C++ CRTP 类层次结构

Question

来自 Wikipedia :

// The Curiously Recurring Template Pattern (CRTP)
template <typename T>
struct base
{
    // ...
};
struct derived : base<derived>
{
    // ...
};

现在如果我想要derived_from_derived，我可以写:

// The Curiously Recurring Template Pattern (CRTP)
template <typename T>
struct base
{
    // ...
};
template <typename T>
struct derived : base<T>
{
    // ...
};
struct derived_from_derived : derived <derived_from_derived>
{
    // ...
};

现在假设我只想要一个derived 对象。这不起作用:

derived<derived> obj;

derived 是否必须是抽象的，或者有没有办法实例化它？

Answer 1

我自己的答案是这样的:

struct base
{
    template <typename T>
    struct type
    {
        // ...
    };
};
struct derived
{
    template <typename T=derived>
    struct type : base::type<T>
    {
        // ...
    };
}
struct derived_from_derived 
{
    template <typename T=derived_from_derived >
    struct type : derived::type<T>
    {
        // ...
    };
};

现在我可以拥有一个derived::type<> obj .此外，参数化继承有效(例如装饰器模式):

template <typename whatever>
struct derived_from_whatever 
{
    template <typename T=derived_from_whatever>
    struct type : whatever::type<T>
    {
        // ...
    };
};

derived_from_whatever<derived_from_derived>::type<> obj_whatever;