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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" class="active">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="条款十六让const成员函数线程安全"><a class="header" href="#条款十六让const成员函数线程安全">条款十六：让<code>const</code>成员函数线程安全</a></h2>
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<p><strong>Item 16: Make <code>const</code> member functions thread safe</strong></p>
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<p>如果我们在数学领域中工作，我们就会发现用一个类表示多项式是很方便的。在这个类中，使用一个函数来计算多项式的根是很有用的，也就是多项式的值为零的时候。这样的一个函数它不会更改多项式。所以，它自然被声明为<code>const</code>函数。</p>
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<pre><code class="language-c++">class Polynomial {
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public:
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using RootsType = //数据结构保存多项式为零的值
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std::vector<double>; //（“using” 的信息查看条款9）
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…
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RootsType roots() const;
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…
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};
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</code></pre>
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<p>计算多项式的根是很复杂的，因此如果不需要的话，我们就不做。如果必须做，我们肯定不想再做第二次。所以，如果必须计算它们，就缓存多项式的根，然后实现<code>roots</code>来返回缓存的值。下面是最基本的实现：</p>
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<pre><code class="language-c++">class Polynomial {
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public:
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using RootsType = std::vector<double>;
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RootsType roots() const
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{
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if (!rootsAreValid) { //如果缓存不可用
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… //计算根
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//用rootVals存储它们
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rootsAreValid = true;
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}
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return rootVals;
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}
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private:
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mutable bool rootsAreValid{ false }; //初始化器（initializer）的
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mutable RootsType rootVals{}; //更多信息请查看条款7
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};
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</code></pre>
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<p>从概念上讲，<code>roots</code>并不改变它所操作的<code>Polynomial</code>对象。但是作为缓存的一部分，它也许会改变<code>rootVals</code>和<code>rootsAreValid</code>的值。这就是<code>mutable</code>的经典使用样例，这也是为什么它是数据成员声明的一部分。</p>
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<p>假设现在有两个线程同时调用<code>Polynomial</code>对象的<code>roots</code>方法:</p>
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<pre><code class="language-c++">Polynomial p;
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…
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/*------ Thread 1 ------*/ /*-------- Thread 2 --------*/
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auto rootsOfp = p.roots(); auto valsGivingZero = p.roots();
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</code></pre>
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<p>这些用户代码是非常合理的。<code>roots</code>是<code>const</code>成员函数，那就表示着它是一个读操作。在没有同步的情况下，让多个线程执行读操作是安全的。它最起码应该做到这点。在本例中却没有做到线程安全。因为在<code>roots</code>中，这些线程中的一个或两个可能尝试修改成员变量<code>rootsAreValid</code>和<code>rootVals</code>。这就意味着在没有同步的情况下，这些代码会有不同的线程读写相同的内存，这就是数据竞争（<em>data race</em>）的定义。这段代码的行为是未定义的。</p>
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<p>问题就是<code>roots</code>被声明为<code>const</code>，但不是线程安全的。<code>const</code>声明在C++11中与在C++98中一样正确（检索多项式的根并不会更改多项式的值），因此需要纠正的是线程安全的缺乏。</p>
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<p>解决这个问题最普遍简单的方法就是——使用<code>mutex</code>（互斥量）：</p>
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<pre><code class="language-c++">class Polynomial {
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public:
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using RootsType = std::vector<double>;
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RootsType roots() const
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{
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std::lock_guard<std::mutex> g(m); //锁定互斥量
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if (!rootsAreValid) { //如果缓存无效
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… //计算/存储根值
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rootsAreValid = true;
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}
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return rootsVals;
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} //解锁互斥量
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private:
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mutable std::mutex m;
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mutable bool rootsAreValid { false };
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mutable RootsType rootsVals {};
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};
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</code></pre>
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<p><code>std::mutex m</code>被声明为<code>mutable</code>，因为锁定和解锁它的都是non-<code>const</code>成员函数。在<code>roots</code>（<code>const</code>成员函数）中，<code>m</code>却被视为<code>const</code>对象。</p>
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<p>值得注意的是，因为<code>std::mutex</code>是一种只可移动类型（<em>move-only type</em>，一种可以移动但不能复制的类型），所以将<code>m</code>添加进<code>Polynomial</code>中的副作用是使<code>Polynomial</code>失去了被复制的能力。不过，它仍然可以移动。</p>
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<p>在某些情况下，互斥量的副作用显会得过大。例如，如果你所做的只是计算成员函数被调用了多少次，使用<code>std::atomic</code> 修饰的计数器（保证其他线程视它的操作为不可分割的整体，参见<a href="https://github.com/kelthuzadx/EffectiveModernCppChinese/blob/master/7.TheConcurrencyAPI/item40.md">item40</a>）通常会是一个开销更小的方法。（然而它是否轻量取决于你使用的硬件和标准库中互斥量的实现。）以下是如何使用<code>std::atomic</code>来统计调用次数。</p>
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<pre><code class="language-c++">class Point { //2D点
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public:
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…
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double distanceFromOrigin() const noexcept //noexcept的使用
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{ //参考条款14
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++callCount; //atomic的递增
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return std::sqrt((x * x) + (y * y));
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}
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private:
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mutable std::atomic<unsigned> callCount{ 0 };
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double x, y;
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};
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</code></pre>
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<p>与<code>std::mutex</code>一样，<code>std::atomic</code>是只可移动类型，所以在<code>Point</code>中存在<code>callCount</code>就意味着<code>Point</code>也是只可移动的。</p>
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<p>因为对<code>std::atomic</code>变量的操作通常比互斥量的获取和释放的消耗更小，所以你可能会过度倾向与依赖<code>std::atomic</code>。例如，在一个类中，缓存一个开销昂贵的<code>int</code>，你就会尝试使用一对<code>std::atomic</code>变量而不是互斥量。</p>
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<pre><code class="language-c++">class Widget {
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public:
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…
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int magicValue() const
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{
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if (cacheValid) return cachedValue;
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else {
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auto val1 = expensiveComputation1();
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auto val2 = expensiveComputation2();
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cachedValue = val1 + val2; //第一步
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cacheValid = true; //第二步
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return cachedValid;
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}
|
||
}
|
||
|
||
private:
|
||
mutable std::atomic<bool> cacheValid{ false };
|
||
mutable std::atomic<int> cachedValue;
|
||
};
|
||
</code></pre>
|
||
<p>这是可行的，但难以避免有时出现重复计算的情况。考虑：</p>
|
||
<ul>
|
||
<li>一个线程调用<code>Widget::magicValue</code>，将<code>cacheValid</code>视为<code>false</code>，执行这两个昂贵的计算，并将它们的和分配给<code>cachedValue</code>。</li>
|
||
<li>此时，第二个线程调用<code>Widget::magicValue</code>，也将<code>cacheValid</code>视为<code>false</code>，因此执行刚才完成的第一个线程相同的计算。（这里的“第二个线程”实际上可能是其他<strong>几个</strong>线程。）</li>
|
||
</ul>
|
||
<p>这种行为与使用缓存的目的背道而驰。将<code>cachedValue</code>和<code>CacheValid</code>的赋值顺序交换可以解决这个问题，但结果会更糟：</p>
|
||
<pre><code class="language-c++">class Widget {
|
||
public:
|
||
…
|
||
int magicValue() const
|
||
{
|
||
if (cacheValid) return cachedValue;
|
||
else {
|
||
auto val1 = expensiveComputation1();
|
||
auto val2 = expensiveComputation2();
|
||
cacheValid = true; //第一步
|
||
return cachedValue = val1 + val2; //第二步
|
||
}
|
||
}
|
||
…
|
||
}
|
||
</code></pre>
|
||
<p>假设<code>cacheValid</code>是false，那么：</p>
|
||
<ul>
|
||
<li>一个线程调用<code>Widget::magicValue</code>，刚执行完将<code>cacheValid</code>设置<code>true</code>的语句。</li>
|
||
<li>在这时，第二个线程调用<code>Widget::magicValue</code>，检查<code>cacheValid</code>。看到它是<code>true</code>，就返回<code>cacheValue</code>，即使第一个线程还没有给它赋值。因此返回的值是不正确的。</li>
|
||
</ul>
|
||
<p>这里有一个坑。对于需要同步的是单个的变量或者内存位置，使用<code>std::atomic</code>就足够了。不过，一旦你需要对两个以上的变量或内存位置作为一个单元来操作的话，就应该使用互斥量。对于<code>Widget::magicValue</code>是这样的。</p>
|
||
<pre><code class="language-c++">class Widget {
|
||
public:
|
||
…
|
||
int magicValue() const
|
||
{
|
||
std::lock_guard<std::mutex> guard(m); //锁定m
|
||
|
||
if (cacheValid) return cachedValue;
|
||
else {
|
||
auto val1 = expensiveComputation1();
|
||
auto val2 = expensiveComputation2();
|
||
cachedValue = val1 + val2;
|
||
cacheValid = true;
|
||
return cachedValue;
|
||
}
|
||
} //解锁m
|
||
…
|
||
|
||
private:
|
||
mutable std::mutex m;
|
||
mutable int cachedValue; //不再用atomic
|
||
mutable bool cacheValid{ false }; //不再用atomic
|
||
};
|
||
|
||
</code></pre>
|
||
<p>现在，这个条款是基于，多个线程可以同时在一个对象上执行一个<code>const</code>成员函数这个假设的。如果你不是在这种情况下编写一个<code>const</code>成员函数——你可以<strong>保证</strong>在一个对象上永远不会有多个线程执行该成员函数——该函数的线程安全是无关紧要的。比如，为独占单线程使用而设计的类的成员函数是否线程安全并不重要。在这种情况下，你可以避免因使用互斥量和<code>std::atomics</code>所消耗的资源，以及包含它们的类只能使用移动语义带来的副作用。然而，这种线程无关的情况越来越少见，而且很可能会越来越少。可以肯定的是，<code>const</code>成员函数应支持并发执行，这就是为什么你应该确保<code>const</code>成员函数是线程安全的。</p>
|
||
<p><strong>请记住：</strong></p>
|
||
<ul>
|
||
<li>确保<code>const</code>成员函数线程安全，除非你<strong>确定</strong>它们永远不会在并发上下文（<em>concurrent context</em>）中使用。</li>
|
||
<li>使用<code>std::atomic</code>变量可能比互斥量提供更好的性能，但是它只适合操作单个变量或内存位置。</li>
|
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