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<a class="post-title-link" href="/2018/10/29/more-eff-5/" itemprop="url">技术（一）</a></h1>
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<ol>
<li><p>将 constructor 和 non-member functions 虚化</p>
<ul>
<li><p>所谓 virtual constructor 是某种函数，视其获得的输入产生不同类型的对象。一种特别的是virtual copy constructor，返回一个调用者的新副本。例如：</p>
<figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br></pre></td><td class="code"><pre><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">NLComponent</span>{</span></span><br><span class="line"> <span class="keyword">public</span>:</span><br><span class="line"> <span class="function"><span class="keyword">virtual</span> NLComponent* <span class="title">clone</span><span class="params">()</span> <span class="keyword">const</span> </span>= <span class="number">0</span>;</span><br><span class="line">};</span><br><span class="line"></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">TextBlock</span>:</span> <span class="keyword">public</span> NLComponent{</span><br><span class="line"> <span class="keyword">public</span>:</span><br><span class="line"> <span class="function"><span class="keyword">virtual</span> TextBlock* <span class="title">clone</span><span class="params">()</span> <span class="keyword">const</span></span>{<span class="comment">// virtual copy constructor</span></span><br><span class="line"> <span class="keyword">return</span> <span class="keyword">new</span> TextBlock(*<span class="keyword">this</span>);</span><br><span class="line"> }</span><br><span class="line">};</span><br><span class="line"></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Graphic</span>:</span> <span class="keyword">public</span> NLComponent{</span><br><span class="line"> <span class="keyword">public</span>:</span><br><span class="line"> <span class="function"><span class="keyword">virtual</span> Graphic* <span class="title">clone</span><span class="params">()</span> <span class="keyword">const</span></span>{<span class="comment">// virtual copy constructor</span></span><br><span class="line"> <span class="keyword">return</span> <span class="keyword">new</span> Graphic(*<span class="keyword">this</span>);</span><br><span class="line"> }</span><br><span class="line">};</span><br></pre></td></tr></table></figure>
<p>这里利用了 “虚函数返回类型” 规则的宽松点，当 derived class 重新定义 base class 的虚函数时，不需要一定得声明与原本相同的返回类型。</p>
</li>
<li><p>non-member functions 的虚化方法：写一个虚函数做实际工作，再写一个什么都不做的非虚函数，调用虚函数。例如：</p>
<figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br></pre></td><td class="code"><pre><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">NLComponent</span>{</span></span><br><span class="line"> <span class="keyword">public</span>:</span><br><span class="line"> <span class="function"><span class="keyword">virtual</span> ostream& <span class="title">print</span><span class="params">(ostream& s)</span> <span class="keyword">const</span> </span>= <span class="number">0</span>;</span><br><span class="line">};</span><br><span class="line"></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">TextBlock</span>:</span> <span class="keyword">public</span> NLComponent{</span><br><span class="line"> <span class="keyword">public</span>:</span><br><span class="line"> <span class="function"><span class="keyword">virtual</span> ostream& <span class="title">print</span><span class="params">(ostream& s)</span> <span class="keyword">const</span></span>{</span><br><span class="line"> ...</span><br><span class="line"> }</span><br><span class="line">};</span><br><span class="line"></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Graphic</span>:</span> <span class="keyword">public</span> NLComponent{</span><br><span class="line"> <span class="keyword">public</span>:</span><br><span class="line"> <span class="function"><span class="keyword">virtual</span> ostream& <span class="title">print</span><span class="params">(ostream& s)</span> <span class="keyword">const</span></span>{</span><br><span class="line"> ...</span><br><span class="line"> }</span><br><span class="line">};</span><br><span class="line"><span class="keyword">inline</span> ostream& <span class="keyword">operator</span><<(ostream& s, <span class="keyword">const</span> NLComponent& c){</span><br><span class="line"> <span class="keyword">return</span> c.print(s);</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
</li>
</ul>
</li>
<li><p>限制某个 class 所能产生的对象数量</p>
<ul>
<li><p>允许零个或一个对象，构造函数私有和单例。</p>
</li>
<li><p>不同的对象构造状态：自己、派生物的 base 成分、内嵌于其它对象。</p>
</li>
<li><p>实现一个用来计算对象个数的 base class</p>
<figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">template</span> <<span class="class"><span class="keyword">class</span> <span class="title">BeingCounted</span>></span></span><br><span class="line"><span class="class"><span class="title">class</span> <span class="title">Counted</span>{</span></span><br><span class="line"> <span class="keyword">public</span>:</span><br><span class="line"> <span class="class"><span class="keyword">class</span> <span class="title">TooManyObjects</span>{</span>};</span><br><span class="line"> <span class="function"><span class="keyword">static</span> <span class="keyword">int</span> <span class="title">objectCount</span><span class="params">()</span></span>{<span class="keyword">return</span> numObjects;}</span><br><span class="line"> <span class="keyword">protected</span>:</span><br><span class="line"> Counted();</span><br><span class="line"> Counted(<span class="keyword">const</span> Counted&);</span><br><span class="line"> ~Counted(){--numObjects;}</span><br><span class="line"> <span class="keyword">private</span>:</span><br><span class="line"> <span class="keyword">static</span> <span class="keyword">int</span> numObjects;</span><br><span class="line"> <span class="keyword">static</span> <span class="keyword">const</span> <span class="keyword">size_t</span> maxObjects;</span><br><span class="line"> <span class="function"><span class="keyword">void</span> <span class="title">init</span><span class="params">()</span></span>{</span><br><span class="line"> <span class="keyword">if</span>(numObjects >= maxObjects) <span class="keyword">throw</span> TooManyObjects();</span><br><span class="line"> ++numObjects;</span><br><span class="line"> }</span><br><span class="line">};</span><br></pre></td></tr></table></figure>
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<li><p>谨记 80-20 法则</p>
<ul>
<li>一个程序 80% 的资源用于 20% 的代码身上。</li>
<li>找出瓶颈的可行之道是完全根据观察或实验来识别出那 20% 代码，而辨别之道就是借助某个程序分析器。</li>
</ul>
</li>
<li><p>考虑使用 lazy evaluation</p>
<ul>
<li>引用计数，避免非必要的对象复制</li>
<li>区分读和写，区别 operator [] 的读取和写操作</li>
<li>缓式取出，避免非必要的数据库读取动作</li>
<li>表达式缓评估，避免非必要的数值计算动作</li>
</ul>
</li>
<li><p>分期摊还预期的计算成本</p>
<ul>
<li>caching技术，将常用的数据缓存下来</li>
<li>prefetching技术，比如容器的扩容</li>
<li>本条款体现空间可以交换时间</li>
</ul>
</li>
<li><p>了解临时对象的来源</p>
<ul>
<li>C++ 真正的所谓临时对象是不可见的——不会在你的源代码中出现。只要你产生一个 non-heap object 而没有为它命名，变产生一个临时对象。</li>
<li>此等匿名对象通常发生于两种情况：一是当隐式类型转换被施行起来以求函数调用能够成功；二是但函数返回对象的时候。</li>
<li>只有当对象以 by value 方式传递，或是当对象被传递给一个 reference-to-const 参数时，这些转换才会发生。如果对象被传递给一个 reference-to-non-const 参数，并不会发生此类转换。原因是发生隐式类型转换时会产生临时对象，对引用的修改实际是修改的临时对象，而不是修改的引用，与传引用参数的目的不符。</li>
<li>任何时候只要看到一个 reference-to-const 参数，就极可能会有一个临时对象被产生出来绑定至该参数上。任何时候只要看到函数返回一个对象，就会产生临时对象。</li>
</ul>
</li>
<li><p>协助完成 “返回值优化（RVO）”</p>
<ul>
<li><p>如果函数一定得以 by-value 返回对象，你绝对无法消除之。你需要做的是努力降低被返回对象的成本。</p>
</li>
<li><p>返回所谓的 constructor arguments 以取代对象。这笔买卖似乎不见得多划算，因为还是必须为 “函数内的临时对象” 的构造和析构付出代价，还是必须为 “函数返回对象” 的构造和析构付出代价。但是 C++ 允许编译器将临时对象优化，使它们不存在，将 return 表达式所定义的对象构造于内存内。这样临时对象的总成本为 0 。可以将此函数声明为 inline。例如：</p>
<figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br></pre></td><td class="code"><pre><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Rational</span>{</span></span><br><span class="line"><span class="keyword">public</span>:</span><br><span class="line"> Rational(<span class="keyword">int</span> numerator = <span class="number">0</span>, <span class="keyword">int</span> denominator = <span class="number">1</span>);</span><br><span class="line"> <span class="function"><span class="keyword">int</span> <span class="title">numerator</span><span class="params">()</span><span class="keyword">const</span></span>;</span><br><span class="line"> <span class="function"><span class="keyword">int</span> <span class="title">denominator</span><span class="params">()</span><span class="keyword">const</span></span>;</span><br><span class="line"> </span><br><span class="line"> <span class="comment">//返回对象最有效率的做法</span></span><br><span class="line"> <span class="keyword">inline</span> <span class="keyword">const</span> Rational <span class="keyword">operator</span>*(<span class="keyword">const</span> Rational& lhs, <span class="keyword">const</span> Rational& rhs){</span><br><span class="line"> <span class="keyword">return</span> Rational(lhs.numerator() * rhs.numerator(), lhs.denominator() * rhs.denominator()); </span><br><span class="line"> }</span><br><span class="line"><span class="keyword">private</span>:</span><br><span class="line"> <span class="keyword">int</span> numerator;</span><br><span class="line"> <span class="keyword">int</span> denominator;</span><br><span class="line">};</span><br><span class="line"></span><br><span class="line">Rational a = <span class="number">10</span>;</span><br><span class="line"><span class="function">Rational <span class="title">b</span><span class="params">(<span class="number">1</span>,<span class="number">2</span>)</span></span>;</span><br><span class="line">Rational c = a * b;<span class="comment">//编译器的返回值优化</span></span><br></pre></td></tr></table></figure>
</li>
</ul>
</li>
<li><p>利用重载技术避免隐式类型转换</p>
</li>
<li><p>考虑以操作符复合形式 （op=）取代其独身形式 （op）</p>
<ul>
<li><p>可以以前者为基础实现后者。例如：</p>
<figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">template</span> <<span class="class"><span class="keyword">class</span> <span class="title">T</span>></span></span><br><span class="line"><span class="class"><span class="title">const</span> <span class="title">T</span> <span class="title">operator</span> +（<span class="title">const</span> <span class="title">T</span>& <span class="title">lhs</span>, <span class="title">const</span> <span class="title">T</span>& <span class="title">rhs</span>){</span></span><br><span class="line"> <span class="keyword">return</span> T(lhs) += rhs;<span class="comment">//返回值优化</span></span><br><span class="line">}</span><br></pre></td></tr></table></figure>
</li>
<li><p>一般而言，复合操作符比起独身版本效率高，因为独身版本通常必须返回一个新对象，因此必须负担一个临时对象的构造和析构成本。至于复合版本则是直接将结果写入其左端自变量，不需要产生一个临时对象。</p>
</li>
<li><p>允许客户在效率和便利性之间取舍。</p>
</li>
</ul>
</li>
<li><p>考虑使用其他程序库</p>
</li>
<li><p>了解 virtual functions、multiple inheritance、virtual base classes、runtime type identification 的成本</p>
</li>
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<li>利用 destructors 避免泄漏资源<ul>
<li>把资源封装在对象内，通常便可以在 exceptions 出现时避免泄漏资源。</li>
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</li>
<li>在 constructors 内阻止资源泄漏<ul>
<li>如果 constructors 中抛出异常，则对象没有完全构造好，C++ 拒绝为其调用 destructor。</li>
<li>通常只需将所有可能的 exceptions 捕捉起来，执行某种清理工作，然后重新抛出 exception，使它继续传播出去。</li>
<li>使用智能指针代替 pointer class members 也可免除 constructor 内因 exceptions 的出现而发生的资源泄漏。</li>
</ul>
</li>
<li>禁止异常流出 destructors 之外<ul>
<li>它可以避免 terminate 函数在 exception 传播过程中的栈展开机制中被调用。</li>
<li>他可以协助确保 destructors 完成其应该完成的所有事情。</li>
</ul>
</li>
<li>了解 “抛出一个 exception” 与 “传递一个参数” 或 “调用一个虚函数” 之间的差异<ul>
<li>一个对象被抛出作为 exception 时，总是会发生复制。因为对象可能离开了命名空间调用了 destructor。</li>
<li>一般而言，必须使用 throw; 语句才能重新抛出当前的 exception，它不会改变被传播的 exception 的类型。此外，它也比较有效率，因为不需要产生新的 exception object。</li>
<li>“抛出一个 exception” 与 “传递一个参数” 相比，前者会多构造一个 “被抛出物” 的副本。</li>
<li>千万不要抛出一个指向局部对象的指针，因为该局部对象会在 exception 离开其 scope 时被销毁。</li>
<li>exception 与 catch 字句相匹配过程中，仅有两种转换可以发生。一种是继承架构中的类转换，一个针对 base class exception 而编写的 catch 字句，可以处理类型为 derived class 的 exceptions。第二个允许的转换是从一个 “有型指针” 转为 “无型指针”，例如对 const void* 指针设计的 catch 字句，可捕捉所有指针类型的 exception。</li>
<li>与调用虚函数不同，虚函数采用 best fit 策略，而 exception 处理遵循 first fit 策略。catch 子句总是依出现的顺序做匹配，因此针对 derived class 设计的 catch 子句如果出现在 base class 的 catch 子句之后，则不会被调用。</li>
</ul>
</li>
<li>以 by reference 方式捕捉 exceptions<ul>
<li>catch by pointer 会面临捕捉的指针指向不存在对象的问题。</li>
<li>catch by value 则会面临复制两次的问题，一次是必定会产生的 exception object，一次是传递对象的拷贝。另一个问题是引起切割问题，derived class 被视为 base class，丧失其派生成分。</li>
<li>catch by reference 可以避免对象删除问题，也可以避开切割问题，也约束了 exception object 需被复制的次数。</li>
</ul>
</li>
<li>明智运用 exception specifications<ul>
<li>它们对希望函数抛出怎样的 exception 提供了说明，但在违反 exception specification 时会导致调用默认的unexpected，默认的 unexpected 会调用 terminate 进而调用 abort 导致程序停止。</li>
<li>它们很容易被违反，可能会妨碍更上层的 exception 处理函数。</li>
</ul>
</li>
<li>了解异常处理的成本<ul>
<li>为了能够在运行期处理 exceptions，需要成本，即使你从未使用关键词 try，throw 或 catch。</li>
<li>空间成本，放置某些数据结构，记录哪些对象已被完全构造妥当。</li>
<li>时间成本，随时保持那些数据结构的正确性。</li>
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<li><p>对定制的 ”类型转换函数“ 保持警觉</p>
<ul>
<li><p>两种函数允许编译器执行这样的转换：单自变量 constructors 和隐式类型转换操作符。例如：</p>
<figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Name</span>{</span></span><br><span class="line"> <span class="keyword">public</span>:</span><br><span class="line"> Name(<span class="keyword">const</span> <span class="built_in">string</span>& s); <span class="comment">//可以把 string 转换为 Name </span></span><br><span class="line">}；</span><br><span class="line"></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Rational</span>{</span></span><br><span class="line"> <span class="keyword">public</span>:</span><br><span class="line"> Rational(<span class="keyword">int</span> num = <span class="number">0</span>, <span class="keyword">int</span> den = <span class="number">1</span>);<span class="comment">// 可以把 int 转换为 Rational</span></span><br><span class="line">};</span><br></pre></td></tr></table></figure>
</li>
<li><p>隐式类型转换操作符解决办法比较简单，以功能对等的另一个函数取代类型转换符即可。例如</p>
<figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Rational</span>{</span></span><br><span class="line"> <span class="keyword">public</span>:</span><br><span class="line"> <span class="function"><span class="keyword">operator</span> <span class="title">double</span><span class="params">()</span> <span class="keyword">const</span></span>;<span class="comment">//将 Rational 转换为 double</span></span><br><span class="line"> <span class="comment">//替换为下边</span></span><br><span class="line"> <span class="function"><span class="keyword">double</span> <span class="title">asDouble</span><span class="params">()</span> <span class="keyword">const</span></span>;<span class="comment">//将 Rational 转换为 double</span></span><br><span class="line">};</span><br></pre></td></tr></table></figure>
</li>
<li><p>通过单自变量 constructors 完成的隐式转换较难消除，有两种解决办法：一个是简易法，另一个在编译器不支持简易法时使用。</p>
<ul>
<li><p>简易法是使用 C++ 关键字 explicit。只要将 constructors 声明为 explicit，编译器便不能因隐式转换的需要而调用它们。</p>
</li>
<li><p>隐式转换有一条规则：没有任何一个转换程序可以内含一个以上的 ”用户定制转换行为“。可以利用这项规则。例如：</p>
<figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">template</span><<span class="class"><span class="keyword">class</span> <span class="title">T</span>></span></span><br><span class="line"><span class="class"><span class="title">class</span> <span class="title">Array</span>{</span></span><br><span class="line"> <span class="keyword">public</span>:</span><br><span class="line"> <span class="class"><span class="keyword">class</span> <span class="title">ArraySize</span>{</span></span><br><span class="line"> <span class="keyword">public</span>:</span><br><span class="line"> ArraySize(<span class="keyword">int</span> n):size(n){}</span><br><span class="line"> <span class="function"><span class="keyword">int</span> <span class="title">size</span><span class="params">()</span><span class="keyword">const</span></span>{<span class="keyword">return</span> size;}</span><br><span class="line"> <span class="keyword">private</span>:</span><br><span class="line"> <span class="keyword">int</span> size;</span><br><span class="line"> };</span><br><span class="line"> Array(ArraySize size); <span class="comment">//新的构造函数</span></span><br><span class="line">};</span><br></pre></td></tr></table></figure>
<p>本例对 ArraySize class 的利用，往往被称为 proxy classes，之后再详细学习。</p>
</li>
</ul>
</li>
</ul>
</li>
<li><p>区别 increment/decrement 操作符的前置和后置形式</p>