可以联合模板吗？ [英] Can union be templated?

问题描述

似乎联合可以在c ++ 11中模板化，它们用于例如 std :: optional的引用实现。

在c ++ 11之前是否可能？

解决方案

是的，似乎这是一直允许的。联合是一个类，模板是一个函数或类模板。

标准的相关部分：

• [temp]

  
  

  <...>

  
  / blockquote>




• [class]

  
  

  >是用类键定义的类

  





（所以人们可能会认为新类型特征 std :: is_class 是一个小小的错误; traits应该分割类型的空间，因此 is_union 是一个单独的，互相排斥的特质。）

It seems unions can be templated in c++11, they are used for example in the reference implementation of std::optional.

Was that possible before c++11 ?

解决方案

Yes, it seems that this has always been allowed. A union is a class, and a template is either a function or a class template.

Relevant parts of the standards:

• [temp]

  The declaration in a template-declaration shall

  — declare or define a function or a class, [...]

• [class]

  A union is a class defined with the class-key union

(So one might argue that the new type trait std::is_class is a slight misnomer; the traits are supposed to partition the space of types, and so is_union is a separate, mutually exclusive trait.)

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