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<ol class="chapter"><li class="chapter-item expanded "><a href="../Introduction.html">Introduction</a></li><li class="chapter-item expanded "><div>Chapter 1. Deducing Types</div></li><li><ol class="section"><li class="chapter-item expanded "><a href="../1.DeducingTypes/item1.html">Item 1: Understanding template type deduction</a></li><li class="chapter-item expanded "><a href="../1.DeducingTypes/item2.html">Item 2: Understand auto type deduction</a></li><li class="chapter-item expanded "><a href="../1.DeducingTypes/item3.html">Item 3: Understand decltype</a></li><li class="chapter-item expanded "><a href="../1.DeducingTypes/item4.html">Item 4: Know how to view deduced types</a></li></ol></li><li class="chapter-item expanded "><div>Chapter 2. auto</div></li><li><ol class="section"><li class="chapter-item expanded "><a href="../2.Auto/item5.html">Item 5: Prefer auto to explicit type declarations</a></li><li class="chapter-item expanded "><a href="../2.Auto/item6.html">Item 6: Use the explicitly typed initializer idiom when auto deduces undesired types</a></li></ol></li><li class="chapter-item expanded "><div>Chapter 3. Moving to Modern C++</div></li><li><ol class="section"><li class="chapter-item expanded "><a href="../3.MovingToModernCpp/item7.html">Item 7: Distinguish between () and {} when creating objects</a></li><li class="chapter-item expanded "><a href="../3.MovingToModernCpp/item8.html">Item 8: Prefer nullptr to 0 or NULL</a></li><li class="chapter-item expanded "><a href="../3.MovingToModernCpp/item9.html">Item 9: Prefer alias declarations to typedefs</a></li><li class="chapter-item expanded "><a href="../3.MovingToModernCpp/item10.html">Item 10: Prefer scoped enums to unscoped enums</a></li><li class="chapter-item expanded "><a href="../3.MovingToModernCpp/item11.html">Item 11: Prefer deleted functions to private undefined ones</a></li><li class="chapter-item expanded "><a href="../3.MovingToModernCpp/item12.html">Item 12: Declare overriding functions override</a></li><li class="chapter-item expanded "><a href="../3.MovingToModernCpp/item13.html">Item 13: Prefer const_iterators to iterators</a></li><li class="chapter-item expanded "><a href="../3.MovingToModernCpp/item14.html">Item 14: Declare functions noexcept if they won't emit exceptions</a></li><li class="chapter-item expanded "><a href="../3.MovingToModernCpp/item15.html">Item 15: Use constexpr whenever possible</a></li><li class="chapter-item expanded "><a href="../3.MovingToModernCpp/item16.html">Item 16: Make const member functions thread safe</a></li><li class="chapter-item expanded "><a href="../3.MovingToModernCpp/item17.html">Item 17: Understand special member funciton generation</a></li></ol></li><li class="chapter-item expanded "><div>Chapter 4. Smart Pointer</div></li><li><ol class="section"><li class="chapter-item expanded "><a href="../4.SmartPointers/item18.html">Item 18: Use std::unique_ptr for exclusive-ownership resource management</a></li><li class="chapter-item expanded "><a href="../4.SmartPointers/item19.html">Item 19: Use std::shared_ptr for shared-ownership resource management</a></li><li class="chapter-item expanded "><a href="../4.SmartPointers/item20.html">Item 20: Use std::weak_ptr for std::shared_ptr like pointers that can dangle</a></li><li class="chapter-item expanded "><a href="../4.SmartPointers/item21.html">Item 21: Prefer std::make_unique and std::make_shared to direct use of new</a></li><li class="chapter-item expanded "><a href="../4.SmartPointers/item22.html">Item 22: When using the Pimpl Idiom, define special member functions in the implementation file</a></li></ol></li><li class="chapter-item expanded "><div>Chapter 5. Rvalue References, Move Semantics, and Perfect Forwarding</div></li><li><ol class="section"><li class="chapter-item expanded "><a href="../5.RRefMovSemPerfForw/item23.html">Item 23: Understand std::move and std::forward</a></li><li class="chapter-item expanded "><a href="../5.RRefMovSemPerfForw/item24.html" class="active">Item 24: Distinguish universal references from rvalue references</a
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<h2 id="条款二十四区分通用引用与右值引用"><a class="header" href="#条款二十四区分通用引用与右值引用">条款二十四：区分通用引用与右值引用</a></h2>
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<p><strong>Item 24: Distinguish universal references from rvalue references</strong></p>
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<p>据说，真相使人自由，然而在特定的环境下，一个精心挑选的谎言也同样使人解放。这一条款就是这样一个谎言。因为我们在和软件打交道，然而，让我们避开“谎言（lie）”这个词，不妨说，本条款包含了一种“抽象（abstraction）”。</p>
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<p>为了声明一个指向某个类型<code>T</code>的右值引用，你写下了<code>T&&</code>。由此，一个合理的假设是，当你看到一个“<code>T&&</code>”出现在源码中，你看到的是一个右值引用。唉，事情并不如此简单:</p>
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<pre><code class="language-cpp">void f(Widget&& param); //右值引用
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Widget&& var1 = Widget(); //右值引用
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auto&& var2 = var1; //不是右值引用
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template<typename T>
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void f(std::vector<T>&& param); //右值引用
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template<typename T>
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void f(T&& param); //不是右值引用
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</code></pre>
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<p>事实上，“<code>T&&</code>”有两种不同的意思。第一种，当然是右值引用。这种引用表现得正如你所期待的那样：它们只绑定到右值上，并且它们主要的存在原因就是为了识别可以移动操作的对象。</p>
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<p>“<code>T&&</code>”的另一种意思是，它既可以是右值引用，也可以是左值引用。这种引用在源码里看起来像右值引用（即“<code>T&&</code>”），但是它们可以表现得像是左值引用（即“<code>T&</code>”）。它们的二重性使它们既可以绑定到右值上（就像右值引用），也可以绑定到左值上（就像左值引用）。 此外，它们还可以绑定到<code>const</code>或者non-<code>const</code>的对象上，也可以绑定到<code>volatile</code>或者non-<code>volatile</code>的对象上，甚至可以绑定到既<code>const</code>又<code>volatile</code>的对象上。它们可以绑定到几乎任何东西。这种空前灵活的引用值得拥有自己的名字。我把它叫做<strong>通用引用</strong>（<em>universal references</em>）。（<a href="https://github.com/kelthuzadx/EffectiveModernCppChinese/blob/master/5.RRefMovSemPerfForw/item25.md">Item25</a>解释了<code>std::forward</code>几乎总是可以应用到通用引用上，并且在这本书即将出版之际，一些C++社区的成员已经开始将这种通用引用称之为<strong>转发引用</strong>（<em>forwarding references</em>））。</p>
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<p>在两种情况下会出现通用引用。最常见的一种是函数模板形参，正如在之前的示例代码中所出现的例子：</p>
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<pre><code class="language-cpp">template<typename T>
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void f(T&& param); //param是一个通用引用
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</code></pre>
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<p>第二种情况是<code>auto</code>声明符，它是从以上示例中拿出的：</p>
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<pre><code class="language-cpp">auto&& var2 = var1; //var2是一个通用引用
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</code></pre>
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<p>这两种情况的共同之处就是都存在<strong>类型推导</strong>（<em>type deduction</em>）。在模板<code>f</code>的内部，<code>param</code>的类型需要被推导，而在变量<code>var2</code>的声明中，<code>var2</code>的类型也需要被推导。同以下的例子相比较（同样来自于上面的示例代码），下面的例子不带有类型推导。如果你看见“<code>T&&</code>”不带有类型推导，那么你看到的就是一个右值引用：</p>
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<pre><code class="language-cpp">void f(Widget&& param); //没有类型推导，
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//param是一个右值引用
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Widget&& var1 = Widget(); //没有类型推导，
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//var1是一个右值引用
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</code></pre>
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<p>因为通用引用是引用，所以它们必须被初始化。一个通用引用的初始值决定了它是代表了右值引用还是左值引用。如果初始值是一个右值，那么通用引用就会是对应的右值引用，如果初始值是一个左值，那么通用引用就会是一个左值引用。对那些是函数形参的通用引用来说，初始值在调用函数的时候被提供：</p>
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<pre><code class="language-cpp">template<typename T>
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void f(T&& param); //param是一个通用引用
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Widget w;
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f(w); //传递给函数f一个左值；param的类型
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//将会是Widget&，也即左值引用
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f(std::move(w)); //传递给f一个右值；param的类型会是
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//Widget&&，即右值引用
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</code></pre>
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<p>对一个通用引用而言，类型推导是必要的，但是它还不够。引用声明的<strong>形式</strong>必须正确，并且该形式是被限制的。它必须恰好为“<code>T&&</code>”。再看看之前我们已经看过的代码示例：</p>
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<pre><code class="language-cpp">template <typename T>
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void f(std::vector<T>&& param); //param是一个右值引用
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</code></pre>
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<p>当函数<code>f</code>被调用的时候，类型<code>T</code>会被推导（除非调用者显式地指定它，这种边缘情况我们不考虑）。但是<code>param</code>的类型声明并不是<code>T&&</code>，而是一个<code>std::vector<T>&&</code>。这排除了<code>param</code>是一个通用引用的可能性。<code>param</code>因此是一个右值引用——当你向函数<code>f</code>传递一个左值时，你的编译器将会乐于帮你确认这一点:</p>
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<pre><code class="language-cpp">std::vector<int> v;
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f(v); //错误！不能将左值绑定到右值引用
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</code></pre>
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<p>即使一个简单的<code>const</code>修饰符的出现，也足以使一个引用失去成为通用引用的资格:</p>
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<pre><code class="language-cpp">template <typename T>
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void f(const T&& param); //param是一个右值引用
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</code></pre>
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<p>如果你在一个模板里面看见了一个函数形参类型为“<code>T&&</code>”，你也许觉得你可以假定它是一个通用引用。错！这是由于在模板内部并不保证一定会发生类型推导。考虑如下<code>push_back</code>成员函数，来自<code>std::vector</code>：</p>
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<pre><code class="language-cpp">template<class T, class Allocator = allocator<T>> //来自C++标准
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class vector
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{
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public:
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void push_back(T&& x);
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…
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}
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</code></pre>
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|||
|
|
<p><code>push_back</code>函数的形参当然有一个通用引用的正确形式，然而，在这里并没有发生类型推导。因为<code>push_back</code>在有一个特定的<code>vector</code>实例之前不可能存在，而实例化<code>vector</code>时的类型已经决定了<code>push_back</code>的声明。也就是说，</p>
|
|||
|
|
<pre><code class="language-cpp">std::vector<Widget> v;
|
|||
|
|
</code></pre>
|
|||
|
|
<p>将会导致<code>std::vector</code>模板被实例化为以下代码：</p>
|
|||
|
|
<pre><code class="language-cpp">class vector<Widget, allocator<Widget>> {
|
|||
|
|
public:
|
|||
|
|
void push_back(Widget&& x); //右值引用
|
|||
|
|
…
|
|||
|
|
};
|
|||
|
|
</code></pre>
|
|||
|
|
<p>现在你可以清楚地看到，函数<code>push_back</code>不包含任何类型推导。<code>push_back</code>对于<code>vector<T></code>而言（有两个函数——它被重载了）总是声明了一个类型为rvalue-reference-to-<code>T</code>的形参。</p>
|
|||
|
|
<p>作为对比，<code>std::vector</code>内的概念上相似的成员函数<code>emplace_back</code>，却确实包含类型推导:</p>
|
|||
|
|
<pre><code class="language-cpp">template<class T, class Allocator = allocator<T>> //依旧来自C++标准
|
|||
|
|
class vector {
|
|||
|
|
public:
|
|||
|
|
template <class... Args>
|
|||
|
|
void emplace_back(Args&&... args);
|
|||
|
|
…
|
|||
|
|
};
|
|||
|
|
</code></pre>
|
|||
|
|
<p>这儿，类型参数（<em>type parameter</em>）<code>Args</code>是独立于<code>vector</code>的类型参数<code>T</code>的，所以<code>Args</code>会在每次<code>emplace_back</code>被调用的时候被推导。（好吧，<code>Args</code>实际上是一个<a href="https://en.cppreference.com/w/cpp/language/parameter_pack"><em>parameter pack</em></a>，而不是一个类型参数，但是为了方便讨论，我们可以把它当作是一个类型参数。）</p>
|
|||
|
|
<p>虽然函数<code>emplace_back</code>的类型参数被命名为<code>Args</code>，但是它仍然是一个通用引用，这补充了我之前所说的，通用引用的格式必须是“<code>T&&</code>”。你使用的名字<code>T</code>并不是必要的。举个例子，如下模板接受一个通用引用，因为形式（“<code>type&&</code>”）是正确的，并且<code>param</code>的类型将会被推导（重复一次，不考虑边缘情况，即当调用者明确给定类型的时候）。</p>
|
|||
|
|
<pre><code class="language-cpp">template<typename MyTemplateType> //param是通用引用
|
|||
|
|
void someFunc(MyTemplateType&& param);
|
|||
|
|
</code></pre>
|
|||
|
|
<p>我之前提到，类型为<code>auto</code>的变量可以是通用引用。更准确地说，类型声明为<code>auto&&</code>的变量是通用引用，因为会发生类型推导，并且它们具有正确形式(<code>T&&</code>)。<code>auto</code>类型的通用引用不如函数模板形参中的通用引用常见，但是它们在C++11中常常突然出现。而它们在C++14中出现得更多，因为C++14的<em>lambda</em>表达式可以声明<code>auto&&</code>类型的形参。举个例子，如果你想写一个C++14标准的<em>lambda</em>表达式，来记录任意函数调用的时间开销，你可以这样写：</p>
|
|||
|
|
<pre><code class="language-cpp">auto timeFuncInvocation =
|
|||
|
|
[](auto&& func, auto&&... params) //C++14
|
|||
|
|
{
|
|||
|
|
start timer;
|
|||
|
|
std::forward<decltype(func)>(func)( //对params调用func
|
|||
|
|
std::forward<delctype(params)>(params)...
|
|||
|
|
);
|
|||
|
|
stop timer and record elapsed time;
|
|||
|
|
};
|
|||
|
|
</code></pre>
|
|||
|
|
<p>如果你对<em>lambda</em>里的代码“<code>std::forward<decltype(blah blah blah)></code>”反应是“这是什么鬼...?!”，只能说你可能还没有读<a href="https://github.com/kelthuzadx/EffectiveModernCppChinese/blob/master/6.LambdaExpressions/item33.md">Item33</a>。别担心。在本条款，重要的事是<em>lambda</em>表达式中声明的<code>auto&&</code>类型的形参。<code>func</code>是一个通用引用，可以被绑定到任何可调用对象，无论左值还是右值。<code>args</code>是0个或者多个通用引用（即它是个通用引用<em>parameter pack</em>），它可以绑定到任意数目、任意类型的对象上。多亏了<code>auto</code>类型的通用引用，函数<code>timeFuncInvocation</code>可以对<strong>近乎任意</strong>（pretty much any）函数进行计时。(如果你想知道任意（any）和近乎任意（pretty much any）的区别，往后翻到<a href="https://github.com/kelthuzadx/EffectiveModernCppChinese/blob/master/5.RRefMovSemPerfForw/item30.md">Item30</a>)。</p>
|
|||
|
|
<p>牢记整个本条款——通用引用的基础——是一个谎言，噢不，是一个“抽象”。其底层真相被称为<strong>引用折叠</strong>（<em>reference collapsing</em>），<a href="https://github.com/kelthuzadx/EffectiveModernCppChinese/blob/master/5.RRefMovSemPerfForw/item28.md">Item28</a>的专题将致力于讨论它。但是这个真相并不降低该抽象的有用程度。区分右值引用和通用引用将会帮助你更准确地阅读代码（“究竟我眼前的这个<code>T&&</code>是只绑定到右值还是可以绑定任意对象呢？”），并且，当你在和你的合作者交流时，它会帮助你避免歧义（“在这里我在用一个通用引用，而非右值引用”）。它也可以帮助你弄懂<a href="https://github.com/kelthuzadx/EffectiveModernCppChinese/blob/master/5.RRefMovSemPerfForw/item25.md">Item25</a>和<a href="https://github.com/kelthuzadx/EffectiveModernCppChinese/blob/master/5.RRefMovSemPerfForw/item26.md">26</a>，它们依赖于右值引用和通用引用的区别。所以，拥抱这份抽象，陶醉于它吧。就像牛顿的力学定律（本质上不正确），比起爱因斯坦的广义相对论（这是真相）而言，往往更简单，更易用。所以通用引用的概念，相较于穷究引用折叠的细节而言，是更合意之选。</p>
|
|||
|
|
<p><strong>请记住：</strong></p>
|
|||
|
|
<ul>
|
|||
|
|
<li>如果一个函数模板形参的类型为<code>T&&</code>，并且<code>T</code>需要被推导得知，或者如果一个对象被声明为<code>auto&&</code>，这个形参或者对象就是一个通用引用。</li>
|
|||
|
|
<li>如果类型声明的形式不是标准的<code>type&&</code>，或者如果类型推导没有发生，那么<code>type&&</code>代表一个右值引用。</li>
|
|||
|
|
<li>通用引用，如果它被右值初始化，就会对应地成为右值引用；如果它被左值初始化，就会成为左值引用。</li>
|
|||
|
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