了解类模板专业化示例的隐式实例化

Understanding an Implicit Instantiation of Class Template Specialization Example

本文关键字：实例化 专业化 了解      更新时间：2023-10-16

，因此标准的这一部分给出了此示例（我的问题是内联）：

template<class T, class U>
struct Outer {
    template<class X, class Y> struct Inner;      // Where are X and Y from? Is this defining a struct?
    template<class Y> struct Inner<T, Y>;         // Is this defining a struct specialization? If so what is Y?
    template<class Y> struct Inner<T, Y> { };     // I feel like this is a redefinition of the line above, could it ever not be?
    template<class Y> struct Inner<U, Y> { };
};

诚然，我无法理解标准的链接部分，因为我无法理解这里发生了什么。我认为我只是对所有template的飞行感到困惑，但是如果有人可以排队并告诉我发生的事情将非常有帮助。

template<class X, class Y> struct Inner;      
// Where are X and Y from? Is this defining a struct?

这是模板的声明结构Inner，X和Y是模板参数。

template<class Y> struct Inner<T, Y>;         
// Is this defining a struct specialization? If so what is Y?

这是部分专业化的声明， Y是模板参数。

template<class Y> struct Inner<T, Y> { };     
// I feel like this is a redefinition of the line above, could it ever not be?

这是部分专业化的定义，因此在这里没有重新定义。

然后将它们用作：

Outer<foo1, foo2>::Inner<foo3, foo4>* i1; 
// the primary template is used, with T=foo1, U=foo2, X=foo3, Y=foo4
// btw the primary template is not defined in this example
Outer<foo1, foo2>::Inner<foo1, foo3> i2; 
// the 1st partial specialization is used, with T=foo1, U=foo2, Y=foo3
Outer<foo1, foo2>::Inner<foo2, foo3> i3; 
// the 2st partial specialization is used, with T=foo1, U=foo2, Y=foo3

您去这里：

template<class T, class U>
struct Outer {
    // Forward-declares a struct with two template parameters
    template<class X, class Y> struct Inner;
    // Forward-declares a partial specialization of Inner
    // (Y is its only template parameter)
    template<class Y> struct Inner<T, Y>;
    // Defines the forward-declared partial specialization
    template<class Y> struct Inner<T, Y> { };
    // Declares and defines another partial specialization
    // (Y is its only template parameter)
    template<class Y> struct Inner<U, Y> { };
};
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