Getting a Specific Component in an Entity Component System

Question

I've got component class:

class Component
{
public:
    Component();
    ...
};

then my various components derive from the base Component class

class VelocityCom : public Component
{
public:
    VelocityCom();
    ...
};

and then an entity class that holds the components and is able to return a specific component for manipulation

class Entity
{
public:
    Entity();
    std::unique_ptr<Component> getComponent(...);
private:
    std::vector<Component> components;
};

The problem that I'm having with this approach lies withing the act of getting a specific component. Let's say I want to get a pointer to the velocity component. I can't just have whateverEntity.getComponent(Types::Velocity); because the method returns a Component and not a VelocityCom. I thought about casting the Component pointer to a VelocityCom pointer but it seems that a lot of people don't like the idea of downcasting. So my question is what is a good way to get a specific component in an entity component system? Is downcasting okay in this situation or should I change my implementation to where I don't have to?

Answer 1

First, returning a unique_ptr makes no sense. In your design (which is not an ECS, btw) a unique_ptr expresses ownership. The entity owns the component. If you return a unique_ptr then the entity must necessarily give up its ownership of the component. You don't want that.

Second, just downcast. "A lot of people" can dislike it all they want, but they're wrong. It's perfectly valid when used in the appropriate circumstances. Just about every engine with a component model similar to yours end up with functions like these on their core game object type:

struct GameObject
{
  // get generic component based on generic type identifier
  Component* Get(std::type_index type);

  // get a specific component based on compile-time type
  template <class T> T* Get() { return static_cast<T*>(Get(typeid(T))); }
};