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Visit website Recent posts Updated on $DATE date c++17 Zola2026-07-05T00:00:00+00:00https://shaozk.xyz/tags/c-17/atom.xml 2026-07-05T00:00:00+00:00 2026-07-05T00:00:00+00:00 shaozk https://shaozk.xyz/blog/003-use-string-view-instead-of-string/ <p>C++17 中增加了 std::string_view 新特性，表示非占有数据的字符序列。非占有数据如何理解？</p> <p>主要包含三个关键点：</p> <ol> <li> <p>可以表征字符序列</p> <p>C-Style string 和 C++-Style std::string ，都可以用 std:string_view 表示，默认会进行隐式转换。</p> </li> <li> <p>不占有数据存储空间</p> <p>只存储指向原始数据的指针和长度，不会增加数据拷贝的额外负担。</p> </li> <li> <p>无法对数据进行修改</p> <p>你绝对无法修改其数据内容，这也是名字 <code>view</code> 的由来之一。如果你需要对字符串进行修改，那就不该使用它。</p> </li> </ol> <p>有了这些特性，仅仅使用 std::string_view 作为函数形参，就可以实现类似常量引用的效果，不再需要额外的 const 和 reference 修饰符。</p> <pre class="giallo z-code"><code data-lang="cpp"><span class="giallo-l"><span class="z-punctuation z-definition z-comment z-comment">// old</span></span> <span class="giallo-l"><span class="z-storage z-type">void</span><span class="z-entity z-name z-function"> print_str</span><span class="z-punctuation">(</span><span class="z-storage z-modifier">const</span><span class="z-entity z-name z-scope-resolution z-parameter z-cpp"> std</span><span class="z-punctuation">::</span><span class="z-entity z-name z-type">string</span><span class="z-storage z-modifier z-reference z-cpp">&amp;</span><span class="z-variable z-parameter"> str</span><span class="z-punctuation">) {</span></span> <span class="giallo-l"><span class="z-entity z-name z-scope-resolution z-cpp"> std</span><span class="z-punctuation">::</span><span class="z-source">cout </span><span class="z-keyword z-operator">&lt;&lt;</span><span class="z-source"> str </span><span class="z-keyword z-operator">&lt;&lt;</span><span class="z-entity z-name z-scope-resolution z-cpp"> std</span><span class="z-punctuation">::</span><span class="z-source">endl</span><span class="z-punctuation">;</span></span> <span class="giallo-l"><span class="z-punctuation">}</span></span> <span class="giallo-l"></span> <span class="giallo-l"><span class="z-punctuation z-definition z-comment z-comment">// now</span></span> <span class="giallo-l"><span class="z-storage z-type">void</span><span class="z-entity z-name z-function"> print_sv</span><span class="z-punctuation">(</span><span class="z-entity z-name z-scope-resolution z-parameter z-cpp">std</span><span class="z-punctuation">::</span><span class="z-entity z-name z-type">string_view</span><span class="z-variable z-parameter"> sv</span><span class="z-punctuation">) {</span></span> <span class="giallo-l"><span class="z-entity z-name z-scope-resolution z-cpp"> std</span><span class="z-punctuation">::</span><span class="z-source">cout </span><span class="z-keyword z-operator">&lt;&lt;</span><span class="z-source"> str </span><span class="z-keyword z-operator">&lt;&lt;</span><span class="z-entity z-name z-scope-resolution z-cpp"> std</span><span class="z-punctuation">::</span><span class="z-source">endl</span><span class="z-punctuation">;</span></span> <span class="giallo-l"><span class="z-punctuation">}</span></span> <span class="giallo-l"></span></code></pre> <p>然和在《Effective C++》中学到的经验告诉我们尽量 pass-by-reference 而不是 pass-by-value ，当然前提是传值的开销远大于参数参数时。那么 std::string_view 实例对象占用了多大空间呢？</p> <p>在两大开源编译器 g++ 和 clang++ 的实现中，std:string_view 的数据成员仅有两个，一个是指向字符序列的常量指针，另一个是序列大小。</p> <pre class="giallo z-code"><code data-lang="cpp"><span class="giallo-l"><span class="z-punctuation z-definition z-comment z-comment">// gnu g++</span></span> <span class="giallo-l"><span class="z-storage z-type">private</span><span class="z-punctuation">:</span></span> <span class="giallo-l"><span class="z-storage z-type"> size_t</span><span class="z-source"> _M_len</span><span class="z-punctuation">;</span></span> <span class="giallo-l"><span class="z-storage z-modifier"> const</span><span class="z-source"> _CharT</span><span class="z-keyword z-operator">*</span><span class="z-source"> _M_str</span><span class="z-punctuation">;</span></span> <span class="giallo-l"></span> <span class="giallo-l"><span class="z-punctuation z-definition z-comment z-comment">// llvm clang++</span></span> <span class="giallo-l"><span class="z-storage z-type">private</span><span class="z-punctuation">:</span></span> <span class="giallo-l"><span class="z-source"> const_pointer data_</span><span class="z-punctuation">;</span><span class="z-punctuation z-definition z-comment z-comment"> // exposition only</span></span> <span class="giallo-l"><span class="z-source"> size_type size_</span><span class="z-punctuation">;</span><span class="z-punctuation z-definition z-comment z-comment"> // exposition only</span></span></code></pre> <p>在 64 位机器上，仅占用 128 bit(16 bytes) 的空间。pass-by-value 和 pass-by-reference 的开销相差无几。</p> <p>另外，std::string_view 支持哈希，可以直接在 std::unordered_map 及其他字典容器中作为 key 使用。</p> <h1 id="xue-xi">学习</h1> <p>https://en.cppreference.com/cpp/string/basic_string_view https://www.learncpp.com/cpp-tutorial/introduction-to-stdstring_view/ https://learnmoderncpp.com/2024/10/29/when-to-use-stdstring_view/ https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n3921.html</p>