在C ++ 11标准中，它禁止'template< typename T> class A {...}; template< typename T> A类< int> {...}; （如果任何地方）？ [英] Where in the C++11 standard does it prohibit 'template <typename T> class A {...}; template <typename T> class A<int> {...};' (if anywhere)?

问题描述

我试图通过想象神秘的结构更完全地抓住模板语法和语义。我相信以下语法是不允许的C ++ 11标准：

I am attempting to more fully grasp template syntax and semantics by imagining arcane constructs. I believe that the following syntax is not allowed by the C++11 standard:

template <typename T>
class A
{...};

// phony "specialization"
template <typename T>
class A<int>
{...};

但是，我不能在C ++ 11标准中找不到此语法。

However, I cannot find in the C++11 standard where this syntax is disallowed.

我是否更正了C ++ 11标准不允许显示的语法？如果是这样，可以在哪里找到不允许的语法？

Am I correct that the syntax shown is disallowed by the C++11 standard? If so, where can it be found that the syntax is disallowed?

推荐答案

我很惊讶，在14.5.5 [temp.class.spec]中说，必须在模板参数列表中使用类模板部分特化的所有模板参数。这将使 template< class T>因为 T 在模板参数列表< int>中未使用，所以类A< int& code>。

I'm quite surprised that there is no explicit statement in 14.5.5 [temp.class.spec] saying that all template parameters of a class template partial specialization must be used in the template-argument-list. That would make template<class T> class A<int> invalid because T is not used in the template-argument-list <int>.

我认为你的虚假专业化只是隐含地无效，因为你永远不能匹配它，所以它永远不能使用。如果你实例化 A< int> 那么匹配主模板。它不能匹配您的专业化，因为它有一个额外的模板参数 T ，无法推导（你建议可以通过说 A< int>< double> 但是这不是有效的C ++语法，所以没有帮助。）

I think your phony specialization is only implicitly invalid due to the fact that you can never match it, so it can never be used. If you instantiate A<int> then that matches the primary template. It can't match your specialization, because that has an additional template parameter, T, which cannot be deduced (you suggest it could be provided by saying A<int><double> but that is not valid C++ syntax, so doesn't help).

标准委员会澄清你的虚假专业化为什么是无效的（显然是，但我不能看到它说的是什么）。

I've asked the standards committee for clarification why your phony specialization is invalid (obviously it is, but I can't see where it says so).

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