了解类模板专门化示例的隐式实例化 [英] Understanding an Implicit Instantiation of Class Template Specialization Example

问题描述

因此标准的本部分给出了此示例(我的问题是内联的):

So this section of the standard gives this example (My questions are inline):

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，我不明白标准的链接部分，因为我不明白这里发生了什么.我想我对所有template飞来飞去感到困惑，但是如果有人可以一行一行地告诉我正在发生的事情，那将非常有帮助.

Admittedly I can't understand the linked section of the standard cause I can't understand what's happening here. I think I'm just getting confused by all the templates flying around, but If someone could just go line by line and tell me what's happening that would be very helpful.

推荐答案

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

这是模板结构Inner的声明，X和Y是模板参数.

This is the declaration of template struct Inner, X and Y is the template parameters.

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.

This is the declaration of a partial specialization, Y is the template parameter.

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，因此这里没有重新定义.

This is the definition of the partial specialization, so no redefinition here.

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

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