136 lines
15 KiB
HTML
136 lines
15 KiB
HTML
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<title>std::realloc</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_memory_c_realloc 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::</span>realloc</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-memory.html">memory</a>‎ | <a href="cpp-memory-c.html">c</a></span></div>
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<table class="t-dcl-begin">
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<div>定义于头文件 <code><a href="cpp-header-cstdlib.html"><cstdlib></a></code></div>
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<td></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span><span class="sy2">*</span> realloc<span class="br0">(</span> <span class="kw4">void</span><span class="sy2">*</span> ptr, <a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> new_size <span class="br0">)</span><span class="sy4">;</span></span></div>
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<p>重分配给定的内存区域。它必须是 <span class="t-lc"><a href="cpp-memory-c-malloc.html">std::malloc()</a></span> 、 <span class="t-lc"><a href="cpp-memory-c-calloc.html">std::calloc()</a></span> 或 <code>std::realloc()</code> 先前分配的，且仍未被 <span class="t-lc"><a href="cpp-memory-c-free.html">std::free()</a></span> 释放，否则行为未定义。</p>
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<p>重分配按以下之一进行：</p>
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<div class="t-li1"><span class="t-li">a)</span> 若可能则扩张或收缩 <code>ptr</code> 所指向的既存内存区域。区域的内容直到新旧大小的较小者都保留为更改。若区域扩张，则数组新部分的内容是未定义的。</div>
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<div class="t-li1"><span class="t-li">b)</span> 分配大小为 <code>new_size</code> 字节的新内存块，复制大小等于新旧大小较小者的内存区域，并释放旧块。</div>
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<p>若无足够内存，则不释放旧块并返回空指针。</p>
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<p>若 <code>ptr</code> 是空指针，则行为同调用 <span class="t-lc"><a href="cpp-memory-c-malloc.html">std::malloc</a></span>(<code>new_size</code>) 。</p>
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<p>若 <code>new_size</code> 为零，则行为是实现定义的：可以返回空指针（该情况下可能或可能不释放旧内存块），或可能返回某个不可用于访问存储的非空指针。<span class="t-rev-inl t-since-cxx20"><span>这种用法被弃用（经由 <a rel="nofollow" class="external text" href="http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2396.htm#dr_400">C DR 400</a> ）。</span> <span><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></span></span></p>
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<table class="t-rev-begin">
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<tr class="t-rev t-since-cxx11">
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<td>
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<p>要求下列函数是线程安全的：</p>
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<ul>
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<li><a href="cpp-memory-new-operator_new.html"><tt>operator new</tt></a> 及 <a href="cpp-memory-new-operator_delete.html"><tt>operator delete</tt></a> 的库版本</li>
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<li>全局 <a href="cpp-memory-new-operator_new.html"><tt>operator new</tt></a> 与 <a href="cpp-memory-new-operator_delete.html"><tt>operator delete</tt></a> 的用户替换版本</li>
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<li><span class="t-lc"><a href="cpp-memory-c-calloc.html">std::calloc</a></span> 、 <span class="t-lc"><a href="cpp-memory-c-malloc.html">std::malloc</a></span> 、 <span class="t-lc"><strong class="selflink">std::realloc</strong></span> <span class="t-rev-inl t-since-cxx17"><span>、 <span class="t-lc"><a href="cpp-memory-c-aligned_alloc.html">std::aligned_alloc</a></span></span> <span><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></span></span> 、 <span class="t-lc"><a href="cpp-memory-c-free.html">std::free</a></span></li>
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</ul>
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<p>对这些分配或解分配特定存储单元的函数调用以单独全序出现，并且在此顺序中，每个解分配调用<a href="cpp-atomic-memory_order.html">先发生于</a>下个分配（若存在）。</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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</tr>
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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>ptr</td>
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<td>-</td>
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<td>指向要被重分配的内存区域的指针</td>
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</tr>
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<tr class="t-par">
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<td>new_size</td>
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<td>-</td>
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<td>数组的新大小</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>成功时，返回指向新分配内存起始的指针。为避免内存泄漏，返回的必须以 <span class="t-lc"><a href="cpp-memory-c-free.html">std::free()</a></span> 解分配指针。原指针 <code>ptr</code> 被非法化，且任何对它的访问是<a href="cpp-language-ub.html">未定义行为</a>（即使重分配是原位的）。</p>
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<p>失败时，返回空指针。原指针 <code>ptr</code> 保持合法且可能需要以 <span class="t-lc"><a href="cpp-memory-c-free.html">std::free()</a></span> 解分配。</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>因为重分配可能涉及逐位复制（无论是扩展还是压缩），调用 <code>realloc</code> 后，只有<a href="cpp-named_req-TriviallyCopyable.html"><span style="font-style:italic">可平凡复制</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(TriviallyCopyable)</span></a> 类型才能安全地于内存块的保留位置访问。</p>
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<p>一些非标准库定义类型特性“可逐位移动 (BitwiseMovable) ”或“可重定位 (Relocatable) ”，这描述不拥有下列内容的类型：</p>
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<ul>
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<li>外部引用（例如链表或树的保有另一元素引用的结点），和</li>
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<li>内部引用（例如，可能保有另一成员地址的成员指针）。</li>
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</ul>
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<p>重分配其存储后，能访问这种类型的对象，即使其复制构造函数非平凡。</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 class="t-example-live-link"></div>
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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 <cstdlib></span>
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<span class="co2">#include <new></span>
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<span class="co2">#include <cassert></span>
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<span class="kw1">class</span> MallocDynamicBuffer
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<span class="br0">{</span>
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<span class="kw4">char</span><span class="sy2">*</span> p<span class="sy4">;</span>
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<span class="kw1">public</span><span class="sy4">:</span>
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<span class="kw1">explicit</span> MallocDynamicBuffer<span class="br0">(</span><a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> initial <span class="sy1">=</span> <span class="nu0">0</span><span class="br0">)</span> <span class="sy4">:</span> p<span class="br0">(</span>nullptr<span class="br0">)</span> <span class="br0">{</span>
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resize<span class="br0">(</span>initial<span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span>
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~MallocDynamicBuffer<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <a href="cpp-memory-c-free.html"><span class="kw762">std::<span class="me2">free</span></span></a><span class="br0">(</span>p<span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span>
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<span class="kw4">void</span> resize<span class="br0">(</span><a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> newSize<span class="br0">)</span> <span class="br0">{</span>
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<span class="kw1">if</span><span class="br0">(</span>newSize <span class="sy1">==</span> <span class="nu0">0</span><span class="br0">)</span> <span class="br0">{</span> <span class="co1">// 此检查并非严格需要，但零大小 realloc 于 C 中弃用 </span>
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<a href="cpp-memory-c-free.html"><span class="kw762">std::<span class="me2">free</span></span></a><span class="br0">(</span>p<span class="br0">)</span><span class="sy4">;</span>
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p <span class="sy1">=</span> nullptr<span class="sy4">;</span>
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<span class="br0">}</span> <span class="kw1">else</span> <span class="br0">{</span>
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<span class="kw1">if</span><span class="br0">(</span><span class="kw4">void</span><span class="sy2">*</span> mem <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">realloc</span><span class="br0">(</span>p, newSize<span class="br0">)</span><span class="br0">)</span>
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p <span class="sy1">=</span> <span class="kw1">static_cast</span><span class="sy1"><</span><span class="kw4">char</span><span class="sy2">*</span><span class="sy1">></span><span class="br0">(</span>mem<span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">else</span>
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<span class="kw1">throw</span> <a href="cpp-memory-new-bad_alloc.html"><span class="kw693">std::<span class="me2">bad_alloc</span></span></a><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="br0">}</span>
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<span class="kw4">char</span><span class="sy3">&</span> operator<span class="br0">[</span><span class="br0">]</span><span class="br0">(</span>size_t n<span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> p<span class="br0">[</span>n<span class="br0">]</span><span class="sy4">;</span> <span class="br0">}</span>
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<span class="kw4">char</span> operator<span class="br0">[</span><span class="br0">]</span><span class="br0">(</span>size_t n<span class="br0">)</span> <span class="kw4">const</span> <span class="br0">{</span> <span class="kw1">return</span> p<span class="br0">[</span>n<span class="br0">]</span><span class="sy4">;</span> <span class="br0">}</span>
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<span class="br0">}</span><span class="sy4">;</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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MallocDynamicBuffer buf1<span class="br0">(</span><span class="nu0">1024</span><span class="br0">)</span><span class="sy4">;</span>
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buf1<span class="br0">[</span><span class="nu0">5</span><span class="br0">]</span> <span class="sy1">=</span> <span class="st0">'f'</span><span class="sy4">;</span>
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buf1.<span class="me1">resize</span><span class="br0">(</span><span class="nu0">10</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 收缩</span>
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<a href="cpp-error-assert.html"><span class="kw773">assert</span></a><span class="br0">(</span>buf1<span class="br0">[</span><span class="nu0">5</span><span class="br0">]</span> <span class="sy1">==</span> <span class="st0">'f'</span><span class="br0">)</span><span class="sy4">;</span>
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buf1.<span class="me1">resize</span><span class="br0">(</span><span class="nu0">1024</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 增长</span>
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<a href="cpp-error-assert.html"><span class="kw773">assert</span></a><span class="br0">(</span>buf1<span class="br0">[</span><span class="nu0">5</span><span class="br0">]</span> <span class="sy1">==</span> <span class="st0">'f'</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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