2022-06-30 10:23:03 +08:00
<!DOCTYPE HTML>
< html lang = "zh" class = "sidebar-visible no-js light" >
< head >
<!-- Book generated using mdBook -->
< meta charset = "UTF-8" >
2022-07-02 13:21:48 +08:00
< title > Item 10:优先考虑限域枚举而非未限域枚举 - Effective Modern C++< / title >
2022-06-30 10:23:03 +08:00
<!-- Custom HTML head -->
< meta content = "text/html; charset=utf-8" http-equiv = "Content-Type" >
< meta name = "description" content = "" >
< meta name = "viewport" content = "width=device-width, initial-scale=1" >
< meta name = "theme-color" content = "#ffffff" / >
< link rel = "icon" href = "../favicon.svg" >
< link rel = "shortcut icon" href = "../favicon.png" >
< link rel = "stylesheet" href = "../css/variables.css" >
< link rel = "stylesheet" href = "../css/general.css" >
< link rel = "stylesheet" href = "../css/chrome.css" >
< link rel = "stylesheet" href = "../css/print.css" media = "print" >
<!-- Fonts -->
< link rel = "stylesheet" href = "../FontAwesome/css/font-awesome.css" >
< link rel = "stylesheet" href = "../fonts/fonts.css" >
<!-- Highlight.js Stylesheets -->
< link rel = "stylesheet" href = "../highlight.css" >
< link rel = "stylesheet" href = "../tomorrow-night.css" >
< link rel = "stylesheet" href = "../ayu-highlight.css" >
<!-- Custom theme stylesheets -->
<!-- MathJax -->
< script async type = "text/javascript" src = "https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.1/MathJax.js?config=TeX-AMS-MML_HTMLorMML" > < / script >
< / head >
< body >
<!-- Provide site root to javascript -->
< script type = "text/javascript" >
var path_to_root = "../";
var default_theme = window.matchMedia("(prefers-color-scheme: dark)").matches ? "navy" : "light";
< / script >
<!-- Work around some values being stored in localStorage wrapped in quotes -->
< script type = "text/javascript" >
try {
var theme = localStorage.getItem('mdbook-theme');
var sidebar = localStorage.getItem('mdbook-sidebar');
if (theme.startsWith('"') & & theme.endsWith('"')) {
localStorage.setItem('mdbook-theme', theme.slice(1, theme.length - 1));
}
if (sidebar.startsWith('"') & & sidebar.endsWith('"')) {
localStorage.setItem('mdbook-sidebar', sidebar.slice(1, sidebar.length - 1));
}
} catch (e) { }
< / script >
<!-- Set the theme before any content is loaded, prevents flash -->
< script type = "text/javascript" >
var theme;
try { theme = localStorage.getItem('mdbook-theme'); } catch(e) { }
if (theme === null || theme === undefined) { theme = default_theme; }
var html = document.querySelector('html');
html.classList.remove('no-js')
html.classList.remove('light')
html.classList.add(theme);
html.classList.add('js');
< / script >
<!-- Hide / unhide sidebar before it is displayed -->
< script type = "text/javascript" >
var html = document.querySelector('html');
var sidebar = 'hidden';
if (document.body.clientWidth >= 1080) {
try { sidebar = localStorage.getItem('mdbook-sidebar'); } catch(e) { }
sidebar = sidebar || 'visible';
}
html.classList.remove('sidebar-visible');
html.classList.add("sidebar-" + sidebar);
< / script >
< nav id = "sidebar" class = "sidebar" aria-label = "Table of contents" >
< div class = "sidebar-scrollbox" >
2022-11-18 22:12:20 +08:00
< ol class = "chapter" > < li class = "chapter-item expanded " > < a href = "../Introduction.html" > 简介< / a > < / li > < li class = "chapter-item expanded " > < div > 第一章 类型推导< / div > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../1.DeducingTypes/item1.html" > Item 1:理解模板类型推导< / a > < / li > < li class = "chapter-item expanded " > < a href = "../1.DeducingTypes/item2.html" > Item 2:理解auto类型推导< / a > < / li > < li class = "chapter-item expanded " > < a href = "../1.DeducingTypes/item3.html" > Item 3:理解decltype< / a > < / li > < li class = "chapter-item expanded " > < a href = "../1.DeducingTypes/item4.html" > Item 4:学会查看类型推导结果< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < div > 第二章 auto< / div > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../2.Auto/item5.html" > Item 5:优先考虑auto而非显式类型声明< / a > < / li > < li class = "chapter-item expanded " > < a href = "../2.Auto/item6.html" > Item 6:auto推导若非己愿， < / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < div > 第三章 移步现代C++< / div > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item7.html" > Item 7:区别使用()和{}创建对象< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item8.html" > Item 8:优先考虑nullptr而非0和NULL< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item9.html" > Item 9:优先考虑别名声明而非typedefs< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item10.html" class = "active" > Item 10:优先考虑限域枚举而非未限域枚举< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item11.html" > Item 11:优先考虑使用deleted函数而非使用未定义的私有声明< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item12.html" > Item 12:使用override声明重载函数< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item13.html" > Item 13:优先考虑const_iterator而非iterator< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item14.html" > Item 14:如果函数不抛出异常请使用noexcept< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item15.html" > Item 15:尽可能的使用constexpr< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item16.html" > Item 16:让const成员函数线程安全< / a > < / li > < li class = "chapter-item expanded " > < a href = "../3.MovingToModernCpp/item17.html" > Item 17:理解特殊成员函数函数的生成< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < div > 第四章 智能指针< / div > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../4.SmartPointers/item18.html" > Item 18:对于独占资源使用std::unique_ptr< / a > < / li > < li class = "chapter-item expanded " > < a href = "../4.SmartPointers/item19.html" > Item 19:对于共享资源使用std::shared_ptr< / a > < / li > < li class = "chapter-item expanded " > < a href = "../4.SmartPointers/item20.html" > Item 20:当std::shared_ptr可能悬空时使用std::weak_ptr< / a > < / li > < li class = "chapter-item expanded " > < a href = "../4.SmartPointers/item21.html" > Item 21:优先考虑使用std::make_unique和std::make_shared而非new< / a > < / li > < li class = "chapter-item expanded " > < a href = "../4.SmartPointers/item22.html" > Item 22:当使用Pimpl惯用法， < / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < div > 第五章 右值引用，移动语义，完美转发< / div > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../5.RRefMovSemPerfForw/item23.html" > Item 23:理解std::move和std::forward< / a > < / li > < li class = "chapter-item expanded " > < a href = "../5.RRefMovSemPerfForw/item24.html" > Item 24:区别通用引用和右值引用< / a > < / li > < li class = "chapter-item expanded " > < a href = "../5.RRefMovSemPerfForw/item25.html" > Item 25:对于右值引用使用std::move， < / a > < / li > < li class = "chapter-item expanded " > < a href = "../5
2022-06-30 10:23:03 +08:00
< / div >
< div id = "sidebar-resize-handle" class = "sidebar-resize-handle" > < / div >
< / nav >
< div id = "page-wrapper" class = "page-wrapper" >
< div class = "page" >
< div id = "menu-bar-hover-placeholder" > < / div >
< div id = "menu-bar" class = "menu-bar sticky bordered" >
< div class = "left-buttons" >
< button id = "sidebar-toggle" class = "icon-button" type = "button" title = "Toggle Table of Contents" aria-label = "Toggle Table of Contents" aria-controls = "sidebar" >
< i class = "fa fa-bars" > < / i >
< / button >
< button id = "theme-toggle" class = "icon-button" type = "button" title = "Change theme" aria-label = "Change theme" aria-haspopup = "true" aria-expanded = "false" aria-controls = "theme-list" >
< i class = "fa fa-paint-brush" > < / i >
< / button >
< ul id = "theme-list" class = "theme-popup" aria-label = "Themes" role = "menu" >
< li role = "none" > < button role = "menuitem" class = "theme" id = "light" > Light (default)< / button > < / li >
< li role = "none" > < button role = "menuitem" class = "theme" id = "rust" > Rust< / button > < / li >
< li role = "none" > < button role = "menuitem" class = "theme" id = "coal" > Coal< / button > < / li >
< li role = "none" > < button role = "menuitem" class = "theme" id = "navy" > Navy< / button > < / li >
< li role = "none" > < button role = "menuitem" class = "theme" id = "ayu" > Ayu< / button > < / li >
< / ul >
< button id = "search-toggle" class = "icon-button" type = "button" title = "Search. (Shortkey: s)" aria-label = "Toggle Searchbar" aria-expanded = "false" aria-keyshortcuts = "S" aria-controls = "searchbar" >
< i class = "fa fa-search" > < / i >
< / button >
< / div >
< h1 class = "menu-title" > Effective Modern C++< / h1 >
< div class = "right-buttons" >
< a href = "../print.html" title = "Print this book" aria-label = "Print this book" >
< i id = "print-button" class = "fa fa-print" > < / i >
< / a >
2022-09-08 16:47:52 +08:00
< a href = "https://github.com/CnTransGroup/EffectiveModernCppChinese" title = "Git repository" aria-label = "Git repository" >
< i id = "git-repository-button" class = "fa fa-github" > < / i >
< / a >
2022-06-30 10:23:03 +08:00
< / div >
< / div >
< div id = "search-wrapper" class = "hidden" >
< form id = "searchbar-outer" class = "searchbar-outer" >
< input type = "search" id = "searchbar" name = "searchbar" placeholder = "Search this book ..." aria-controls = "searchresults-outer" aria-describedby = "searchresults-header" >
< / form >
< div id = "searchresults-outer" class = "searchresults-outer hidden" >
< div id = "searchresults-header" class = "searchresults-header" > < / div >
< ul id = "searchresults" >
< / ul >
< / div >
< / div >
<!-- Apply ARIA attributes after the sidebar and the sidebar toggle button are added to the DOM -->
< script type = "text/javascript" >
document.getElementById('sidebar-toggle').setAttribute('aria-expanded', sidebar === 'visible');
document.getElementById('sidebar').setAttribute('aria-hidden', sidebar !== 'visible');
Array.from(document.querySelectorAll('#sidebar a')).forEach(function(link) {
link.setAttribute('tabIndex', sidebar === 'visible' ? 0 : -1);
});
< / script >
< div id = "content" class = "content" >
< main >
< h2 id = "条款十优先考虑限域enum而非未限域enum" > < a class = "header" href = "#条款十优先考虑限域enum而非未限域enum" > 条款十：优先考虑限域< code > enum< / code > 而非未限域< code > enum< / code > < / a > < / h2 >
< p > < strong > Item 10: Prefer scoped < code > enum< / code > s to unscoped < code > enum< / code > s< / strong > < / p >
< p > 通常来说， < code > enum< / code > 中声明的枚举名（译注：< em > enumerator< / em > ，连同下文“枚举名”都指< em > enumerator< / em > ）是不成立的。这些枚举名的名字（译注：< em > enumerator< / em > names， ） < code > enum< / code > 的作用域，这意味着作用域内不能含有相同名字的其他东西：< / p >
< pre > < code class = "language-cpp" > enum Color { black, white, red }; //black, white, red在
//Color所在的作用域
auto white = false; //错误! white早已在这个作用
//域中声明
< / code > < / pre >
< p > 这些枚举名的名字泄漏进它们所被定义的< code > enum< / code > 在的那个作用域，这个事实有一个官方的术语：未限域枚举(< em > unscoped < code > enum< / code > < / em > )。在C++11中它们有一个相似物， < em > scoped < code > enum< / code > < / em > )，它不会导致枚举名泄漏：< / p >
< pre > < code class = "language-cpp" > enum class Color { black, white, red }; //black, white, red
//限制在Color域内
auto white = false; //没问题，
Color c = white; //错误，
Color c = Color::white; //没问题
auto c = Color::white; //也没问题（ ）
< / code > < / pre >
< p > 因为限域< code > enum< / code > 是通过“< code > enum class< / code > ”声明，所以它们有时候也被称为枚举类(< em > < code > enum< / code > classes< / em > )。< / p >
< p > 使用限域< code > enum< / code > 来减少命名空间污染，这是一个足够合理使用它而不是它的同胞未限域< code > enum< / code > 的理由，其实限域< code > enum< / code > 还有第二个吸引人的优点：在它的作用域中，枚举名是强类型。未限域< code > enum< / code > 中的枚举名会隐式转换为整型（现在，也可以转换为浮点类型）。因此下面这种歪曲语义的做法也是完全有效的：< / p >
< pre > < code class = "language-cpp" > enum Color { black, white, red }; //未限域enum
std::vector< std::size_t> //func返回x的质因子
primeFactors(std::size_t x);
Color c = red;
…
if (c < 14.5) { // Color与double比较 (!)
auto factors = // 计算一个Color的质因子(!)
primeFactors(c);
…
}
< / code > < / pre >
< p > 在< code > enum< / code > 后面写一个< code > class< / code > 就可以将非限域< code > enum< / code > 转换为限域< code > enum< / code > ，接下来就是完全不同的故事展开了。现在不存在任何隐式转换可以将限域< code > enum< / code > 中的枚举名转化为任何其他类型：< / p >
< pre > < code class = "language-cpp" > enum class Color { black, white, red }; //Color现在是限域enum
Color c = Color::red; //和之前一样，只是
... //多了一个域修饰符
if (c < 14.5) { //错误！不能比较
//Color和double
auto factors = //错误！
primeFactors(c); //的函数传递Color参数
…
}
< / code > < / pre >
< p > 如果你真的很想执行< code > Color< / code > 到其他类型的转换，和平常一样，使用正确的类型转换运算符扭曲类型系统：< / p >
< pre > < code class = "language-cpp" > if (static_cast< double> (c) < 14.5) { //奇怪的代码，
//但是有效
auto factors = //有问题，但是
primeFactors(static_cast< std::size_t> (c)); //能通过编译
…
}
< / code > < / pre >
< p > 似乎比起非限域< code > enum< / code > 而言，限域< code > enum< / code > 有第三个好处，因为限域< code > enum< / code > 可以被前置声明。也就是说，它们可以不指定枚举名直接声明：< / p >
< pre > < code class = "language-cpp" > enum Color; //错误！
enum class Color; //没问题
< / code > < / pre >
< p > 其实这是一个误导。在C++11中， < code > enum< / code > 也可以被前置声明， ： < code > enum< / code > 都有一个由编译器决定的整型的底层类型。对于非限域< code > enum< / code > 比如< code > Color< / code > ， < / p >
< pre > < code class = "language-cpp" > enum Color { black, white, red };
< / code > < / pre >
< p > 编译器可能选择< code > char< / code > 作为底层类型，因为这里只需要表示三个值。然而，有些< code > enum< / code > 中的枚举值范围可能会大些，比如：< / p >
< pre > < code class = "language-cpp" > enum Status { good = 0,
failed = 1,
incomplete = 100,
corrupt = 200,
indeterminate = 0xFFFFFFFF
};
< / code > < / pre >
< p > 这里值的范围从< code > 0< / code > 到< code > 0xFFFFFFFF< / code > 。除了在不寻常的机器上（比如一个< code > char< / code > 至少有32bits的那种） ， < code > char< / code > 大的整型类型来表示< code > Status< / code > 。< / p >
< p > 为了高效使用内存，编译器通常在确保能包含所有枚举值的前提下为< code > enum< / code > 选择一个最小的底层类型。在一些情况下， ， ， ， ， < code > enum< / code > 定义（所有枚举名全部列出来）；< code > enum< / code > 声明是不被允许的。这使得编译器能在使用之前为每一个< code > enum< / code > 选择一个底层类型。< / p >
< p > 但是不能前置声明< code > enum< / code > 也是有缺点的。最大的缺点莫过于它可能增加编译依赖。再次考虑< code > Status< / code > < code > enum< / code > ： < / p >
< pre > < code class = "language-cpp" > enum Status { good = 0,
failed = 1,
incomplete = 100,
corrupt = 200,
indeterminate = 0xFFFFFFFF
};
< / code > < / pre >
< p > 这种< code > enum< / code > 很有可能用于整个系统，因此系统中每个包含这个头文件的组件都会依赖它。如果引入一个新状态值，< / p >
< pre > < code class = "language-cpp" > enum Status { good = 0,
failed = 1,
incomplete = 100,
corrupt = 200,
audited = 500,
indeterminate = 0xFFFFFFFF
};
< / code > < / pre >
< p > 那么可能整个系统都得重新编译，即使只有一个子系统——或者只有一个函数——使用了新添加的枚举名。这是大家都< strong > 不希望< / strong > 看到的。C++11中的前置声明< code > enum< / code > s可以解决这个问题。比如这里有一个完全有效的限域< code > enum< / code > 声明和一个以该限域< code > enum< / code > 作为形参的函数声明：< / p >
< pre > < code class = "language-cpp" > enum class Status; //前置声明
void continueProcessing(Status s); //使用前置声明enum
< / code > < / pre >
< p > 即使< code > Status< / code > 的定义发生改变，包含这些声明的头文件也不需要重新编译。而且如果< code > Status< / code > 有改动（比如添加一个< code > audited< / code > 枚举名），< code > continueProcessing< / code > 的行为不受影响（比如因为< code > continueProcessing< / code > 没有使用这个新添加的< code > audited< / code > ） ， < code > continueProcessing< / code > 也不需要重新编译。< / p >
< p > 但是如果编译器在使用它之前需要知晓该< code > enum< / code > 的大小， ？ ： < code > enum< / code > 的底层类型总是已知的，而对于非限域< code > enum< / code > ，你可以指定它。< / p >
< p > 默认情况下，限域枚举的底层类型是< code > int< / code > ： < / p >
< pre > < code class = "language-cpp" > enum class Status; //底层类型是int
< / code > < / pre >
< p > 如果默认的< code > int< / code > 不适用，你可以重写它：< / p >
< pre > < code class = "language-cpp" > enum class Status: std::uint32_t;
< cstdint> ）
< / code > < / pre >
< p > 不管怎样，编译器都知道限域< code > enum< / code > 中的枚举名占用多少字节。< / p >
< p > 要为非限域< code > enum< / code > 指定底层类型，你可以同上，结果就可以前向声明：< / p >
< pre > < code class = "language-cpp" > enum Color: std::uint8_t; //非限域enum前向声明
< / code > < / pre >
< p > 底层类型说明也可以放到< code > enum< / code > 定义处：< / p >
< pre > < code class = "language-cpp" > enum class Status: std::uint32_t { good = 0,
failed = 1,
incomplete = 100,
corrupt = 200,
audited = 500,
indeterminate = 0xFFFFFFFF
};
< / code > < / pre >
< p > 限域< code > enum< / code > 避免命名空间污染而且不接受荒谬的隐式类型转换，但它并非万事皆宜，你可能会很惊讶听到至少有一种情况下非限域< code > enum< / code > 是很有用的。那就是牵扯到C++11的< code > std::tuple< / code > 的时候。比如在社交网站中，假设我们有一个< em > tuple< / em > 保存了用户的名字， ， ： < / p >
< pre > < code class = "language-cpp" > using UserInfo = //类型别名，
std::tuple< std::string, //名字
std::string, //email地址
std::size_t> ; //声望
< / code > < / pre >
< p > 虽然注释说明了tuple各个字段对应的意思， ： < / p >
< pre > < code class = "language-cpp" > UserInfo uInfo; //tuple对象
…
auto val = std::get< 1> (uInfo); //获取第一个字段
< / code > < / pre >
< p > 作为一个程序员， ？ < code > enum< / code > 将名字和字段编号关联起来以避免上述需求：< / p >
< pre > < code class = "language-cpp" > enum UserInfoFields { uiName, uiEmail, uiReputation };
UserInfo uInfo; //同之前一样
…
auto val = std::get< uiEmail> (uInfo); //啊，
< / code > < / pre >
< p > 之所以它能正常工作是因为< code > UserInfoFields< / code > 中的枚举名隐式转换成< code > std::size_t< / code > 了，其中< code > std::size_t< / code > 是< code > std::get< / code > 模板实参所需的。< / p >
< p > 对应的限域< code > enum< / code > 版本就很啰嗦了：< / p >
< pre > < code class = "language-cpp" > enum class UserInfoFields { uiName, uiEmail, uiReputation };
UserInfo uInfo; //同之前一样
…
auto val =
std::get< static_cast< std::size_t> (UserInfoFields::uiEmail)>
(uInfo);
< / code > < / pre >
2022-11-25 13:43:31 +08:00
< p > 为避免这种冗长的表示，我们可以写一个函数传入枚举名并返回对应的< code > std::size_t< / code > 值，但这有一点技巧性。< code > std::get< / code > 是一个模板（函数），需要你给出一个< code > std::size_t< / code > 值的模板实参（注意使用< code > < > < / code > 而不是< code > ()< / code > ），因此将枚举名变换为< code > std::size_t< / code > 值的函数必须< strong > 在编译期< / strong > 产生这个结果。如< a href = "../3.MovingToModernCpp/item15.html" > Item15< / a > 提到的，那必须是一个< code > constexpr< / code > 函数。< / p >
< p > 事实上，它也的确该是一个< code > constexpr< / code > 函数模板，因为它应该能用于任何< code > enum< / code > 。如果我们想让它更一般化，我们还要泛化它的返回类型。较之于返回< code > std::size_t< / code > ，我们更应该返回枚举的底层类型。这可以通过< code > std::underlying_type< / code > 这个< em > type trait< / em > 获得。（参见< a href = "../3.MovingToModernCpp/item9.html" > Item9< / a > 关于< em > type trait< / em > 的内容）。最终我们还要再加上< code > noexcept< / code > 修饰（参见< a href = "../3.MovingToModernCpp/item14.html" > Item14< / a > ），因为我们知道它肯定不会产生异常。根据上述分析最终得到的< code > toUType< / code > 函数模板在编译期接受任意枚举名并返回它的值：< / p >
2022-06-30 10:23:03 +08:00
< pre > < code class = "language-cpp" > template< typename E>
constexpr typename std::underlying_type< E> ::type
toUType(E enumerator) noexcept
{
return
static_cast< typename
std::underlying_type< E> ::type> (enumerator);
}
< / code > < / pre >
2022-11-25 13:43:31 +08:00
< p > 在C++14中， < code > toUType< / code > 还可以进一步用< code > std::underlying_type_t< / code > （参见< a href = "../3.MovingToModernCpp/item9.html" > Item9< / a > ）代替< code > typename std::underlying_type< E> ::type< / code > 打磨：< / p >
2022-06-30 10:23:03 +08:00
< pre > < code class = "language-cpp" > template< typename E> //C++14
constexpr std::underlying_type_t< E>
toUType(E enumerator) noexcept
{
return static_cast< std::underlying_type_t< E> > (enumerator);
}
< / code > < / pre >
2022-11-25 13:43:31 +08:00
< p > 还可以再用C++14 < code > auto< / code > （参见< a href = "../1.DeducingTypes/item3.html" > Item3< / a > ）打磨一下代码：< / p >
2022-06-30 10:23:03 +08:00
< pre > < code class = "language-cpp" > template< typename E> //C++14
constexpr auto
toUType(E enumerator) noexcept
{
return static_cast< std::underlying_type_t< E> > (enumerator);
}
< / code > < / pre >
< p > 不管它怎么写，< code > toUType< / code > 现在允许这样访问tuple的字段了： < / p >
< pre > < code class = "language-cpp" > auto val = std::get< toUType(UserInfoFields::uiEmail)> (uInfo);
< / code > < / pre >
< p > 这仍然比使用非限域< code > enum< / code > 要写更多的代码， ， ， （ ） < / p >
< p > < strong > 记住< / strong > < / p >
< ul >
< li > C++98的< code > enum< / code > 即非限域< code > enum< / code > 。< / li >
< li > 限域< code > enum< / code > 的枚举名仅在< code > enum< / code > 内可见。要转换为其它类型只能使用< em > cast< / em > 。< / li >
< li > 非限域/限域< code > enum< / code > 都支持底层类型说明语法，限域< code > enum< / code > 底层类型默认是< code > int< / code > 。非限域< code > enum< / code > 没有默认底层类型。< / li >
< li > 限域< code > enum< / code > 总是可以前置声明。非限域< code > enum< / code > 仅当指定它们的底层类型时才能前置。< / li >
< / ul >
< / main >
< nav class = "nav-wrapper" aria-label = "Page navigation" >
<!-- Mobile navigation buttons -->
< a rel = "prev" href = "../3.MovingToModernCpp/item9.html" class = "mobile-nav-chapters previous" title = "Previous chapter" aria-label = "Previous chapter" aria-keyshortcuts = "Left" >
< i class = "fa fa-angle-left" > < / i >
< / a >
< a rel = "next" href = "../3.MovingToModernCpp/item11.html" class = "mobile-nav-chapters next" title = "Next chapter" aria-label = "Next chapter" aria-keyshortcuts = "Right" >
< i class = "fa fa-angle-right" > < / i >
< / a >
< div style = "clear: both" > < / div >
< / nav >
< / div >
< / div >
< nav class = "nav-wide-wrapper" aria-label = "Page navigation" >
< a rel = "prev" href = "../3.MovingToModernCpp/item9.html" class = "nav-chapters previous" title = "Previous chapter" aria-label = "Previous chapter" aria-keyshortcuts = "Left" >
< i class = "fa fa-angle-left" > < / i >
< / a >
< a rel = "next" href = "../3.MovingToModernCpp/item11.html" class = "nav-chapters next" title = "Next chapter" aria-label = "Next chapter" aria-keyshortcuts = "Right" >
< i class = "fa fa-angle-right" > < / i >
< / a >
< / nav >
< / div >
< script type = "text/javascript" >
window.playground_copyable = true;
< / script >
< script src = "../elasticlunr.min.js" type = "text/javascript" charset = "utf-8" > < / script >
< script src = "../mark.min.js" type = "text/javascript" charset = "utf-8" > < / script >
< script src = "../searcher.js" type = "text/javascript" charset = "utf-8" > < / script >
< script src = "../clipboard.min.js" type = "text/javascript" charset = "utf-8" > < / script >
< script src = "../highlight.js" type = "text/javascript" charset = "utf-8" > < / script >
< script src = "../book.js" type = "text/javascript" charset = "utf-8" > < / script >
<!-- Custom JS scripts -->
< / body >
< / html >
