206 lines
17 KiB
HTML
206 lines
17 KiB
HTML
<!DOCTYPE html>
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<title>std::vector<T,Allocator>::reserve</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_container_vector_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::vector<T,Allocator>::</span>reserve</h1>
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<div id="bodyContent">
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-container.html">container</a>‎ | <a href="cpp-container-vector.html">vector</a></span></div>
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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="br0">)</span><span class="sy4">;</span></span></div>
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<td class="t-dcl-nopad"></td>
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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 class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<p>增加 <code>vector</code> 的容量到大于或等于 <code>new_cap</code> 的值。若 <code>new_cap</code> 大于当前的 <span class="t-lc"><a href="cpp-container-vector-capacity.html">capacity()</a></span> ，则分配新存储，否则该方法不做任何事。</p>
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<p><code>reserve()</code> 不更改 <code>vector</code> 的 <code>size</code> 。</p>
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<p>若 <code>new_cap</code> 大于 <span class="t-lc"><a href="cpp-container-vector-capacity.html">capacity()</a></span> ，则所有迭代器，包含尾后迭代器和所有到元素的引用都被非法化。否则，没有迭代器或引用被非法化。</p>
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<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">参数</span></h3>
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<table class="t-par-begin">
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<tr class="t-par">
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<td>new_cap</td>
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<td>-</td>
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<td>vector 的新容量</td>
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<tr class="t-par-hitem">
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<td colspan="3">类型要求</td>
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<tr class="t-par-req">
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<td colspan="3">-
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<div class="spacer"></div>
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<code>T</code> 必须满足<a href="cpp-named_req-MoveInsertable.html"><span style="font-style:italic">可移动插入</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(MoveInsertable)</span></a> 的要求。</td>
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</tr>
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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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<ul>
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<li>若 <span class="t-c"><span class="mw-geshi cpp source-cpp">new_cap <span class="sy1">></span> max_size<span class="br0">(</span><span class="br0">)</span></span></span> 则为 <span class="t-lc"><a href="cpp-error-length_error.html">std::length_error</a></span> 。</li>
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<li>任何 <code>Allocator::allocate()</code> 所抛的异常（典型为 <span class="t-lc"><a href="cpp-memory-new-bad_alloc.html">std::bad_alloc</a></span> ）</li>
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</ul>
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<p>若抛出异常，则此函数无效果（<a href="cpp-language-exceptions.html">强异常保证</a>）。</p>
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<p>若 <code>T</code> 的移动构造函数不是 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">noexcept</span></span></span> 且 T 非<a href="cpp-named_req-CopyInsertable.html"><span style="font-style:italic">可复制插入</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(CopyInsertable)</span></a> 到 <code>*this</code> ，则 vector 将使用移动构造函数。若它抛出，则摒弃保证，且效果未指定。</p>
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</td>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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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>至多与容器的 <span class="t-lc"><a href="cpp-container-vector-size.html">size()</a></span> 成线性。</p>
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<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">注解</span></h3>
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<p>正确使用 <code>reserve()</code> 能避免不必要的分配，但不适当地使用 <code>reserve()</code> （例如在每次 <span class="t-lc"><a href="cpp-container-vector-push_back.html">push_back()</a></span> 调用前调用它）可能会实际增加重分配的数量（通过导致容量线性而非指数增长）并导致计算复杂度增加，性能下降。例如，按引用接受任意 <code>vector</code> 并后附元素的函数通常<i>不</i>应在 <code>vector</code> 上调用 <code>reserve()</code> ，因为它不知道该 <code>vector</code> 的使用特征。</p>
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<p>插入范围时， <span class="t-lc"><a href="cpp-container-vector-insert.html">insert()</a></span> 的范围版本通常更合适，因为它保持正确的容量增长行为，而不同于 <code>reserve()</code> 后随一系列 <span class="t-lc"><a href="cpp-container-vector-push_back.html">push_back()</a></span> 。</p>
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<p>不能用 <code>reserve()</code> 减少容器容量。为该目的提供的是 <span class="t-lc"><a href="cpp-container-vector-shrink_to_fit.html">shrink_to_fit()</a></span> 。</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 <cstddef></span>
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<span class="co2">#include <new></span>
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<span class="co2">#include <vector></span>
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<span class="co2">#include <iostream></span>
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<span class="co1">// 带调试输出的最小 C++11 分配器</span>
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<span class="kw1">template</span> <span class="sy1"><</span><span class="kw1">class</span> Tp<span class="sy1">></span>
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<span class="kw1">struct</span> NAlloc <span class="br0">{</span>
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<span class="kw1">typedef</span> Tp value_type<span class="sy4">;</span>
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NAlloc<span class="br0">(</span><span class="br0">)</span> <span class="sy1">=</span> <span class="kw1">default</span><span class="sy4">;</span>
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<span class="kw1">template</span> <span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span> NAlloc<span class="br0">(</span><span class="kw4">const</span> NAlloc<span class="sy1"><</span>T<span class="sy1">></span><span class="sy3">&</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
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Tp<span class="sy2">*</span> allocate<span class="br0">(</span><a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> n<span class="br0">)</span>
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<span class="br0">{</span>
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n <span class="sy2">*</span><span class="sy1">=</span> sizeof<span class="br0">(</span>Tp<span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"allocating "</span> <span class="sy1"><<</span> n <span class="sy1"><<</span> <span class="st0">" bytes<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="kw1">return</span> <span class="kw1">static_cast</span><span class="sy1"><</span>Tp<span class="sy2">*</span><span class="sy1">></span><span class="br0">(</span><span class="sy4">::</span><a href="cpp-memory-new-operator_new.html"><span class="kw687">operator new</span></a><span class="br0">(</span>n<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw4">void</span> deallocate<span class="br0">(</span>Tp<span class="sy2">*</span> p, <a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> n<span class="br0">)</span>
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<span class="br0">{</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"deallocating "</span> <span class="sy1"><<</span> n<span class="sy2">*</span>sizeof<span class="br0">(</span>Tp<span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">" bytes<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="sy4">::</span><a href="cpp-memory-new-operator_delete.html"><span class="kw689">operator delete</span></a><span class="br0">(</span>p<span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">template</span> <span class="sy1"><</span><span class="kw1">class</span> T, <span class="kw1">class</span> U<span class="sy1">></span>
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<span class="kw4">bool</span> operator<span class="sy1">==</span><span class="br0">(</span><span class="kw4">const</span> NAlloc<span class="sy1"><</span>T<span class="sy1">></span><span class="sy3">&</span>, <span class="kw4">const</span> NAlloc<span class="sy1"><</span>U<span class="sy1">></span><span class="sy3">&</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="kw2">true</span><span class="sy4">;</span> <span class="br0">}</span>
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<span class="kw1">template</span> <span class="sy1"><</span><span class="kw1">class</span> T, <span class="kw1">class</span> U<span class="sy1">></span>
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<span class="kw4">bool</span> operator<span class="sy3">!</span><span class="sy1">=</span><span class="br0">(</span><span class="kw4">const</span> NAlloc<span class="sy1"><</span>T<span class="sy1">></span><span class="sy3">&</span>, <span class="kw4">const</span> NAlloc<span class="sy1"><</span>U<span class="sy1">></span><span class="sy3">&</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="kw2">false</span><span class="sy4">;</span> <span class="br0">}</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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<span class="kw4">int</span> sz <span class="sy1">=</span> <span class="nu0">100</span><span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"using reserve: <span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="br0">{</span>
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<a href="cpp-container-vector.html"><span class="kw1269">std::<span class="me2">vector</span></span></a><span class="sy1"><</span><span class="kw4">int</span>, NAlloc<span class="sy1"><</span><span class="kw4">int</span><span class="sy1">>></span> v1<span class="sy4">;</span>
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v1.<span class="me1">reserve</span><span class="br0">(</span>sz<span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">for</span><span class="br0">(</span><span class="kw4">int</span> n <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> n <span class="sy1"><</span> sz<span class="sy4">;</span> <span class="sy2">++</span>n<span class="br0">)</span>
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v1.<span class="me1">push_back</span><span class="br0">(</span>n<span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"not using reserve: <span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="br0">{</span>
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<a href="cpp-container-vector.html"><span class="kw1269">std::<span class="me2">vector</span></span></a><span class="sy1"><</span><span class="kw4">int</span>, NAlloc<span class="sy1"><</span><span class="kw4">int</span><span class="sy1">>></span> v1<span class="sy4">;</span>
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<span class="kw1">for</span><span class="br0">(</span><span class="kw4">int</span> n <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> n <span class="sy1"><</span> sz<span class="sy4">;</span> <span class="sy2">++</span>n<span class="br0">)</span>
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v1.<span class="me1">push_back</span><span class="br0">(</span>n<span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="br0">}</span></pre></div>
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</div>
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<p>可能的输出：</p>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="text source-text">
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<pre class="de1">using reserve:
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allocating 400 bytes
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deallocating 400 bytes
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not using reserve:
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allocating 4 bytes
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allocating 8 bytes
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deallocating 4 bytes
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allocating 16 bytes
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deallocating 8 bytes
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allocating 32 bytes
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deallocating 16 bytes
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allocating 64 bytes
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deallocating 32 bytes
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allocating 128 bytes
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deallocating 64 bytes
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allocating 256 bytes
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deallocating 128 bytes
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allocating 512 bytes
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deallocating 256 bytes
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deallocating 512 bytes</pre></div>
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</div>
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</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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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-container-vector-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>返回当前存储空间能够容纳的元素数<br />
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<span class="t-mark">(公开成员函数)</span></td>
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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-container-vector-max_size.html"><span class="t-lines"><span>max_size</span></span></a></div>
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</div>
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</td>
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<td>返回可容纳的最大元素数<br />
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<span class="t-mark">(公开成员函数)</span></td>
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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-container-vector-resize.html"><span class="t-lines"><span>resize</span></span></a></div>
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</div>
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</td>
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<td>改变容器中可存储元素的个数<br />
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<span class="t-mark">(公开成员函数)</span></td>
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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-container-vector-shrink_to_fit.html"><span class="t-lines"><span>shrink_to_fit</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div>
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</div>
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</td>
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<td>通过释放未使用的内存减少内存的使用<br />
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<span class="t-mark">(公开成员函数)</span></td>
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</tr>
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</table>
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</div>
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<div class="visualClear"></div>
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</body>
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