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<!DOCTYPE html>
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<title>富贵山庄</title>
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<p class="site-subtitle" itemprop="description">恒者行远，思者常新</p>
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<link itemprop="mainEntityOfPage" href="https://witgao.com/2023/08/20/av/Web%E7%A1%AC%E4%BB%B6%E7%BC%96%E8%A7%A3%E7%A0%81-WebCodec/">
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<a href="/2023/08/20/av/Web%E7%A1%AC%E4%BB%B6%E7%BC%96%E8%A7%A3%E7%A0%81-WebCodec/" class="post-title-link" itemprop="url">Web硬件编解码-WebCodec</a>
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<span class="post-meta-item-text">发表于</span>
<time title="创建时间：2023-08-20 00:00:00" itemprop="dateCreated datePublished" datetime="2023-08-20T00:00:00+08:00">2023-08-20</time>
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<a href="/categories/%E9%9F%B3%E8%A7%86%E9%A2%91/" itemprop="url" rel="index"><span itemprop="name">音视频</span></a>
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<span class="post-meta-item-text">本文字数：</span>
<span>864</span>
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<span class="post-meta-item-text">阅读时长 ≈</span>
<span>3 分钟</span>
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<div class="post-body" itemprop="articleBody">
<p>WebCodec 是一个较新的 Web API，它为网页应用提供了接近硬件级的编解码能力。使用 WebCodec，开发者可以在浏览器中直接对音频和视频数据进行高效编解码，这对于实现复杂的媒体应用（如实时视频通信、视频编辑和游戏流）非常有用。</p>
<h1 id="WebCodec-编解码流程"><a href="#WebCodec-编解码流程" class="headerlink" title="WebCodec 编解码流程"></a>WebCodec 编解码流程</h1><h2 id="解码"><a href="#解码" class="headerlink" title="解码"></a>解码</h2><h3 id="创建解码器"><a href="#创建解码器" class="headerlink" title="创建解码器"></a>创建解码器</h3><h4 id="设置解码器名称"><a href="#设置解码器名称" class="headerlink" title="设置解码器名称"></a>设置解码器名称</h4><p>WebCodec设置上参数与其它平台不同的一点，在于其名称字段比较繁琐</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><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br><span class="line">51</span><br><span class="line">52</span><br><span class="line">53</span><br><span class="line">54</span><br><span class="line">55</span><br><span class="line">56</span><br><span class="line">57</span><br><span class="line">58</span><br><span class="line">59</span><br><span class="line">60</span><br><span class="line">61</span><br><span class="line">62</span><br><span class="line">63</span><br><span class="line">64</span><br><span class="line">65</span><br><span class="line">66</span><br><span class="line">67</span><br><span class="line">68</span><br><span class="line">69</span><br><span class="line">70</span><br><span class="line">71</span><br></pre></td><td class="code"><pre><span class="line"> <span class="keyword">if</span> (VideoType == H264)</span><br><span class="line"> {</span><br><span class="line"> ...</span><br><span class="line"> std::stringstream stream;</span><br><span class="line"> stream << <span class="string">"avc1."</span>;</span><br><span class="line"> stream << <span class="built_in">decimalToHex</span>(profile_idc);</span><br><span class="line"> stream << <span class="built_in">decimalToHex</span>(constraint_set_flags);</span><br><span class="line"> stream << <span class="built_in">decimalToHex</span>(level_idc);</span><br><span class="line"> codecName = stream.<span class="built_in">str</span>();</span><br><span class="line"> <span class="comment">// codecName = "avc1.640020";</span></span><br><span class="line"> }<span class="keyword">else</span> <span class="keyword">if</span>(VideoType == H265)</span><br><span class="line">{</span><br><span class="line"> std::stringstream stream;</span><br><span class="line"> stream << <span class="string">"hev1."</span>;</span><br><span class="line"> <span class="keyword">switch</span> (general_profile_space) {</span><br><span class="line"> <span class="keyword">case</span> <span class="number">0</span>:</span><br><span class="line"> <span class="keyword">break</span>;</span><br><span class="line"> <span class="keyword">case</span> <span class="number">1</span>:</span><br><span class="line"> stream << <span class="string">'A'</span>;</span><br><span class="line"> <span class="keyword">break</span>;</span><br><span class="line"> <span class="keyword">case</span> <span class="number">2</span>:</span><br><span class="line"> stream << <span class="string">'B'</span>;</span><br><span class="line"> <span class="keyword">break</span>;</span><br><span class="line"> <span class="keyword">case</span> <span class="number">3</span>:</span><br><span class="line"> stream << <span class="string">'C'</span>;</span><br><span class="line"> <span class="keyword">break</span>;</span><br><span class="line"> }</span><br><span class="line"> stream << general_profile_idc;</span><br><span class="line"> stream << <span class="string">'.'</span>;</span><br><span class="line"> </span><br><span class="line"> MUInt32 val = (MUInt32)general_profile_compatibility_flag;</span><br><span class="line"> <span class="type">int</span> reversed = <span class="number">0</span>;</span><br><span class="line"> <span class="keyword">for</span> (<span class="type">int</span> i = <span class="number">0</span>; i < <span class="number">32</span>; i++)</span><br><span class="line"> {</span><br><span class="line"> reversed |= val & <span class="number">1</span>;</span><br><span class="line"> <span class="keyword">if</span> (i == <span class="number">31</span>)</span><br><span class="line"> <span class="keyword">break</span>;</span><br><span class="line"> reversed <<= <span class="number">1</span>;</span><br><span class="line"> val >>= <span class="number">1</span>;</span><br><span class="line"> }</span><br><span class="line"> stream << <span class="built_in">decimalToHex</span>(reversed, <span class="number">0</span>);</span><br><span class="line"> stream << <span class="string">'.'</span>;</span><br><span class="line"> <span class="keyword">if</span> (general_tier_flag == <span class="number">0</span>)</span><br><span class="line"> {</span><br><span class="line"> stream << <span class="string">'L'</span>;</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">else</span></span><br><span class="line"> {</span><br><span class="line"> stream << <span class="string">'H'</span>;</span><br><span class="line"> }</span><br><span class="line"> stream << general_level_idc;</span><br><span class="line"> MBool hasByte = <span class="literal">false</span>;</span><br><span class="line"></span><br><span class="line"> std::string constraint_string = <span class="string">""</span>;</span><br><span class="line"> <span class="keyword">for</span> (<span class="type">int</span> i = <span class="number">5</span>; i >= <span class="number">0</span>; i--)</span><br><span class="line"> {</span><br><span class="line"> <span class="type">int</span> val = <span class="number">0</span>;</span><br><span class="line"> <span class="keyword">if</span> (general_constraint_indicator_flags[i] != <span class="string">'\0'</span>)</span><br><span class="line"> {</span><br><span class="line"> val = general_constraint_indicator_flags[i];</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">if</span> (val != <span class="number">0</span> || hasByte)</span><br><span class="line"> {</span><br><span class="line"> constraint_string = <span class="string">"."</span> + <span class="built_in">decimalToHex</span>(val, <span class="number">0</span>) + constraint_string;</span><br><span class="line"> hasByte = <span class="literal">true</span>;</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> stream << constraint_string;</span><br><span class="line"> codecName = stream.<span class="built_in">str</span>();</span><br><span class="line"> <span class="comment">// codecName = "hev1.1.6.L120.90";</span></span><br><span class="line">}</span><br></pre></td></tr></table></figure>
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<h3 id="std-declval-的作用"><a href="#std-declval-的作用" class="headerlink" title="std::declval 的作用"></a><code>std::declval</code> 的作用</h3><p><code>std::declval</code> 是 C++ 标准库中的一个工具，用于在不创建对象的情况下获得一个类型的引用。其主要作用是在编译时期在表达式中使用一个类型的实例而不实际构造对象。这在模板元编程和 SFINAE 中尤其有用，因为它允许我们在编译时对类型的潜在成员进行推断和检查。</p>
<p><code>std::declval</code> 在 <code><utility></code> 头文件中定义，并且仅在 unevaluated context（未求值上下文，例如 sizeof 和 decltype 中）是合法的，这意味着它不能用于在运行时创建对象。</p>
<h3 id="std-declval-的使用场景"><a href="#std-declval-的使用场景" class="headerlink" title="std::declval 的使用场景"></a><code>std::declval</code> 的使用场景</h3><h4 id="1-类型推导"><a href="#1-类型推导" class="headerlink" title="1. 类型推导"></a>1. 类型推导</h4><p>最常见的场景是在使用 <code>decltype</code> 来推导表达式的类型时，尤其是当你需要推导一个类的成员函数的返回类型，但又不想或不能创建这个类的实例。</p>
<p>例如，下面的代码展示了如何使用 <code>std::declval</code> 来推导成员函数的返回类型：</p>
<figure class="highlight cpp"><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></pre></td><td class="code"><pre><span class="line"><span class="meta">#<span class="keyword">include</span> <span class="string"><utility></span></span></span><br><span class="line"></span><br><span class="line"><span class="keyword">struct</span> <span class="title class_">MyClass</span> {</span><br><span class="line"> <span class="function"><span class="type">int</span> <span class="title">myFunction</span><span class="params">()</span> <span class="type">const</span> </span>{ <span class="keyword">return</span> <span class="number">42</span>; }</span><br><span class="line">};</span><br><span class="line"></span><br><span class="line"><span class="comment">// 不需要创建 MyClass 的实例</span></span><br><span class="line"><span class="keyword">decltype</span>(std::<span class="built_in">declval</span><MyClass>().<span class="built_in">myFunction</span>()) myVar;</span><br></pre></td></tr></table></figure>
<p>在这里，<code>decltype</code> 用来确定 <code>myFunction</code> 的返回类型，而 <code>std::declval<MyClass>()</code> “假装” 有一个 <code>MyClass</code> 的对象来允许我们调用成员函数。</p>
<h4 id="2-SFINAE-和类型萃取"><a href="#2-SFINAE-和类型萃取" class="headerlink" title="2. SFINAE 和类型萃取"></a>2. SFINAE 和类型萃取</h4><p>在 SFINAE（替换失败并非错误）或写类型萃取（trait）时，<code>std::declval</code> 能够确保在没有默认构造函数的情况下依然能进行操作。</p>
<figure class="highlight cpp"><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></pre></td><td class="code"><pre><span class="line"><span class="meta">#<span class="keyword">include</span> <span class="string"><type_traits></span></span></span><br><span class="line"><span class="meta">#<span class="keyword">include</span> <span class="string"><utility></span></span></span><br><span class="line"></span><br><span class="line"><span class="keyword">struct</span> <span class="title class_">MyClass</span> {</span><br><span class="line"> <span class="keyword">using</span> value_type = <span class="type">int</span>;</span><br><span class="line"> <span class="function">value_type <span class="title">myFunction</span><span class="params">()</span> <span class="type">const</span> </span>{ <span class="keyword">return</span> <span class="number">42</span>; }</span><br><span class="line">};</span><br><span class="line"></span><br><span class="line"><span class="comment">// 一个用于萃取成员函数返回类型的类型萃取</span></span><br><span class="line"><span class="keyword">template</span><<span class="keyword">typename</span> T></span><br><span class="line"><span class="keyword">struct</span> <span class="title class_">ReturnType</span> {</span><br><span class="line"> <span class="keyword">using</span> type = <span class="keyword">decltype</span>(std::<span class="built_in">declval</span><T>().<span class="built_in">myFunction</span>());</span><br><span class="line">};</span><br><span class="line"></span><br><span class="line"><span class="built_in">static_assert</span>(std::is_same<ReturnType<MyClass>::type, <span class="type">int</span>>::value, <span class="string">"Type must be int"</span>);</span><br></pre></td></tr></table></figure>
<p>在这个例子中，即使 <code>MyClass</code> 没有默认构造函数，我们也能用 <code>std::declval</code> 来进行编译时检查。</p>
<h4 id="3-重载决策"><a href="#3-重载决策" class="headerlink" title="3. 重载决策"></a>3. 重载决策</h4><p>在某些复杂的重载决策场景中，你可能希望根据类型是否支持某个操作来选择不同的函数重载。<code>std::declval</code> 可以在不创建对象的情况下 “假设” 该对象的存在，以便进行这样的检查。</p>
<figure class="highlight cpp"><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><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">#<span class="keyword">include</span> <span class="string"><iostream></span></span></span><br><span class="line"><span class="meta">#<span class="keyword">include</span> <span class="string"><type_traits></span></span></span><br><span class="line"><span class="meta">#<span class="keyword">include</span> <span class="string"><utility></span></span></span><br><span class="line"></span><br><span class="line"><span class="keyword">struct</span> <span class="title class_">A</span> {};</span><br><span class="line"></span><br><span class="line"><span class="keyword">struct</span> <span class="title class_">B</span> {</span><br><span class="line"> <span class="function"><span class="type">void</span> <span class="title">print</span><span class="params">()</span> <span class="type">const</span> </span>{ std::cout << <span class="string">"B::print"</span> << std::endl; }</span><br><span class="line">};</span><br><span class="line"></span><br><span class="line"><span class="keyword">template</span><<span class="keyword">typename</span> T></span><br><span class="line"><span class="keyword">typename</span> std::enable_if<std::is_member_function_pointer<<span class="keyword">decltype</span>(&T::print)>::value>::<span class="function">type</span></span><br><span class="line"><span class="function"><span class="title">testPrint</span><span class="params">(T&& t)</span> </span>{</span><br><span class="line"> t.<span class="built_in">print</span>();</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">template</span><<span class="keyword">typename</span> T></span></span><br><span class="line"><span class="function"><span class="type">void</span> <span class="title">testPrint</span><span class="params">(T&&)</span> </span>{</span><br><span class="line"> std::cout << <span class="string">"No print function"</span> << std::endl;</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="type">int</span> <span class="title">main</span><span class="params">()</span> </span>{</span><br><span class="line"> A a;</span><br><span class="line"> B b;</span><br><span class="line"> <span class="built_in">testPrint</span>(a); <span class="comment">// 输出 "No print function"</span></span><br><span class="line"> <span class="built_in">testPrint</span>(b); <span class="comment">// 输出 "B::print"</span></span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>这里 <code>std::declval</code> 在编译时帮助确定 <code>T</code> 类型是否有一个名为 <code>print</code> 的成员函数。</p>
<h3 id="小结"><a href="#小结" class="headerlink" title="小结"></a>小结</h3><p><code>std::declval</code> 是模板编程中一个极为有用的工具，它能够让程序员在不实例化对象的情况下引用任意类型，特别是在仅仅需要类型信息而不需要对象的场合。它通常与 <code>decltype</code> 结合使用，以便在编译时对表达式类型进行操作和推导。由于 <code>std::declval</code> 仅在未求值上下文中有效，因此它</p>
<p>主要用于模板元编程和编译时类型推导，而不是在实际运行时代码中创建对象。</p>
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<h3 id="什么是-SFINAE"><a href="#什么是-SFINAE" class="headerlink" title="什么是 SFINAE?"></a>什么是 SFINAE?</h3><p>SFINAE 是 “Substitution Failure Is Not An Error” 的缩写。这是 C++ 模板元编程中的一个重要概念，它允许在模板类型推导过程中，如果某个替换失败，这个失败不会立即导致编译错误。而是使得编译器简单地放弃这次替换，并尝试其他重载或模板特化。</p>
<h3 id="SFINAE-的用途"><a href="#SFINAE-的用途" class="headerlink" title="SFINAE 的用途"></a>SFINAE 的用途</h3><p>SFINAE 最常见的用途包括：</p>
<ul>
<li><strong>编写类型特征</strong>：检查类型是否具有某个成员函数、成员类型或操作符。</li>
<li><strong>函数模板重载决策</strong>：在多个函数模板重载之间选择最适合的一个。</li>
<li><strong>类模板特化</strong>：在多个类模板特化之间选择最适合的一个。</li>
</ul>
<h3 id="如何使用-SFINAE"><a href="#如何使用-SFINAE" class="headerlink" title="如何使用 SFINAE"></a>如何使用 SFINAE</h3><p>在 C++11 及其之后的版本中，SFINAE 最常见的实现方式是使用 <code>decltype</code>、<code>std::enable_if</code> 和类型特征（如 <code>std::is_integral</code> 等）。下面是一些使用 SFINAE 的例子：</p>
<h4 id="例子-1：使用-std-enable-if-禁用某些函数重载"><a href="#例子-1：使用-std-enable-if-禁用某些函数重载" class="headerlink" title="例子 1：使用 std::enable_if 禁用某些函数重载"></a>例子 1：使用 <code>std::enable_if</code> 禁用某些函数重载</h4><figure class="highlight cpp"><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="meta">#<span class="keyword">include</span> <span class="string"><type_traits></span></span></span><br><span class="line"><span class="meta">#<span class="keyword">include</span> <span class="string"><iostream></span></span></span><br><span class="line"></span><br><span class="line"><span class="keyword">template</span><<span class="keyword">typename</span> T></span><br><span class="line"><span class="keyword">typename</span> std::enable_if<std::is_integral<T>::value, T>::<span class="function">type</span></span><br><span class="line"><span class="function"><span class="title">foo</span><span class="params">(T t)</span> </span>{</span><br><span class="line"> std::cout << <span class="string">"Integral version: "</span> << t << std::endl;</span><br><span class="line"> <span class="keyword">return</span> t;</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="keyword">template</span><<span class="keyword">typename</span> T></span><br><span class="line"><span class="keyword">typename</span> std::enable_if<std::is_floating_point<T>::value, T>::<span class="function">type</span></span><br><span class="line"><span class="function"><span class="title">foo</span><span class="params">(T t)</span> </span>{</span><br><span class="line"> std::cout << <span class="string">"Floating point version: "</span> << t << std::endl;</span><br><span class="line"> <span class="keyword">return</span> t;</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="type">int</span> <span class="title">main</span><span class="params">()</span> </span>{</span><br><span class="line"> <span class="built_in">foo</span>(<span class="number">10</span>); <span class="comment">// Calls the integral version</span></span><br><span class="line"> <span class="built_in">foo</span>(<span class="number">3.14</span>); <span class="comment">// Calls the floating point version</span></span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>在这个例子中，<code>foo</code> 函数模板有两个重载。通过 <code>std::enable_if</code>，我们能够让编译器在模板类型是整型时选择第一个重载，在模板类型是浮点型时选择第二个重载。</p>
<h4 id="例子-2：检查类型是否有成员函数"><a href="#例子-2：检查类型是否有成员函数" class="headerlink" title="例子 2：检查类型是否有成员函数"></a>例子 2：检查类型是否有成员函数</h4><figure class="highlight cpp"><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><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">#<span class="keyword">include</span> <span class="string"><type_traits></span></span></span><br><span class="line"><span class="meta">#<span class="keyword">include</span> <span class="string"><iostream></span></span></span><br><span class="line"></span><br><span class="line"><span class="keyword">template</span><<span class="keyword">typename</span>, <span class="keyword">typename</span> T></span><br><span class="line"><span class="keyword">struct</span> <span class="title class_">has_serialize</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="keyword">template</span><<span class="keyword">typename</span> C, <span class="keyword">typename</span> Ret, <span class="keyword">typename</span>... Args></span><br><span class="line"><span class="keyword">struct</span> <span class="title class_">has_serialize</span><C, <span class="built_in">Ret</span>(Args...)> {</span><br><span class="line"><span class="keyword">private</span>:</span><br><span class="line"> <span class="comment">// 如果C类型中，包含serialize函数，并且参数符合Args...，则匹配这条规则</span></span><br><span class="line"> <span class="function"><span class="keyword">template</span><<span class="keyword">typename</span> T></span></span><br><span class="line"><span class="function"> <span class="type">static</span> <span class="keyword">constexpr</span> <span class="keyword">auto</span> <span class="title">check</span><span class="params">(T*)</span></span></span><br><span class="line"><span class="function"> -> <span class="keyword">typename</span> std::is_same<</span></span><br><span class="line"><span class="function"> <span class="title">decltype</span><span class="params">(std::declval<T>().serialize(std::declval<Args>()...))</span>,</span></span><br><span class="line"><span class="function"> Ret </span></span><br><span class="line"><span class="function"> >::type</span>; <span class="comment">// 当返回类型为auto时，在参数后通过->来指定返回类型</span></span><br><span class="line"> </span><br><span class="line"> <span class="comment">// 否则匹配这条规则</span></span><br><span class="line"> <span class="function"><span class="keyword">template</span><<span class="keyword">typename</span>></span></span><br><span class="line"><span class="function"> <span class="type">static</span> <span class="keyword">constexpr</span> std::false_type <span class="title">check</span><span class="params">(...)</span></span>;</span><br><span class="line"> </span><br><span class="line"> <span class="comment">// 得到C类型check后的结果类型，false_type or true_type</span></span><br><span class="line"> <span class="function"><span class="keyword">typedef</span> <span class="title">decltype</span><span class="params">(check<C>(<span class="number">0</span>))</span> type</span>;</span><br><span class="line"></span><br><span class="line"><span class="keyword">public</span>:</span><br><span class="line"> <span class="type">static</span> <span class="keyword">constexpr</span> <span class="type">bool</span> value = type::value;</span><br><span class="line">};</span><br><span class="line"></span><br><span class="line"><span class="keyword">struct</span> <span class="title class_">MyType</span> {</span><br><span class="line"> <span class="function"><span class="type">void</span> <span class="title">serialize</span><span class="params">(<span class="type">int</span>)</span> </span>{}</span><br><span class="line">};</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="type">int</span> <span class="title">main</span><span class="params">()</span> </span>{</span><br><span class="line"> std::cout << has_serialize<MyType, <span class="built_in">void</span>(<span class="type">int</span>)>::value << std::endl; </span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>在这个例子中，我们使用模板特化和 <code>decltype</code> 来检查 <code>MyType</code> 是否有一个接受 <code>int</code> 参数的 <code>serialize</code> 成员函数，<code>decltype</code>检测函数返回类型时，函数只需声明即可，无需实现。</p>
<h3 id="C-17-中的-SFINAE-友好的特性"><a href="#C-17-中的-SFINAE-友好的特性" class="headerlink" title="C++17 中的 SFINAE 友好的特性"></a>C++17 中的 SFINAE 友好的特性</h3><p>C++17 引入了 <code>if constexpr</code> 和 <code>auto</code> 类型推导的新特性，让 SFINAE 使用起来更为方便。</p>
<h4 id="例子-3：使用-if-constexpr"><a href="#例子-3：使用-if-constexpr" class="headerlink" title="例子 3：使用 if constexpr"></a>例子 3：使用 <code>if constexpr</code></h4><figure class="highlight cpp"><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></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">template</span><<span class="keyword">typename</span> T></span></span><br><span class="line"><span class="function"><span class="keyword">auto</span> <span class="title">foo</span><span class="params">(T t)</span> </span>{</span><br><span class="line"> <span class="function"><span class="keyword">if</span> <span class="title">constexpr</span> <span class="params">(std::is_integral_v<T>)</span> </span>{</span><br><span class="line"> std::cout << <span class="string">"Integral version: "</span> << t << std::endl;</span><br><span class="line"> } <span class="keyword">else</span> <span class="keyword">if</span> <span class="built_in">constexpr</span> (std::is_floating_point_v<T>) {</span><br><span class="line"> std::cout << <span class="string">"Floating point version: "</span> << t << std::endl;</span><br><span class="line"> } <span class="keyword">else</span> {</span><br><span class="line"> <span class="built_in">static_assert</span>(<span class="literal">false</span>, <span class="string">"Unsupported type"</span>);</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>这个例子演示了如何在函数模板内部使用 `</p>
<p>if constexpr` 来在编译时选择执行路径，这样可以在不生成错误的情况下根据类型条件编译代码。</p>
<h3 id="小结"><a href="#小结" class="headerlink" title="小结"></a>小结</h3><p>SFINAE 是 C++ 模板编程中一项强大的技术，可以用来在编译时根据类型特性来选择不同的代码路径。虽然 SFINAE 可以使代码变得非常灵活，但它也可能使得代码难以阅读和维护。随着 C++ 标准的发展，引入了更简洁的特性如 <code>if constexpr</code> 和概念（C++20 中的 concepts），它们可以使得之前需要 SFINAE 来实现的功能变得更简单和直观。</p>
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<h1 id="const"><a href="#const" class="headerlink" title="const"></a>const</h1><h2 id="修饰变量"><a href="#修饰变量" class="headerlink" title="修饰变量"></a>修饰变量</h2><p>const关键字可以用来防止对象发生变化。一个const对象必须有以下特征:</p>
<ul>
<li>必须已经初始化</li>
<li>不能被修改</li>
<li>是线程安全的</li>
<li>只能调用const成员函数</li>
</ul>
<h3 id="值"><a href="#值" class="headerlink" title="值"></a>值</h3><figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="type">const</span> <span class="type">int</span> ci = <span class="number">1</span>;</span><br></pre></td></tr></table></figure>
<h3 id="指针"><a href="#指针" class="headerlink" title="指针"></a>指针</h3><h4 id="常量指针"><a href="#常量指针" class="headerlink" title="常量指针"></a>常量指针</h4><p>指向常量的指针，简称常量指针。</p>
<figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="type">const</span> <span class="type">int</span> ci = <span class="number">1</span>;</span><br><span class="line"><span class="type">const</span> <span class="type">int</span>* pci = &ci;</span><br></pre></td></tr></table></figure>
<h4 id="指针常量"><a href="#指针常量" class="headerlink" title="指针常量"></a>指针常量</h4><p>指针本身是一个常量。</p>
<figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="type">int</span> i = <span class="number">1</span>;</span><br><span class="line"><span class="type">int</span> <span class="type">const</span>* cpi = &i;</span><br></pre></td></tr></table></figure>
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<h1 id="为什么要了解类型推断？"><a href="#为什么要了解类型推断？" class="headerlink" title="为什么要了解类型推断？"></a>为什么要了解类型推断？</h1><p><img src="/2022/11/24/c++/c++%E7%B1%BB%E5%9E%8B%E6%8E%A8%E6%96%AD/zhihu_c++50.png"></p>
<h2 id="C-98"><a href="#C-98" class="headerlink" title="C++98"></a>C++98</h2><p>在c++98的年代，只有模板才有类型推断，如果你不使用模板，那么你完全不需要理解类型推断。</p>
<h2 id="C-11"><a href="#C-11" class="headerlink" title="C++11"></a>C++11</h2><p>在C++11中，类型推断在模板之外被大量应用。</p>
<ul>
<li>auto</li>
<li>lambda captures</li>
<li>function return</li>
<li>decltype</li>
</ul>
<p>想要熟练的使用上述新特性，那么就必须要了解类型推断的规则。</p>
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<h1 id="片段着色器（Fragment-Shader）"><a href="#片段着色器（Fragment-Shader）" class="headerlink" title="片段着色器（Fragment Shader）"></a>片段着色器（Fragment Shader）</h1><p>片段着色器的主要目的是计算每个像素应该输出什么颜色，在片段着色器中可以接收到顶点着色器输出的变量。</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></pre></td><td class="code"><pre><span class="line"><span class="meta">#version 330 core</span></span><br><span class="line">out vec4 FragColor;</span><br><span class="line"></span><br><span class="line">in vec3 ourColor;</span><br><span class="line">in vec2 TexCoord;</span><br><span class="line"></span><br><span class="line"><span class="comment">// texture sampler</span></span><br><span class="line">uniform sampler2D texture1;</span><br><span class="line"></span><br><span class="line"><span class="type">void</span> <span class="title function_">main</span><span class="params">()</span></span><br><span class="line">{</span><br><span class="line"> FragColor = texture(texture1, TexCoord);</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
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<h1 id="顶点"><a href="#顶点" class="headerlink" title="顶点"></a>顶点</h1><p>顶点用于图元装配，OpenGL支持的图元有三种：点、线、三角形，对于点只需要一个顶点，对于三角形需要三个顶点。</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></pre></td><td class="code"><pre><span class="line"><span class="type">float</span> vertices[] = {</span><br><span class="line"> <span class="number">-0.5f</span>, <span class="number">-0.5f</span>, <span class="number">0.0f</span>, <span class="comment">// left </span></span><br><span class="line"> <span class="number">0.5f</span>, <span class="number">-0.5f</span>, <span class="number">0.0f</span>, <span class="comment">// right </span></span><br><span class="line"> <span class="number">0.0f</span>, <span class="number">0.5f</span>, <span class="number">0.0f</span> <span class="comment">// top </span></span><br><span class="line">}; </span><br></pre></td></tr></table></figure>
<h2 id="顶点坐标"><a href="#顶点坐标" class="headerlink" title="顶点坐标"></a>顶点坐标</h2><p>OpenGL不是简单的把所有的3D坐标变换成屏幕上的2D像素，仅当3D坐标在3个轴上-1.0到1.0的范围内时才处理它，所有在这个范围内的坐标叫做标准化</p>
<p><img src="/opengl-ndc.png" alt="NDC"><br>(0,0)是坐标的中心，y轴正方向向上，x轴正方向向右。</p>
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