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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" class="active">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">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="条款八优先考虑nullptr而非0和null"><a class="header" href="#条款八优先考虑nullptr而非0和null">条款八：优先考虑<code>nullptr</code>而非<code>0</code>和<code>NULL</code></a></h2>
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<p><strong>Item 8: Prefer <code>nullptr</code> to <code>0</code> and <code>NULL</code></strong></p>
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<p>你看这样对不对：字面值<code>0</code>是一个<code>int</code>不是指针。如果C++发现在当前上下文只能使用指针，它会很不情愿的把<code>0</code>解释为指针，但是那是最后的退路。一般来说C++的解析策略是把<code>0</code>看做<code>int</code>而不是指针。</p>
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<p>实际上，<code>NULL</code>也是这样的。但在<code>NULL</code>的实现细节有些不确定因素，因为实现被允许给<code>NULL</code>一个除了<code>int</code>之外的整型类型（比如<code>long</code>）。这不常见，但也算不上问题所在。这里的问题不是<code>NULL</code>没有一个确定的类型，而是<code>0</code>和<code>NULL</code>都不是指针类型。</p>
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<p>在C++98中，对指针类型和整型进行重载意味着可能导致奇怪的事情。如果给下面的重载函数传递<code>0</code>或<code>NULL</code>，它们绝不会调用指针版本的重载函数：</p>
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<pre><code class="language-cpp">void f(int); //三个f的重载函数
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void f(bool);
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void f(void*);
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f(0); //调用f(int)而不是f(void*)
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f(NULL); //可能不会被编译，一般来说调用f(int)，
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//绝对不会调用f(void*)
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</code></pre>
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<p>而<code>f(NULL)</code>的不确定行为是由<code>NULL</code>的实现不同造成的。如果<code>NULL</code>被定义为<code>0L</code>（指的是<code>0</code>为<code>long</code>类型），这个调用就具有二义性，因为从<code>long</code>到<code>int</code>的转换或从<code>long</code>到<code>bool</code>的转换或<code>0L</code>到<code>void*</code>的转换都同样好。有趣的是源代码<strong>表现出</strong>的意思（“我使用空指针<code>NULL</code>调用<code>f</code>”）和<strong>实际表达出</strong>的意思（“我是用整型数据而不是空指针调用<code>f</code>”）是相矛盾的。这种违反直觉的行为导致C++98程序员都将避开同时重载指针和整型作为编程准则（译注：请务必注意结合上下文使用这条规则）。在C++11中这个编程准则也有效，因为尽管我这个条款建议使用<code>nullptr</code>，可能很多程序员还是会继续使用<code>0</code>或<code>NULL</code>，哪怕<code>nullptr</code>是更好的选择。</p>
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<p><code>nullptr</code>的优点是它不是整型。老实说它也不是一个指针类型，但是你可以把它认为是<strong>所有</strong>类型的指针。<code>nullptr</code>的真正类型是<code>std::nullptr_t</code>，在一个完美的循环定义以后，<code>std::nullptr_t</code>又被定义为<code>nullptr</code>。<code>std::nullptr_t</code>可以隐式转换为指向任何内置类型的指针，这也是为什么<code>nullptr</code>表现得像所有类型的指针。</p>
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<p>使用<code>nullptr</code>调用<code>f</code>将会调用<code>void*</code>版本的重载函数，因为<code>nullptr</code>不能被视作任何整型：</p>
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<pre><code class="language-cpp">f(nullptr); //调用重载函数f的f(void*)版本
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</code></pre>
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<p>使用<code>nullptr</code>代替<code>0</code>和<code>NULL</code>可以避开了那些令人奇怪的函数重载决议，这不是它的唯一优势。它也可以使代码表意明确，尤其是当涉及到与<code>auto</code>声明的变量一起使用时。举个例子，假如你在一个代码库中遇到了这样的代码：</p>
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<pre><code class="language-cpp">auto result = findRecord( /* arguments */ );
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if (result == 0) {
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…
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}
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</code></pre>
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<p>如果你不知道<code>findRecord</code>返回了什么（或者不能轻易的找出），那么你就不太清楚到底<code>result</code>是一个指针类型还是一个整型。毕竟，<code>0</code>（用来测试<code>result</code>的值的那个）也可以像我们之前讨论的那样被解析。但是换一种假设如果你看到这样的代码：</p>
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<pre><code class="language-cpp">auto result = findRecord( /* arguments */ );
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if (result == nullptr) {
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…
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}
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</code></pre>
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<p>这就没有任何歧义：<code>result</code>的结果一定是指针类型。</p>
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<p>当模板出现时<code>nullptr</code>就更有用了。假如你有一些函数只能被合适的已锁互斥量调用。每个函数都有一个不同类型的指针：</p>
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<pre><code class="language-cpp">int f1(std::shared_ptr<Widget> spw); //只能被合适的
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double f2(std::unique_ptr<Widget> upw); //已锁互斥量
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bool f3(Widget* pw); //调用
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</code></pre>
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<p>如果这样传递空指针：</p>
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<pre><code class="language-cpp">std::mutex f1m, f2m, f3m; //用于f1，f2，f3函数的互斥量
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using MuxGuard = //C++11的typedef，参见Item9
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std::lock_guard<std::mutex>;
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…
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{
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MuxGuard g(f1m); //为f1m上锁
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auto result = f1(0); //向f1传递0作为空指针
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} //解锁
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…
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{
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MuxGuard g(f2m); //为f2m上锁
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auto result = f2(NULL); //向f2传递NULL作为空指针
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} //解锁
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…
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{
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MuxGuard g(f3m); //为f3m上锁
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auto result = f3(nullptr); //向f3传递nullptr作为空指针
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} //解锁
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</code></pre>
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<p>令人遗憾前两个调用没有使用<code>nullptr</code>，但是代码可以正常运行，这也许对一些东西有用。但是重复的调用代码——为互斥量上锁，调用函数，解锁互斥量——更令人遗憾。它让人很烦。模板就是被设计于减少重复代码，所以让我们模板化这个调用流程：</p>
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<pre><code class="language-cpp">template<typename FuncType,
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typename MuxType,
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typename PtrType>
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auto lockAndCall(FuncType func,
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MuxType& mutex,
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PtrType ptr) -> decltype(func(ptr))
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{
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MuxGuard g(mutex);
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return func(ptr);
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}
|
||
</code></pre>
|
||
<p>如果你对函数返回类型（<code>auto ... -> decltype(func(ptr))</code>）感到困惑不解，<a href="../1.DeducingTypes/item3.html">Item3</a>可以帮助你。在C++14中代码的返回类型还可以被简化为<code>decltype(auto)</code>：</p>
|
||
<pre><code class="language-cpp">template<typename FuncType,
|
||
typename MuxType,
|
||
typename PtrType>
|
||
decltype(auto) lockAndCall(FuncType func, //C++14
|
||
MuxType& mutex,
|
||
PtrType ptr)
|
||
{
|
||
MuxGuard g(mutex);
|
||
return func(ptr);
|
||
}
|
||
</code></pre>
|
||
<p>可以写这样的代码调用<code>lockAndCall</code>模板（两个版本都可）：</p>
|
||
<pre><code class="language-cpp">auto result1 = lockAndCall(f1, f1m, 0); //错误！
|
||
...
|
||
auto result2 = lockAndCall(f2, f2m, NULL); //错误！
|
||
...
|
||
auto result3 = lockAndCall(f3, f3m, nullptr); //没问题
|
||
</code></pre>
|
||
<p>代码虽然可以这样写，但是就像注释中说的，前两个情况不能通过编译。在第一个调用中存在的问题是当<code>0</code>被传递给<code>lockAndCall</code>模板，模板类型推导会尝试去推导实参类型，<code>0</code>的类型总是<code>int</code>，所以这就是这次调用<code>lockAndCall</code>实例化出的<code>ptr</code>的类型。不幸的是，这意味着<code>lockAndCall</code>中<code>func</code>会被<code>int</code>类型的实参调用，这与<code>f1</code>期待的<code>std::shared_ptr<Widget></code>形参不符。传递<code>0</code>给<code>lockAndCall</code>本来想表示空指针，但是实际上得到的一个普通的<code>int</code>。把<code>int</code>类型看做<code>std::shared_ptr<Widget></code>类型给<code>f1</code>自然是一个类型错误。在模板<code>lockAndCall</code>中使用<code>0</code>之所以失败是因为在模板中，传给的是<code>int</code>但实际上函数期待的是一个<code>std::shared_ptr<Widget></code>。</p>
|
||
<p>第二个使用<code>NULL</code>调用的分析也是一样的。当<code>NULL</code>被传递给<code>lockAndCall</code>，形参<code>ptr</code>被推导为整型（译注：由于依赖于具体实现所以不一定是整数类型，所以用整型泛指<code>int</code>，<code>long</code>等类型），然后当<code>ptr</code>——一个<code>int</code>或者类似<code>int</code>的类型——传递给<code>f2</code>的时候就会出现类型错误，<code>f2</code>期待的是<code>std::unique_ptr<Widget></code>。</p>
|
||
<p>然而，使用<code>nullptr</code>是调用没什么问题。当<code>nullptr</code>传给<code>lockAndCall</code>时，<code>ptr</code>被推导为<code>std::nullptr_t</code>。当<code>ptr</code>被传递给<code>f3</code>的时候，隐式转换使<code>std::nullptr_t</code>转换为<code>Widget*</code>，因为<code>std::nullptr_t</code>可以隐式转换为任何指针类型。</p>
|
||
<p>模板类型推导将<code>0</code>和<code>NULL</code>推导为一个错误的类型（即它们的实际类型，而不是作为空指针的隐含意义），这就导致在当你想要一个空指针时，它们的替代品<code>nullptr</code>很吸引人。使用<code>nullptr</code>，模板不会有什么特殊的转换。另外，使用<code>nullptr</code>不会让你受到同重载决议特殊对待<code>0</code>和<code>NULL</code>一样的待遇。当你想用一个空指针，使用<code>nullptr</code>，不用<code>0</code>或者<code>NULL</code>。</p>
|
||
<p><strong>记住</strong></p>
|
||
<ul>
|
||
<li>优先考虑<code>nullptr</code>而非<code>0</code>和<code>NULL</code></li>
|
||
<li>避免重载指针和整型</li>
|
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