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-rw-r--r--2023/05/03/Rust-NewType-模式/index.html2
-rw-r--r--2023/05/04/C-的-Trait/index.html294
-rw-r--r--2023/05/11/C-vector-的-push-back-和-emplace-back/index.html279
-rw-r--r--2023/05/12/C-20-实现-string-split/index.html274
-rw-r--r--2023/05/24/Rust-闭包-lifetime-may-not-live-long-enough-问题/index.html2
5 files changed, 2 insertions, 849 deletions
diff --git a/2023/05/03/Rust-NewType-模式/index.html b/2023/05/03/Rust-NewType-模式/index.html
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- C++ 的 Trait
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- May 4, 2023
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- <blockquote>
-<p>C++ 的 traits 技术,是一种约定俗称的技术方案,用来为同一类数据(包括自定义数据类型和内置数据类型)提供统一的类型名(traits),这样可以统一的操作函数,例如 advance(), swap(), encode()&#x2F;decode() 等。</p>
-</blockquote>
-<p>例如,拥有义类型Foo, Bar,以及编译器自带类型 int, double, string,我们想要为这些不同的类型提供统一的编码函数 decode() 。</p>
-<p>除了使用 trait 技术之外,函数重载和模板函数 + 内置字段也可以实现,前者每增加一种数据类型就需要重新实现一个函数,而同一类数据(int, unsinged int)可以使用同样的编码方法。我们想要的是针对同一种数据类型,只编写一个函数。后者对于系统自定义变量 int, double 而言,是无法在其内部定义 type 的。</p>
-<p>traits 技术的关键在于,使用另外的模板类 type_traits 来保存不同数据类型的 type,这样就可以兼容自定义数据类型和内置数据类型:</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="comment">// 定义数据 type 类</span></span><br><span class="line"><span class="keyword">enum</span> <span class="title class_">Type</span> &#123;</span><br><span class="line"> TYPE_1,</span><br><span class="line"> TYPE_2,</span><br><span class="line"> TYPE_3</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
-
-<p>对于自定义类型,在类内部定义 type,然后在 traits 类中定义同样的 type:</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="comment">// 自定义数据类型</span></span><br><span class="line"><span class="keyword">class</span> <span class="title class_">Foo</span> &#123;</span><br><span class="line"><span class="keyword">public</span>:</span><br><span class="line"> Type type = TYPE_1;</span><br><span class="line">&#125;;</span><br><span class="line"><span class="keyword">class</span> <span class="title class_">Bar</span> &#123;</span><br><span class="line"><span class="keyword">public</span>:</span><br><span class="line"> Type type = TYPE_2;</span><br><span class="line">&#125;;</span><br><span class="line"><span class="keyword">template</span>&lt;<span class="keyword">typename</span> T&gt;</span><br><span class="line"><span class="keyword">struct</span> <span class="title class_">type_traits</span> &#123;</span><br><span class="line"> Type type = T::type;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
-
-<p>对于内置数据类型,使用模板类的特化为自定义类型生成独有的 type_traits:</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="comment">// 内置数据类型</span></span><br><span class="line"><span class="keyword">template</span>&lt;<span class="keyword">typename</span> <span class="type">int</span>&gt;</span><br><span class="line"><span class="keyword">struct</span> <span class="title class_">type_traits</span> &#123;</span><br><span class="line"> Type type = Type::TYPE_1;</span><br><span class="line">&#125;</span><br><span class="line"><span class="keyword">template</span>&lt;<span class="keyword">typename</span> <span class="type">double</span>&gt;</span><br><span class="line"><span class="keyword">struct</span> <span class="title class_">type_traits</span> &#123;</span><br><span class="line"> Type type = Type::TYPE_3;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
-
-<p>这样就可以为不同数据类型生成统一的模板函数</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></pre></td><td class="code"><pre><span class="line"><span class="comment">// 统一的编码函数</span></span><br><span class="line"><span class="keyword">template</span>&lt;<span class="keyword">typename</span> T&gt;</span><br><span class="line"><span class="type">void</span> decode&lt;<span class="type">const</span> T&amp; data, <span class="type">char</span>* buf) &#123;</span><br><span class="line"> <span class="keyword">if</span>(type_traits&lt;T&gt;::type == Type::TYPE_1) &#123;</span><br><span class="line"> ...</span><br><span class="line"> &#125;</span><br><span class="line"> <span class="keyword">else</span> <span class="keyword">if</span>(type_traits&lt;T&gt;::type == Type::TYPE_2) &#123;</span><br><span class="line"> ...</span><br><span class="line"> &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
-
-<p>总结</p>
-<ul>
-<li>traits 技术的关键在于使用第三方模板类 traits,利用模板特化的功能, 实现对自定义数据和编译器内置数据的统一</li>
-<li>这个例子使用了枚举变量来表示数据类型,而实际操作中通常使用不同的类来表示不同的类型,这样可以在编写模板函数时更好的优化。</li>
-<li>tratis 技术常见于标准库的实现中,但对日常开发中降低代码冗余也有很好的借鉴意义</li>
-<li>C++20 提供了Concept 的特性,使用Concept 可以使得实现类似的功能更加方便</li>
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- <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="comment">/// Inserts a new element at the end of the vector, right after its current last element. This new element is constructed in place using args as the arguments for its constructor.</span></span><br><span class="line"><span class="comment">/// This effectively increases the container size by one, which causes an automatic reallocation of the allocated storage space if -and only if- the new vector size surpasses the current vector capacity.</span></span><br><span class="line"><span class="comment">/// The element is constructed in-place by calling allocator_traits::construct with args forwarded.</span></span><br><span class="line"><span class="comment">///A similar member function exists, push_back, which either copies or moves an existing object into the container.</span></span><br><span class="line"><span class="keyword">template</span> &lt;<span class="keyword">class</span>... Args&gt;</span><br><span class="line"><span class="function"><span class="type">void</span> <span class="title">emplace_back</span> <span class="params">(Args&amp;&amp;... args)</span></span>;</span><br></pre></td></tr></table></figure>
-
-<p><code>push_back</code> 会构造一个临时对象,这个临时对象会被拷贝或者移入到容器中,然而 <code>emplace_back</code> 会直接根据传入的参数在容器的适当位置进行构造而避免拷贝或者移动。</p>
-<p>传统观点认为 <code>push_back</code> 会构造一个临时对象,这个临时对象会被移入到 v 中,然而 <code>emplace_back</code> 会直接根据传入的参数在适当位置进行构造而避免拷贝或者移动。从标准库代码的实现角度来说这是对的,但是对于提供了优化的编译器来讲,上面示例中最后两行表达式生成的代码其实没有区别。</p>
-<p>真正的区别在于,<code>emplace_back</code> 更加强大,它可以调用任何类型的(只要存在)构造函数。而 push_back 会更加严谨,它只调用隐式构造函数。隐式构造函数被认为是安全的。如果能够通过对象 T 隐式构造对象 U,就认为 U 能够完整包含 T 的所有内容,这样将 T 传递给 U 通常是安全的。正确使用隐式构造的例子是用 <code>std::uint32_t</code> 对象构造 <code>std::uint64_t</code> 对象,错误使用隐式构造的例子是用 <code>double</code> 构造 <code>std::uint8_t</code>。</p>
-<p>如果想要调用显示构造函数,那么就调用 <code>emplace_back</code>。如果只希望调用隐式构造函数,那么请使用更加安全的 <code>push_back</code>:</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">std::vector&lt;std::unique_ptr&lt;T&gt;&gt; v;</span><br><span class="line">T a;</span><br><span class="line">v.<span class="built_in">emplace_back</span>(std::<span class="built_in">addressof</span>(a)); <span class="comment">// compiles</span></span><br><span class="line">v.<span class="built_in">push_back</span>(std::<span class="built_in">addressof</span>(a)); <span class="comment">// fails to compile</span></span><br></pre></td></tr></table></figure>
-
-<p><code>std::unique_ptr&lt;T&gt;</code> 包含了显示构造函数通过 <code>T*</code> 进行构造。因为 <code>emplace_back</code> 能够调用显示构造函数,所以传递一个裸指针并不会产生编译错误。然而,当 v 超出了作用域,<code>std::unique_ptr&lt;T&gt;</code> 的析构函数会尝试 delete 类型 <code>T*</code> 的指针,而类型 <code>T*</code> 的指针并不是通过 new 来分配的,因为它保存的是栈对象的地址,因此 delete 行为是未定义的。</p>
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- C++ 20 实现 string split
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- May 12, 2023
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- <p>C++20引入了范围库ranges,其中提供的两个范围适配器std::split、std::lazy_split可以使我们以一种更为优雅的形式实现split:</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><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">#<span class="keyword">include</span> <span class="string">&lt;concept&gt;</span></span></span><br><span class="line"><span class="meta">#<span class="keyword">include</span> <span class="string">&lt;ranges&gt;</span></span></span><br><span class="line"><span class="meta">#<span class="keyword">include</span> <span class="string">&lt;algorithm&gt;</span></span></span><br><span class="line"><span class="meta">#<span class="keyword">include</span> <span class="string">&lt;format&gt;</span></span></span><br><span class="line"><span class="meta">#<span class="keyword">include</span> <span class="string">&lt;iostream&gt;</span></span></span><br><span class="line"></span><br><span class="line"><span class="meta">#<span class="keyword">define</span> stdr std::ranges</span></span><br><span class="line"><span class="meta">#<span class="keyword">define</span> stdrv std::ranges::views</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">template</span>&lt;<span class="keyword">template</span>&lt;<span class="keyword">typename</span>&gt; <span class="keyword">typename</span> Container = std::vector, <span class="keyword">typename</span> Arg = std::string_view&gt;</span><br><span class="line"><span class="keyword">auto</span> <span class="built_in">Split</span>(std::string_view str, std::string_view delimiter)</span><br><span class="line">&#123;</span><br><span class="line"> Container&lt;Arg&gt; myCont;</span><br><span class="line"> <span class="keyword">auto</span> temp = str </span><br><span class="line"> | stdrv::<span class="built_in">split</span>(delimiter)</span><br><span class="line"> | stdrv::<span class="built_in">transform</span>([](<span class="keyword">auto</span>&amp;&amp; r)</span><br><span class="line"> &#123;</span><br><span class="line"> <span class="keyword">return</span> <span class="built_in">Arg</span>(std::<span class="built_in">addressof</span>(*r.<span class="built_in">begin</span>()), stdr::<span class="built_in">distance</span>(r));</span><br><span class="line"> &#125;);</span><br><span class="line"> <span class="keyword">auto</span> iter = std::<span class="built_in">inserter</span>(myCont, myCont.<span class="built_in">end</span>());</span><br><span class="line"> stdr::for_each(temp, [&amp;](<span class="keyword">auto</span>&amp;&amp; x) &#123; iter = &#123;x.<span class="built_in">begin</span>(), x.<span class="built_in">end</span>()&#125;; &#125;);</span><br><span class="line"> <span class="keyword">return</span> myCont;</span><br><span class="line">&#125;</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="function"></span>&#123;</span><br><span class="line"> std::string str = <span class="string">&quot;Hello233C++20233and233New233Spilt&quot;</span>;</span><br><span class="line"> std::string delimiter = <span class="string">&quot;233&quot;</span>;</span><br><span class="line"> <span class="keyword">auto</span>&amp;&amp; strCont = <span class="built_in">Split</span>&lt;std::list, std::string&gt;(str, delimiter);</span><br><span class="line"> stdr::for_each(strCont, [](<span class="keyword">auto</span>&amp;&amp; x) &#123; std::cout &lt;&lt; std::format(<span class="string">&quot;&#123;&#125; &quot;</span>, x); &#125;);</span><br><span class="line">&#125;</span><br><span class="line"><span class="comment">//output: Hello C++20 and New Spilt</span></span><br></pre></td></tr></table></figure>
-
-<p>C++20没有提供关键的 <code>ranges::to&lt;container&gt;</code>函数,导致demo中还需要额外封装并手写for_each来写入数据,等到C++23实装了该函数,split的实现会比现在简洁优雅的多,真正做到方便泛用、无需封装:</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></pre></td><td class="code"><pre><span class="line"><span class="keyword">auto</span>&amp;&amp; strCont = str</span><br><span class="line"> | stdrv::<span class="built_in">lazy_split</span>(delimiter)</span><br><span class="line"> | stdr ::to&lt;std::vector&lt;std::string&gt;&gt;;</span><br></pre></td></tr></table></figure>
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