问题描述

我知道在 C++11 中我们现在可以使用 using 来编写类型别名，比如 typedefs:

I know that in C++11 we can now use using to write type alias, like typedefs:

typedef int MyInt;

据我所知，相当于:

using MyInt = int;

这种新的语法是为了有一种表达模板类型定义"的方法而产生的:

And that new syntax emerged from the effort to have a way to express "template typedef":

template< class T > using MyType = AnotherType< T, MyAllocatorType >;

但是，对于前两个非模板示例，标准中是否还有其他细微差别?例如，typedefs 在弱"代码中做别名处理.道路.也就是说，它不会创建新类型，而只会创建新名称(这些名称之间的转换是隐式的).

But, with the first two non-template examples, are there any other subtle differences in the standard? For example, typedefs do aliasing in a "weak" way. That is it does not create a new type but only a new name (conversions are implicit between those names).

和using一样还是生成新的类型?有什么不同吗?

Is it the same with using or does it generate a new type? Are there any differences?

推荐答案

_{以下所有标准参考参考 N4659:2017 年 3 月 Kona 后工作草案/C++17 DIS.}

但是，对于前两个非模板示例，标准中还有其他细微差别吗?

But, with the first two non-template examples, are there any other subtle differences in the standard?

• 差异语义:无.
• 差异在允许的上下文中:一些⁽¹⁾.
• Differences in semantics: none.
• Differences in allowed contexts: some⁽¹⁾.

^{(1) 除了别名模板的例子，在原帖中已经提到.}

^{(1) In addition to the examples of alias templates, which has already been mentioned in the original post.}

受 [dcl.typedef]/2 管辖[摘录，强调我的]

As governed by [dcl.typedef]/2 [extract, emphasis mine]

[dcl.typedef]/2typedef-name也可以通过介绍别名声明.using 关键字后面的 identifier 变成了typedef-name 和 identifier 后面的可选 attribute-specifier-seq 属于那个 typedef-name.这样的typedef-name 与由 typedef 说明符引入的语义相同. [...]

[dcl.typedef]/2 A typedef-name can also be introduced by an alias-declaration. The identifier following the using keyword becomes a typedef-name and the optional attribute-specifier-seq following the identifier appertains to that typedef-name. Such a typedef-name has the same semantics as if it were introduced by the typedef specifier. [...]

由别名声明引入的typedef-name与typedef<引入具有相同的语义/code> 声明.

a typedef-name introduced by an alias-declaration has the same semantics as if it were introduced by the typedef declaration.

然而，这并不暗示这两种变体在使用环境方面具有相同的限制.事实上，尽管是极端情况，typedef 声明 是一个 init-statement 因此可以在允许初始化语句的上下文中使用

However, this does not imply that the two variations have the same restrictions with regard to the contexts in which they may be used. And indeed, albeit a corner case, a typedef declaration is an init-statement and may thus be used in contexts which allow initialization statements

// C++11 (C++03) (init. statement in for loop iteration statements).
for(typedef int Foo; Foo{} != 0;) {}

// C++17 (if and switch initialization statements).
if (typedef int Foo; true) { (void)Foo{}; }
//  ^^^^^^^^^^^^^^^ init-statement

switch(typedef int Foo; 0) { case 0: (void)Foo{}; }
//     ^^^^^^^^^^^^^^^ init-statement

// C++20 (range-based for loop initialization statements).
std::vector<int> v{1, 2, 3};
for(typedef int Foo; Foo f : v) { (void)f; }
//  ^^^^^^^^^^^^^^^ init-statement

for(typedef struct { int x; int y;} P;
//  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ init-statement
    auto [x, y] : {P{1, 1}, {1, 2}, {3, 5}}) { (void)x; (void)y; }

而 别名声明不是一个init-statement，因此不能在允许初始化语句的上下文中使用

whereas an alias-declaration is not an init-statement, and thus may not be used in contexts which allows initialization statements

// C++ 11.
for(using Foo = int; Foo{} != 0;) {}
//  ^^^^^^^^^^^^^^^ error: expected expression

// C++17 (initialization expressions in switch and if statements).
if (using Foo = int; true) { (void)Foo{}; }
//  ^^^^^^^^^^^^^^^ error: expected expression

switch(using Foo = int; 0) { case 0: (void)Foo{}; }
//     ^^^^^^^^^^^^^^^ error: expected expression

// C++20 (range-based for loop initialization statements).
std::vector<int> v{1, 2, 3};
for(using Foo = int; Foo f : v) { (void)f; }
//  ^^^^^^^^^^^^^^^ error: expected expression

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