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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">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" class="active">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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<h1 class="menu-title">Effective Modern C++</h1>
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<h2 id="条款三十三对auto形参使用decltype以stdforward它们"><a class="header" href="#条款三十三对auto形参使用decltype以stdforward它们">条款三十三：对<code>auto&&</code>形参使用<code>decltype</code>以<code>std::forward</code>它们</a></h2>
<p><strong>Item 33: Use <code>decltype</code> on <code>auto&&</code> parameters to <code>std::forward</code> them</strong></p>
<p><strong>泛型<em>lambda</em></strong>（<em>generic lambdas</em>）是C++14中最值得期待的特性之一——因为在<em>lambda</em>的形参中可以使用<code>auto</code>关键字。这个特性的实现是非常直截了当的：即在闭包类中的<code>operator()</code>函数是一个函数模版。例如存在这么一个<em>lambda</em>，</p>
<pre><code class="language-c++">auto f = [](auto x){ return func(normalize(x)); };
</code></pre>
<p>对应的闭包类中的函数调用操作符看来就变成这样：</p>
<pre><code class="language-c++">class SomeCompilerGeneratedClassName {
public:
template<typename T> //auto返回类型见条款3
auto operator()(T x) const
{ return func(normalize(x)); }
… //其他闭包类功能
};
</code></pre>
<p>在这个样例中，<em>lambda</em>对变量<code>x</code>做的唯一一件事就是把它转发给函数<code>normalize</code>。如果函数<code>normalize</code>对待左值右值的方式不一样，这个<em>lambda</em>的实现方式就不大合适了，因为即使传递到<em>lambda</em>的实参是一个右值，<em>lambda</em>传递进<code>normalize</code>的总是一个左值（形参<code>x</code>）。</p>
<p>实现这个<em>lambda</em>的正确方式是把<code>x</code>完美转发给函数<code>normalize</code>。这样做需要对代码做两处修改。首先，<code>x</code>需要改成通用引用（见<a href="../5.RRefMovSemPerfForw/item24.html">Item24</a>），其次，需要使用<code>std::forward</code>将<code>x</code>转发到函数<code>normalize</code>（见<a href="../5.RRefMovSemPerfForw/item25.html">Item25</a>）。理论上，这都是小改动：</p>
<pre><code class="language-c++">auto f = [](auto&& x)
{ return func(normalize(std::forward<???>(x))); };
</code></pre>
<p>在理论和实际之间存在一个问题：你应该传递给<code>std::forward</code>的什么类型，即确定我在上面写的<code>???</code>该是什么。</p>
<p>一般来说，当你在使用完美转发时，你是在一个接受类型参数为<code>T</code>的模版函数里，所以你可以写<code>std::forward<T></code>。但在泛型<em>lambda</em>中，没有可用的类型参数<code>T</code>。在<em>lambda</em>生成的闭包里，模版化的<code>operator()</code>函数中的确有一个<code>T</code>，但在<em>lambda</em>里却无法直接使用它，所以也没什么用。</p>
<p><a href="../5.RRefMovSemPerfForw/item28.html">Item28</a>解释过如果一个左值实参被传给通用引用的形参，那么形参类型会变成左值引用。传递的是右值，形参就会变成右值引用。这意味着在这个<em>lambda</em>中，可以通过检查形参<code>x</code>的类型来确定传递进来的实参是一个左值还是右值，<code>decltype</code>就可以实现这样的效果（见<a href="../1.DeducingTypes/item3.html">Item3</a>）。传递给<em>lambda</em>的是一个左值，<code>decltype(x)</code>就能产生一个左值引用；如果传递的是一个右值，<code>decltype(x)</code>就会产生右值引用。</p>
<p><a href="../5.RRefMovSemPerfForw/item28.html">Item28</a>也解释过在调用<code>std::forward</code>时，惯例决定了类型实参是左值引用时来表明要传进左值，类型实参是非引用就表明要传进右值。在前面的<em>lambda</em>中，如果<code>x</code>绑定的是一个左值，<code>decltype(x)</code>就能产生一个左值引用。这符合惯例。然而如果<code>x</code>绑定的是一个右值，<code>decltype(x)</code>就会产生右值引用，而不是常规的非引用。</p>
<p>再看一下<a href="../5.RRefMovSemPerfForw/item28.html">Item28</a>中关于<code>std::forward</code>的C++14实现：</p>
<pre><code class="language-c++">template<typename T> //在std命名空间
T&& forward(remove_reference_t<T>& param)
{
return static_cast<T&&>(param);
}
</code></pre>
<p>如果用户想要完美转发一个<code>Widget</code>类型的右值时，它会使用<code>Widget</code>类型（即非引用类型）来实例化<code>std::forward</code>，然后产生以下的函数：</p>
<pre><code class="language-c++">Widget&& forward(Widget& param) //当T是Widget时的std::forward实例
{
return static_cast<Widget&&>(param);
}
</code></pre>
<p>思考一下如果用户代码想要完美转发一个<code>Widget</code>类型的右值，但没有遵守规则将<code>T</code>指定为非引用类型，而是将<code>T</code>指定为右值引用，这会发生什么。也就是，思考将<code>T</code>换成<code>Widget&&</code>会如何。在<code>std::forward</code>实例化、应用了<code>std::remove_reference_t</code>后，引用折叠之前，<code>std::forward</code>看起来像这样：</p>
<pre><code class="language-c++">Widget&& && forward(Widget& param) //当T是Widget&&时的std::forward实例
{ //（引用折叠之前）
return static_cast<Widget&& &&>(param);
}
</code></pre>
<p>应用了引用折叠之后（右值引用的右值引用变成单个右值引用），代码会变成：</p>
<pre><code class="language-c++">Widget&& forward(Widget& param) //当T是Widget&&时的std::forward实例
{ //（引用折叠之后）
return static_cast<Widget&&>(param);
}
</code></pre>
<p>对比这个实例和用<code>Widget</code>设置<code>T</code>去实例化产生的结果，它们完全相同。表明用右值引用类型和用非引用类型去初始化<code>std::forward</code>产生的相同的结果。</p>
<p>那是一个很好的消息，因为当传递给<em>lambda</em>形参<code>x</code>的是一个右值实参时，<code>decltype(x)</code>可以产生一个右值引用。前面已经确认过，把一个左值传给<em>lambda</em>时，<code>decltype(x)</code>会产生一个可以传给<code>std::forward</code>的常规类型。而现在也验证了对于右值，把<code>decltype(x)</code>产生的类型传递给<code>std::forward</code>是非传统的，不过它产生的实例化结果与传统类型相同。所以无论是左值还是右值，把<code>decltype(x)</code>传递给<code>std::forward</code>都能得到我们想要的结果，因此<em>lambda</em>的完美转发可以写成：</p>
<pre><code class="language-c++">auto f =
[](auto&& param)
{
return
func(normalize(std::forward<decltype(param)>(param)));
};
</code></pre>
<p>再加上6个点，就可以让我们的<em>lambda</em>完美转发接受多个形参了，因为C++14中的<em>lambda</em>也可以是可变形参的：</p>
<pre><code class="language-c++">auto f =
[](auto&&... params)
{
return
func(normalize(std::forward<decltype(params)>(params)...));
};
</code></pre>
<p><strong>请记住：</strong></p>
<ul>
<li>对<code>auto&&</code>形参使用<code>decltype</code>以<code>std::forward</code>它们。</li>
</ul>
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