does it's stuff, then call's its base classes render, which just delegates to the decorated npc member.

You will have a call stack like:

Elite::render
{
    // this == Elite * @ 0x12345678
    cout << "Elite ";
    NPCDecorator::render
    {
        Shaman::render
        {
            // this == Shaman * @ 0x56781234
            cout << "Shaman ";
            NPCDecorator::render
            {
                NPC::render
                {
                    // this == NPC * @ 0x78563412
                    cout << name; // "Goblin"
                }
            }
        }
    }
}

Upvotes: 1

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