152 lines
9.1 KiB
HTML
152 lines
9.1 KiB
HTML
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<title>std::basic_string<CharT,Traits,Allocator>::reserve</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_string_basic_string_reserve skin-cppreference2 action-view cpp-navbar">
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::basic_string<CharT,Traits,Allocator>::</span>reserve</h1>
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-string.html">string</a>‎ | <a href="cpp-string-basic_string.html">basic string</a></span></div>
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<td rowspan="142">(1)</td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span> reserve<span class="br0">(</span> size_type new_cap <span class="sy1">=</span> <span class="nu0">0</span> <span class="br0">)</span><span class="sy4">;</span></span></div>
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<td><span class="t-mark-rev t-until-cxx20">(C++20 前)</span></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span> <span class="kw4">void</span> reserve<span class="br0">(</span> size_type new_cap <span class="br0">)</span><span class="sy4">;</span></span></div>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span> reserve<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span></span></div>
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</td>
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<td>(2)</td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span><br />
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<span class="t-mark">(弃用)</span></td>
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<div class="t-li1"><span class="t-li">1)</span> 告诉 <code>std::basic_string</code> 对象大小的有计划更改，使得它能准确地管理存储分配。</div>
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<dd>
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<li>若 <code>new_cap</code> 大于当前 <span class="t-lc"><a href="cpp-string-basic_string-capacity.html">capacity()</a></span> ，则分配新存储，并令 <span class="t-lc"><a href="cpp-string-basic_string-capacity.html">capacity()</a></span> 大于或等于 <code>new_cap</code> 。</li>
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<li>若 <code>new_cap</code> 小于当前 <span class="t-lc"><a href="cpp-string-basic_string-capacity.html">capacity()</a></span> ，则这是非强制的收缩请求。</li>
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<li>若 <code>new_cap</code> 小于当前 <span class="t-lc"><a href="cpp-string-basic_string-size.html">size()</a></span> ，则这是非强制的收缩到适合 (shrink-to-fit) 请求<span class="t-rev-inl t-since-cxx11"><span>，等价于 <span class="t-lc"><a href="cpp-string-basic_string-shrink_to_fit.html">shrink_to_fit()</a></span></span> <span><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></span></span>。</li>
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</dd>
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<td><span class="t-mark-rev t-until-cxx20">(C++20 前)</span></td>
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<li>若 <code>new_cap</code> 小于或等于当前 <span class="t-lc"><a href="cpp-string-basic_string-capacity.html">capacity()</a></span> ，则无效果。</li>
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</dd>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<div class="t-li1"><span class="t-li"></span> 若发生容量更改，则非法化所有迭代器与引用，包含尾后迭代器。</div>
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<div class="t-li1"><span class="t-li">2)</span> 以无参数调用 <code>reserve</code> 是非强制的收缩到适合请求。此调用后， <span class="t-lc"><a href="cpp-string-basic_string-capacity.html">capacity()</a></span> 拥有大于或等于 <span class="t-lc"><a href="cpp-string-basic_string-size.html">size()</a></span> 的未指定值。</div>
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</td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">参数</span></h3>
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<td>new_cap</td>
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<td>-</td>
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<td>string 的新容量</td>
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</tr>
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</table>
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<h3><span class="mw-headline" id=".E8.BF.94.E5.9B.9E.E5.80.BC">返回值</span></h3>
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<p>（无）</p>
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<h3><span class="mw-headline" id=".E5.BC.82.E5.B8.B8">异常</span></h3>
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<p>若 <code>new_cap</code> 大于 <span class="t-lc"><a href="cpp-string-basic_string-max_size.html">max_size()</a></span> 则抛出 <span class="t-lc"><a href="cpp-error-length_error.html">std::length_error</a></span></p>
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<p>可能抛出任何 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-memory-allocator_traits.html"><span class="kw701">std::<span class="me2">allocator_traits</span></span></a><span class="sy1"><</span>Allocator<span class="sy1">></span><span class="sy4">::</span><span class="me2">allocate</span><span class="br0">(</span><span class="br0">)</span></span></span> 所抛的异常，如 <span class="t-lc"><a href="cpp-memory-new-bad_alloc.html">std::bad_alloc</a></span> 。</p>
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<h3><span class="mw-headline" id=".E5.A4.8D.E6.9D.82.E5.BA.A6">复杂度</span></h3>
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<p>至多与 string 的 <span class="t-lc"><a href="cpp-string-basic_string-size.html">size()</a></span> 成线性</p>
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<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
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<div class="t-example">
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="cpp source-cpp">
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<pre class="de1"><span class="co2">#include <cassert></span>
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<span class="co2">#include <string></span>
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<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
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<span class="br0">{</span>
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<a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> s<span class="sy4">;</span>
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std<span class="sy4">::</span><span class="me2">string</span><span class="sy4">::</span><span class="me2">size_type</span> new_capacity<span class="br0">{</span> 100u <span class="br0">}</span><span class="sy4">;</span>
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<a href="cpp-error-assert.html"><span class="kw774">assert</span></a><span class="br0">(</span>new_capacity <span class="sy1">></span> s.<span class="me1">capacity</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
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s.<span class="me1">reserve</span><span class="br0">(</span>new_capacity<span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-error-assert.html"><span class="kw774">assert</span></a><span class="br0">(</span>new_capacity <span class="sy1"><=</span> s.<span class="me1">capacity</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span></pre></div>
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<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
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<table class="t-dsc-begin">
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<div class="t-dsc-member-div">
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<div><a href="cpp-string-basic_string-capacity.html"><span class="t-lines"><span>capacity</span></span></a></div>
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</div>
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</td>
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<td>返回当前对象分配的存储空间能保存的字符数量<br />
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<span class="t-mark">(公开成员函数)</span></td>
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