class 模板，在定义中引用自己的类型

Question

我想知道 visual studio 2017 和 GCC 中关于标准的以下情况哪个是正确的。问题是在 class 模板第二个中，visual studio 中的标识符 'second' 总是引用具体类型，但在 gcc 中它似乎是上下文相关的。

GCC 示例

template <typename...>
struct type_list{};

template<template <typename...> typename tmpl> 
struct tmpl_c
{
    template <typename...Ts> using type = tmpl<Ts...>;
};

template<typename> struct template_of;
template <template <typename...> typename tmpl, typename... Ts>
struct template_of<tmpl<Ts...>>{
    using type = tmpl_c<tmpl>; 

};


template <typename T>
struct first{};


template <typename T>
struct second
{
    // 'second' here refers to second<int>
    using test = second; 

    // 'second' here refers to the template second
    // is this due to the context? ie that the tmpl_c is expecting a template not a concrete type?
    using types = type_list<tmpl_c<first>, tmpl_c<second> >; 


    // second here refers to the concrete type 'second<int>'
    // this workaround is needed for visual studio as it seems second always 
    // refers to the concrete type, not the template.
    using types2 = type_list<tmpl_c<first>, typename template_of<second>::type >; 
};

Demo

和 Visual Studio 示例（只有不同的位）

template <typename T>
struct second
{
    // 'second' here refers to second<int>
    using test = second; 

    // 'second' here refers to second<int>
    // this doesn't compile in visual studio as second<int> not the template.
    //using types = type_list<tmpl_c<first>, tmpl_c<second> >; 


    // second here refers to the concrete type 'second<int>'
    // this workaround is needed for visual studio as it seems second always 
    // refers to the concrete type, not the template.
    using types2 = type_list<tmpl_c<first>, typename template_of<second>::type >; 
};

Demo

由于 template_of 解决方法似乎在两者中都能正常运行，它目前是我唯一的选择..但我仍然想知道哪个是正确的，或者是否有其他解决方法..

fixed in visual studio 15.8 Preview 2

Answer 1

Gcc 是正确的。根据injected-class-name for class template的使用规则，injected-class-name也可以用作template-name或type-name。

（强调我的）

Like other classes, class templates have an injected-class-name. The injected-class-name can be used as a template-name or a type-name.

In the following cases, the injected-class-name is treated as a template-name of the class template itself:

it is followed by <

it is used as a template argument that corresponds to a template template parameter

it is the final identifier in the elaborated class specifier of a friend class template declaration.

Otherwise, it is treated as a type-name, and is equivalent to the template-name followed by the template-parameters of the class template enclosed in <>.

也就是说，

template <typename T>
struct second
{
    
    // second is same as second<T>
    using test = second; 

    // second is considered as a template-name
    // it's used as template argument for a template template parameter
    using types = type_list<tmpl_c<first>, tmpl_c<second> >; 

    // second is same as second<T>
    using types2 = type_list<tmpl_c<first>, typename template_of<second>::type >; 
};

class 模板，在定义中引用自己的类型

class template, referring to its own type in definition

c++

templates

metaprogramming

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