CRTP:为什么获取派生类的嵌套类型和嵌套方法之间存在差异? [英] CRTP: why a difference between getting a nested type and nested method of the derived class?

问题描述

CRTP 模式中的基类可以访问派生类的成员函数，但不能访问派生类中的嵌套类型.

The base class in the CRTP pattern can access the member functions of the derived class, but it can't access a nested type in the derived class.

为什么会有这种差异?

为了说明，请考虑以下代码:

To illustrate, consider the following piece of code:

template<typename Derived>
struct crtp_base
{
    void crtp_method() { return static_cast<Derived&>(*this).method(); } // compiles

    using crtp_type = typename Derived::type; // doesn't compile
};

struct X : public crtp_base<X>
{
    void method() {}

    using type = int;
};

int main()
{

}

crtp_type 导致编译错误，而 crtp_method 编译正常，尽管两者都尝试访问 Derived 类中定义的内容.解释这种差异的 C++ 规范是什么?

crtp_type causes a compilation error, while crtp_method compiles fine, although both attempt to access something defined in the Derived class. What is the C++ specification that explains that difference?

推荐答案

这里的区别在于方法的实例化仅在您实际使用它时发生，而 crtp_base 的实例化发生在 public crtp_base<X> 其中类型 X 仍然不完整.解决方法是使用类型特征:

The difference here is that instantiation of method happens only when you actually use it while instantiation of crtp_base happens at public crtp_base<X> where type X is still incomplete. The workaround would be to use type traits:

template<typename x_Target>
struct Trait;

template<typename Derived>
struct crtp_base
{
    void crtp_method() { return static_cast<Derived&>(*this).method(); }

    using crtp_type = typename Trait<Derived>::type;
};

struct X;

template<>
struct Trait<X>
{
    using type = int;
};

struct X : public crtp_base<X>
{
    void method() {}

    using type = Trait<X>::type;
};

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