Component based architecture c++

Question

I'm having trouble figuring out a way to make a component based engine architecture in c++. But i cant figure out a way to combine a vector of components with a class that derives from component.

I want to override components virtual function. But the only way i can make it call the overrided function is to make the component-derived class a pointer, but i want every gameobject to contain its own components in a vector and not outside the class as a pointer.

I tried to remove as much unnecessary code as possible.

My structure:

//GameObject class, contains components and other objects
class GameObject
{
public:

    GameObject(){}
    ~GameObject(){}

    void AddChild(GameObject child)
    {
       children.push_back(child);
    }
    void AddComponent(Component component)
    {
       components.push_back(component);
    }

    void Input(){}
    void Update(){}
    void Render()
    {
       for(unsigned int i = 0; i < components.size(); i++)
            components[i].Render();
    }

private:

    std::vector<GameObject> children;
    std::vector<Component> components;
};

//base class component
class Component
{
public:
    Component(){}
    ~Component(){}

    virtual void Input(){}
    virtual void Update(){}
    virtual void Render(){ cout << "Component -> Render()" << endl; }

};

class MeshRenderer : public Component
{
public:
    MeshRenderer(Mesh _mesh, Material _material)
    {
        mesh = _mesh;
        material = _material
    }

    ~MeshRenderer(){}

    //override components virtual Render()
    void Render(Transform transform)
    {
        mesh.Render(material);
        cout << "MeshRenderer -> Render()" << endl;
    }

private:
    Mesh mesh;
    Material material;
};

GameObject* root = new GameObject();
MeshRenderer meshRenderer(mesh, material);

root->AddComponent(meshRenderer);

//GameLoop
while(!quit)
{
   root->Render();
}

Maybe use a vector of pointers? Or perhaps even a vector of smart pointers? — Galik, Nov 10 '14 at 19:59
Just wondering. How do you plan to retrieve components with this design? — Veritas, Nov 10 '14 at 20:21

score 2 · Answer 1 · edited May 23 '17 at 11:47

Looks like you want to pass your objects by reference, use

void AddComponent(Component& component);

to avoid any slicing.

For proper usage with std::vector<>'s and polymorphic inheritance, you'll need smart pointers, e.g. std::unique_ptr<Component> to preserve ownership, or std::shared_ptr<Component> for shared ownership (raw pointers as Component* might work as well, but are far harder to manage correctly).

void AddComponent(std::unique_ptr<Component> componentPtr); // Unique ownership

or

void AddComponent(std::shared_ptr<Component> componentPtr); // Shared ownership

and accordingly

std::vector<std::unique_ptr<Component>> components;

or

std::vector<std::shared_ptr<Component>> components;

It depends on your actual use cases if these Component instances should be uniquely owned by their aggregating parent GameObject class, or not.

To use std::shared<> pointers, that could expire outside their usages scope you may consider using std::weak_ptr<>.

As mentioned, it totally depends on your use cases, and how you want these aggregated components being accessible from outside of the GameObject class.

score 2 · Answer 2 · answered Nov 10 '14 at 20:17

It would be the best if you could use unique_ptr:

void AddComponent(std::unique_ptr<Component> component) {
    components.push_back(std::move(component));
}

std::vector<std::unique_ptr<Component>> components;

Thus by calling AddComponent() you transfer ownership of the component to containing GameObject.

Component based architecture c++

2 Answers2