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<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，使用显式类型初始化惯用法</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，请在实现文件中定义特殊成员函数</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，对于通用引用使用std::forward</a></li><li class="chapter-item expanded "><a href="../5.RRefMovSemPerfForw/item26.html">Item 26:避免重载通用引用</a></li><li class="chapter-item expanded "><a href="../5.RRefMovSemPerfForw/item27.html">Item 27:熟悉重载通用引用的替代品</a></li><li class="chapter-item expanded "><a href="../5.RRefMovSemPerfForw/item28.html">Item 28:理解引用折叠</a></li><li class="chapter-item expanded "><a href="../5.RRefMovSemPerfForw/item29.html">Item 29:认识移动操作的缺点</a></li><li class="chapter-item expanded "><a href="../5.RRefMovSemPerfForw/item30.html">Item 30:熟悉完美转发失败的情况</a></li></ol></li><li class="chapter-item expanded "><div>第六章 Lambda表达式</div></li><li><ol class="section"><li class="chapter-item expanded "><a href="../6.LambdaExpressions/item31.html">Item 31:避免使用默认捕获模式</a></li><li class="chapter-item expanded "><a href="../6.LambdaExpressions/item32.html">Item 32:使用初始化捕获来移动对象到闭包中</a></li><li class="chapter-item expanded "><a href="../6.LambdaExpressions/item33.html">Item 33:对于std::forward的auto&&形参使用decltype</a></li><li class="chapter-item expanded "><a href="../6.LambdaExpressions/item34.html">Item 34:优先考虑lambda表达式而非std::bind</a></li></ol></li><li class="chapter-item expanded "><div>第七章 并发API</div></li><li><ol class="section"><li class="chapter-item expanded "><a href="../7.TheConcurrencyAPI/Item35.html">Item 35:优先考虑基于任务的编程而非基于线程的编程</a></li><li class="chapter-item expanded "><a href="../7.TheConcurrencyAPI/item36.html">Item 36:如果有异步的必要请指定std::launch::threads</a></li><li class="chapter-item expanded "><a href="../7.TheConcurrencyAPI/item37.html">Item 37:从各个方面使得std::threads unjoinable</a></li><li class="chapter-item expanded "><a href="../7.TheConcurrencyAPI/item38.html">Item 38:关注不同线程句柄析构行为</a></li><li class="chapter-item expanded "><a href="../7.TheConcurrencyAPI/item39.html">Item 39:考虑对于单次事件通信使用void</a></li><li class="chapter-item expanded "><a href="../7.TheConcurrencyAPI/item40.html">Item 40:对于并发使用std::atomic，volatile用于特殊内存区</a></li></ol></li><li class="chapter-item expanded "><div>第八章 微调</div></li><li><ol class="section"><li class="chapter-item expanded "><a href="../8.Tweaks/item41.html">Item 41:对于那些可移动总是被拷贝的形参使用传值方式</a></li><li class="chapter-item expanded "><a href="../8.Tweaks/item42.html">Item 42:考虑就地创建而非插入</a></li></ol></li></ol>
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<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>
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<p><strong>Item 10: Prefer scoped <code>enum</code>s to unscoped <code>enum</code>s</strong></p>
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<p>通常来说，在花括号中声明一个名字会限制它的作用域在花括号之内。但这对于C++98风格的<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，连同下文“名字”都指names）属于包含这个<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>
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<pre><code class="language-cpp">enum Color { black, white, red }; //black, white, red在
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//Color所在的作用域
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auto white = false; //错误! white早已在这个作用
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//域中声明
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</code></pre>
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<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，限域枚举(<em>scoped <code>enum</code></em>)，它不会导致枚举名泄漏：</p>
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<pre><code class="language-cpp">enum class Color { black, white, red }; //black, white, red
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//限制在Color域内
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auto white = false; //没问题，域内没有其他“white”
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Color c = white; //错误，域中没有枚举名叫white
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Color c = Color::white; //没问题
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auto c = Color::white; //也没问题（也符合Item5的建议）
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</code></pre>
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<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>
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<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>
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<pre><code class="language-cpp">enum Color { black, white, red }; //未限域enum
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std::vector<std::size_t> //func返回x的质因子
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primeFactors(std::size_t x);
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Color c = red;
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…
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if (c < 14.5) { // Color与double比较 (!)
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auto factors = // 计算一个Color的质因子(!)
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primeFactors(c);
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…
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}
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</code></pre>
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<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>
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<pre><code class="language-cpp">enum class Color { black, white, red }; //Color现在是限域enum
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Color c = Color::red; //和之前一样，只是
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... //多了一个域修饰符
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if (c < 14.5) { //错误！不能比较
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//Color和double
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auto factors = //错误！不能向参数为std::size_t
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primeFactors(c); //的函数传递Color参数
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…
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}
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</code></pre>
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<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>
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<pre><code class="language-cpp">if (static_cast<double>(c) < 14.5) { //奇怪的代码，
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//但是有效
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auto factors = //有问题，但是
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primeFactors(static_cast<std::size_t>(c)); //能通过编译
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…
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}
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</code></pre>
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<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>
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<pre><code class="language-cpp">enum Color; //错误！
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enum class Color; //没问题
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</code></pre>
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<p>其实这是一个误导。在C++11中，非限域<code>enum</code>也可以被前置声明，但是只有在做一些其他工作后才能实现。这些工作来源于一个事实：在C++中所有的<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>
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<pre><code class="language-cpp">enum Color { black, white, red };
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</code></pre>
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<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>
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<pre><code class="language-cpp">enum Status { good = 0,
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failed = 1,
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incomplete = 100,
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corrupt = 200,
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indeterminate = 0xFFFFFFFF
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};
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</code></pre>
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<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），编译器都会选择一个比<code>char</code>大的整型类型来表示<code>Status</code>。</p>
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<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，编译器将会优化速度，舍弃大小，这种情况下它可能不会选择最小的底层类型，而是选择对优化大小有帮助的类型。为此，C++98只支持<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>
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<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>
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<pre><code class="language-cpp">enum Status { good = 0,
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failed = 1,
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incomplete = 100,
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corrupt = 200,
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indeterminate = 0xFFFFFFFF
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};
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</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，该怎么声明才能让C++11做到C++98不能做到的事情呢？答案很简单：限域<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; //Status的底层类型
|
||
//是std::uint32_t
|
||
//（需要包含 <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
|
||
//底层类型为
|
||
//std::uint8_t
|
||
</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，email地址，声望值：</p>
|
||
<pre><code class="language-cpp">using UserInfo = //类型别名，参见Item9
|
||
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，但当你在另一文件遇到下面的代码那之前的注释就不是那么有用了：</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，你有很多工作要持续跟进。你应该记住第一个字段代表用户的email地址吗？我认为不。可以使用非限域<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，获取用户email字段的值
|
||
</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>
|
||
<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>
|
||
<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>
|
||
<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>
|
||
<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>
|
||
<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>
|
||
<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，但同时它也避免命名空间污染，防止不经意间使用隐式转换。大多数情况下，你应该会觉得多敲几个（几行）字符作为避免使用未限域枚举这种老得和2400波特率猫同时代技术的代价是值得的。</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>
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