"Cannot convert from 'A' to 'B&'

Question

I'm building an Entity-Component system using template metaprogramming. I keep getting either Cannot convert from [base type] to [type user requested]& or Cannot convert NullComponent to [type user requested]& errors:

class Entity {
public:
    Entity() = default;
    ~Entity() = default;

    template<typename C, typename... Args>
    void AddComponent(Args&&... args);

    template<typename C>
    C& GetComponent();

protected:
private:
    //...add/get helper methods here...

    unsigned int _id;
    std::vector<std::unique_ptr<IComponent>> _components;
};

template<typename C>
C& Entity::GetComponent() {
    for(auto c : _components) {
        if(std::is_base_of<a2de::IComponent&, C&>().value && std::is_same<decltype(c), C&>().value) {
            return *c; //<-- error here
        }
    }
    return NullComponent(); //<-- and here
}

EDIT

These options seem to work for now.

template<typename C>
const C& Entity::GetComponent() const {
    for(auto& uc : _components) {
        auto* c = dynamic_cast<C*>(uc.get());
        if(c && std::is_base_of<a2de::IComponent&, C&>().value && std::is_same<decltype(c), C&>().value) {
            return *c;
        }
    }
    throw std::runtime_error(std::string("Component not available."));
}

OR

class Entity {
public:
    //same as before...
protected:
private:
    //same as before...
    a2de::NullComponent _null_component;
};

template<typename C>
const C& Entity::GetComponent() const {
    for(auto& uc : _components) {
        auto* c = dynamic_cast<C*>(uc.get());
        if(c && std::is_base_of<a2de::IComponent&, C&>().value && std::is_same<decltype(c), C&>().value) {
            return *c;
        }
    }
    return _null_component;
}

Answer 1

At least three things:

• In GetComponent() you iterate over unique_ptr elements and compare their type (always std::unique_ptr<IComponent>) with something else in the std::is_same. You probably don't want that.
• You are returning a reference to a temporary in the final return, it seems.
• return *c needs a dynamic_cast unless C == IComponent.

EDIT

Also:

• std::is_base_of makes no sense with references. Even with class NullComponent : IComponent {};, you would still get std::is_base_of<IComponent&, NullComponent&>::value == false.
• And you do not check for nullptr

In the end, it seems to me that you should replace your for loop with

for(auto& component : _components) {
  auto* c = dynamic_cast<C*>(component.get());
  if (c)
  {
    return *c;
  }
}