第2章 类模板：2.8 类型别名

2.8 Type Aliases

2.8 类型别名

You can make using a class template more convenient by defining a new name for the whole type.

通过为整个类型定义一个新的名字，可以让类模板的使用更加方便。

Typedefs and Alias Declarations

Typedefs和别名声明(using)

To simply define a new name for a complete type, there are two ways to do it:

要简单地为完整类型定义一个新名称，有两种方法：

1. By using the keyword typedef:

1. 使用typedef关键字

typedef Stack
typedef Stack<int> IntStack; // typedef
void foo (IntStack const& s); // s is stack of ints
IntStack istack[10]; // istack is array of 10 stacks of ints

We call this declaration a typedef and the resulting name is called a typedef-name.

我们将此声明称为typedef，并将得到的名称叫做”typedef名称”。

2. By using the keyword using (since C++11):

2.使用关键字using（从C++11起）

using IntStack = Stack <int>; // 别名声明
void foo (IntStack const& s); // s is stack of ints
IntStack istack[10]; // istack is array of 10 stacks of ints

Introduced by [DosReisMarcusAliasTemplates], this is called an alias declaration.

这是由[DosReisMarcusAliasTemplates]引入，称为“别名声明”。

Note that in both cases we define a new name for an existing type rather than a new type.

注意，在以上两种情况下，我们都是为己经存在的类型定义一个新的名字，而不是定义一个新的类型。

Thus, after the typedef

因此，经过typedef之后

typedef Stack <int> IntStack;

or

或者

using IntStack = Stack <int>;

IntStack and Stack<int> are two interchangeable notations for the same type.

IntStack和Stack<int>是同一类型的两个可互换的符号。

As a common term for both alternatives to define a new name for an existing type, we use the term type alias declaration. The new name is a type alias then.

作为两个替代方案的通用术语，为现有类型定义新名称，我们称之为“类型别名声明”。

Because it is more readable (always having the defined type name on the left side of the =), for the remainder of this book, we prefer the alias declaration syntax when declaring an type alias.

因为它更具可读性（经常将定义的类型名称放在“=”等号左边）。对于本书的其余部分，当声明一个类型别名时，我们更喜欢使用“别名声明”语法。

Alias Templates

别名模板

Unlike a typedef, an alias declaration can be templated to provide a convenient name for a family of types. This is also available since C++11 and is called an alias template.

与typedef不同，别名声明(using)可以被模板化，以便为类型族提供方便的名称。从C++11开始提供该功能，我们称之为“别名模板”。

The following alias template DequeStack, parameterized over the element type T, expands to a Stack that stores its elements in a std::deque:

下面的DequeStack别名模板(在元素类型T上参数化)，扩展到一个栈。该栈将其元素存储在std::queue中：

template<typename T>
using DequeStack = Stack<T, std::deque<T>>;

Thus, both class templates and alias templates can be used as a parameterized type.

因此，类模板和别名模板都可以作为一个将被参数化的类型。

But again, an alias template simply gives a new name to an existing type, which can still be used. Both DequeStack<int> and Stack<int, std::deque<int>> represent the same type.

但是同样，别名模板只是简单地为一个己经存在的类型起个新名字。该名字仍可以使用。DequeStack<int>和Stack<int, std::deque<int>>表示相同的类型。

Note again that, in general, templates can only be declared and defined in global/namespace scope or inside class declarations.

再次注意，通常，模板只能在全局/名称空间作域或者类内部声明。

Alias Templates for Member Types

成员类型的别名模板

Alias templates are especially helpful to define shortcuts for types that are members of class templates. After

别名模板对于定义类函数成员的快捷定义特别有用。之后：

struct C {
typedef … iterator;
    …
};

or:

或者

struct MyType {
using iterator = …;
    …
};

a definition such as

如下的定义：

template<typename T>
using MyTypeIterator = typename MyType<T>::iterator;

allows the use of

允许使用

MyTypeIterator< int> pos;

instead of the following:

来代替下面的：

typename MyType<T>::iterator pos;

Type Traits Suffix_t

类型萃取后缀_t

Since C++14, the standard library uses this technique to define shortcuts for all type traits in the standard library that yield a type. For example, to be able to write

从C++14开始，标准库使用一项快捷定义所有类型萃取的技术来产生一个类型。例如，可以用

std::add_const_t<T> // 从C++14开始

instead of

来替换

typename std::add_const<T>::type // 从 C++11开始

the standard library defines:

标准库定义：

namespace std {

    template<typename T>
    using add_const_t = typename add_const<T>::type;

}