C ++：无法将参数1从子非模板类转换为父模板类

Question

I know there is a lot of these "cannot convert argument" questions here, but I promise I looked around a lot and I think my particular case hasn't been asked yet, plus I have tried debugging this for quite a few hours but can't seem to get it.

Basically, I am creating a Component system for a 2D game engine. I have separated my logic into two parts: Component and ComponentSystem . The Component class will store only data, while the ComponentSystem will update and manipulate that data accordingly.

The ComponentSystem base class is abstract with a template to specify which Component the system will be using. Here is the .h file:

// =================================================================================================
// == COMPONENT SYSTEM CLASS =======================================================================
// =================================================================================================
template <class T>
class ComponentSystem
{
public:
    // -- ctor & dtor
    ComponentSystem<T>();
    ~ComponentSystem<T>();

    // -- update components
    virtual void UpdateComponents(float dt) = 0;
    // -- populates the components of this systems
    virtual void PopulateFromCurrentLevel() = 0;
    // -- clears the components of this system
    virtual void ClearFromCurrentLevel() = 0;

protected:
    // -- actual list of components
    std::vector<T*> components;
};

I then also have a SpriteComponentSystem , which derives from ComponentSystem , but with the SpriteComponent as the template type:

class SpriteComponentSystem : public ComponentSystem<SpriteComponent>
{
public:
    SpriteComponentSystem();
    ~SpriteComponentSystem();
    virtual void UpdateComponents(float dt);
    virtual void PopulateFromCurrentLevel();
    virtual void ClearFromCurrentLevel();
};

Finally, in my Game class I am storing a vector of the base class, like so:

std::vector<ComponentSystem<Component*>*> systems;

However, when I try to push a SpriteComponentSystem* into systems , I get the following error:

C2664 - 'void std::vector<HSZGame::ComponentSystem<HSZGame::Component *> 
*,std::allocator<_Ty> >::push_back(_Ty &&)': cannot convert argument 1 from 
'HSZGame::SpriteComponentSystem *' to 
'HSZGame::ComponentSystem<HSZGame::Component *> *const &'  

I have tried implementing a specific cast from one object to the other, and also tried doing a dynamic_cast which worked for compile time but the object was then nullptr at runtime.

In case anyone is wondering, SpriteComponent does indeed inherit from Component , which is the base class. Thanks again for your help everyone!

Answer 1

First of all you are initializing a std::vector<ComponentSystem<Component*>*> that takes a ComponentSystem with a Component* template parameter. This means that your vector components that is contained within ComponentSystem is holding a pointer to a pointer to a Component . This might be an error on your part unless you actually mean to hold a pointer to a pointer. If you don't your vector should be initialized std::vector<ComponentSystem<Component>*> .

Secondly it seems you would like to use runtime polymorphism so that your vector systems can hold not only ComponentSystem<Component> objects but also SpriteComponentSystem (AKA ComponentSystem<SpriteComponent> objects).

Unfortunately the language does not allow this since templates are instantiated at compile time and each instantiation is its own type. The language views ComponentSystem<Component> and ComponentSystem<SpriteComponent> as separate types and hence the compiler is complaining.

I think you would need to rethink this design philosophy.