从基类的静态模板方法中调用受保护的ctor继承类失败 [英] Calling protected ctor of inheriting class from within static template method of base class fails

问题描述

我有一个组件类定义一个静态模板方法如何创建一个组件：

I have a component class that defines a static template method of how a Component should be created in general:

class Component {
protected:
    uint32_t id;

    Component(uint32_t id) :
            id(id) {
    }

    template<typename T, uint32_t C>
    static T* createComponent() {
        // content here not relevant
        return new T(someParameter);
    }

};

然后有一个实现，例如 Button 。这个类的构造函数不应该直接使用，而是有一个静态方法调用 Component :: createComponent 模板函数。

Then there is an implementation, for example a Button. The constructor of this class should not be used directly, instead there is a static method that calls the Component::createComponent template function.

class Button: public Component {
protected:
    Button(uint32_t id) :
            Component(id) {
    }
public:
    static Button* create();
};

实现看起来像这样，将类型传递给实例化和创建时使用的常量： p>

The implementation looks like this, passing the type to instantiate and a constant thats used in creation:

Button* Button::create() {
    return createComponent<Button, UI_COMPONENT_BUTTON>();
}

现在的问题是，编译器抱怨 'Button :: Button（uint32_t）'is protected。对我的理解，这个构造函数调用应该是OK，因为 Button extends Component ，但这似乎是一个问题。

Now the problem is, that the compiler complains with "error: 'Button::Button(uint32_t)' is protected". For my understanding, this constructor call should be OK as Button extends Component, but this seems to be a problem here.

我如何解决这个问题？

推荐答案

c $ c> create（）函数将不能处理进一步继承的类，你可以利用这个和不创建一个按钮，而是一个可以访问您的 protected 构造函数的通用派生的，受保护的派生类：

Since your create() function won't be able to deal with further inherited classes you can take advantage of this and not create a Button but, instead, a generic derived, protected, derived class which would have access to your protected constructor:

class Component {
    uint32_t id;
    template <typename T>
    struct Concrete: T {
        Concrete(uint32_t id): T(id) {}
    };
protected:
    Component(uint32_t id) :
        id(id) {
    }

    template<typename T, uint32_t C>
    static T* createComponent() {
        // content here not relevant
        return new Concrete<T>(C);
    }
};

class Button:
    public Component {
protected:
    Button(uint32_t id): Component(id) {}
public:
    static Button* create() {
         return createComponent<Button, UI_COMPONENT_BUTTON>();
    }
};

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