如何从ECS中的模板方法获取组件？

Question

我正在开发一个实体组件系统游戏引擎，但在从实体中检索组件时遇到了麻烦。

这是我对实体的实现：

class Entity {
public:
    Entity(int l_id);

// ...

template <typename T>
std::shared_ptr<T> GetComponent() { // T = CRender, CInput, etc
    for (auto& component : m_components) {
        if (typeid(component.second.get()) == typeid(T*)) {
            return std::dynamic_pointer_cast<T>(component.second);
        }
    }
    return nullptr;
}

private:
    int m_id;
    std::string m_name;
    std::unordered_map<ComponentType, std::shared_ptr<Component>> m_components;

ComponentType 是包含在组件 class 中的枚举 class。实体指向的组件是静态类型的Component，动态类型是CRender、CInput等。也就是说，我有一堆组件作为组件 class 的子组件。此外，我在不同的 class 中为每个组件保留了一些 smart_pointers。代码的思路是检查map中的每一个组件，检查智能指针指向的指针类型

GetComponent() 方法是我正在努力工作的方法。这里的 objective 是为了能够通过执行以下操作来检索任何组件（渲染组件的 CRender 等）：

std::shared_ptr renderComponent = entity.GetComponent<CRender>();

很难弄清楚如何去做，因为我将组件存储在地图中，因此每种类型只能有 1 个组件。

我做错了什么？我也欢迎您提出任何更好的实施设计想法。提前致谢！

Answer 1

This seems to be the problem here:

if (typeid(component.second.get()) == typeid(T*))

The main issue is that typeid when used with pointers will not get you the derived type like you are expecting. It’s going to give you the typeid of “pointer-to-Component”, so you will always be evaluating typeid(pointer-to-Component) == typeid(pointer-to-DerivedComponent).

As for improvements, in your code you are not utilizing the lookup by key functionality of std::map. Instead of looping through the map, you could change your GetComponent function to a non template function that takes the enum value as a parameter and use that to perform the map lookup.

Since you originally wrote a template function, as an alternative, you might benefit from changing the key type of your map to std::type_index. This would allow you to do:

std::map<std::type_index, std::shared_ptr<Component>> components;

template <typename T>
std::shared_ptr<T> GetComponent() {
    return std::dynamic_pointer_cast<T>(components[typeid(T)]);
}

And you should include logic to handle the case where the component is not found, like you do in your original code.

如何从ECS中的模板方法获取组件？

How to get component from template method in ECS?

c++

templates

components

shared-ptr

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