• 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;
    
    }
  • 相关阅读:
    求树的某一层的节点最多
    快排 PAT 1101
    PAT 1098
    PAt 1099
    多项式求和
    pat 1100
    getline 函数
    数字的标准写法
    线性回归
    HTTP 中状态码 302的使用场景
  • 原文地址:https://www.cnblogs.com/5iedu/p/12709191.html
Copyright © 2020-2023  润新知