新的“使用”解决了什么问题，模板typedefs的语法？ [英] What was the issue solved by the new "using" syntax for template typedefs?

问题描述

在C ++ 11中，您可以通过执行

In C++11 you can create a "type alias" by doing something like

template <typename T>
using stringpair = std::pair<std::string, T>;

但这是一个偏离你期望模板typedef看起来像：

But this is a deviation from what you'd expect a template typedef would look like:

template <typename T>
typedef std::pair<std::string, T> stringpair;

这就引出了一个问题 - 他们为什么需要提出一个新的语法？它是不是用旧的 typedef 语法工作？

So this raises the question - why did they need to come up with a new syntax? what was it that did not work with the old typedef syntax?

我意识到最后一位不编译但是为什么不能编译呢？

I realize the last bit doesn't compile but why can't it be made to compile?

推荐答案

从WG21提案 N1489模板别名 （由Stroustrup和Dos Reis发布）：

From the WG21 proposal N1489 Template aliases (by Stroustrup and Dos Reis):

已建议（重新）使用关键字 typedef b paper [4]引入模板别名：

It has been suggested to (re)use the keyword typedef as done in the paper [4] to introduce template aliases:

 template<class T> 
 typedef std::vector<T, MyAllocator<T> > Vec;

该符号的优点是使用已知的
引入一个类型别名。然而，它还显示几个不好的
，其中混淆使用已知的引入
别名的关键字在上下文中的别名不指定
a类型的类型名称，但是模板; Vec不是类型的别名，不应该为typedef名称采用
。名称Vec是系列的名称
std :: vector< o，MyAllocator< o> > 其中项目符号是类型名称的占位符
。因此，我们不建议使用typedef语法。

That notation has the advantage of using a keyword already known to introduce a type alias. However, it also displays several disavantages among which the confusion of using a keyword known to introduce an alias for a type-name in a context where the alias does not designate a type, but a template; Vec is not an alias for a type, and should not be taken for a typedef-name. The name Vec is a name for the family std::vector<o, MyAllocator<o> > where the bullet is a placeholder for a type-name. Consequently we do not propose the typedef syntax.

另一方面，

template<class T> 
using Vec = std::vector<T, MyAllocator<T> >;

可以读取/解释为：从现在开始，我将使用 Vec< T> 作为
同义词 std :: vector< T，MyAllocator< T& > 。有了这个阅读，
新语法的混叠看起来是合理的。

can be read/interpreted as: from now on, I'll be using Vec<T> as a synonym for std::vector<T, MyAllocator<T> >. With that reading, the new syntax for aliasing seems reasonably logical.

上面提到的论文[4]报价是以前的提案WG21 N1406建议添加到C ++：Typedef模板 （由Herb Sutter提供）。它使用不同的语法（ typedef vs 使用）以及不同的命名法（typedef模板vs模板别名） 。 Herb提出的语法没有实现，但有时可以在非正式讨论中找到命名。

The paper [4] referred to in the above quote was a prior proposal WG21 N1406 Proposed Addition to C++: Typedef Templates (by Herb Sutter). It uses both a different syntax (typedef vs using) as well as a different nomenclature (typedef templates vs template aliases). Herb's proposed syntax didn't make it, but the nomenclature can sometimes be found in informal discussions.

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