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<div id="contentSub"><span class="subpages">&lt; <a href="cpp.html">cpp</a>&lrm; | <a href="cpp-numeric.html">numeric</a>&lrm; | <a href="cpp-numeric-fenv.html">fenv</a></span></div>
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<div>&#23450;&#20041;&#20110;&#22836;&#25991;&#20214; <code><a href="cpp-header-cfenv.html">&lt;cfenv&gt;</a></code></div>
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<div><span class="mw-geshi cpp source-cpp"><span class="co2">#define FE_DFL_ENV &nbsp;/*implementation defined*/</span></span></div>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 &#36215;)</span></td>
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<p>&#23439;&#24120;&#37327; <span class="t-lc"><strong class="selflink">FE_DFL_ENV</strong></span> &#23637;&#24320;&#25104; <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span> std<span class="sy4">::</span><span class="me2">fenv_t</span><span class="sy2">*</span></span></span> &#31867;&#22411;&#34920;&#36798;&#24335;&#65292;&#23427;&#25351;&#21521;&#40664;&#35748;&#28014;&#28857;&#29615;&#22659;&#65292;&#21363;&#22312;&#31243;&#24207;&#21152;&#36733;&#26102;&#30340;&#28014;&#28857;&#29615;&#22659;&#30340;&#23436;&#25972;&#21103;&#26412;&#12290;</p>
<p>&#23454;&#29616;&#20134;&#21487;&#25903;&#25345;&#38468;&#21152;&#30340;&#20197; <code>FE_</code> &#21518;&#38543;&#22823;&#20889;&#23383;&#27597;&#65292;&#24182;&#25317;&#26377; <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span> std<span class="sy4">::</span><span class="me2">fenv_t</span><span class="sy2">*</span></span></span> &#31867;&#22411;&#30340;&#23439;&#12290;</p>
<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">&#31034;&#20363;</span></h3>
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<div class="cpp source-cpp">
<pre class="de1"><span class="co2">#include &lt;iostream&gt;</span>
<span class="co2">#include &lt;cfenv&gt;</span>
<span class="co2">#pragma STDC FENV_ACCESS ON</span>
<span class="kw4">void</span> show_env<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
<span class="kw4">int</span> e <span class="sy1">=</span> <a href="cpp-numeric-fenv-fetestexcept.html"><span class="kw1451">std::<span class="me2">fetestexcept</span></span></a><span class="br0">(</span><a href="cpp-numeric-fenv-FE_exceptions.html"><span class="kw1461">FE_ALL_EXCEPT</span></a><span class="br0">)</span><span class="sy4">;</span>
<span class="kw1">if</span><span class="br0">(</span>e <span class="sy3">&amp;</span> <a href="cpp-numeric-fenv-FE_exceptions.html"><span class="kw1462">FE_DIVBYZERO</span></a><span class="br0">)</span> <a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;division by zero is raised<span class="es1">\n</span>&quot;</span><span class="sy4">;</span>
<span class="kw1">if</span><span class="br0">(</span>e <span class="sy3">&amp;</span> <a href="cpp-numeric-fenv-FE_exceptions.html"><span class="kw1463">FE_INEXACT</span></a><span class="br0">)</span> <a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;inexact is raised<span class="es1">\n</span>&quot;</span><span class="sy4">;</span>
<span class="kw1">if</span><span class="br0">(</span>e <span class="sy3">&amp;</span> <a href="cpp-numeric-fenv-FE_exceptions.html"><span class="kw1464">FE_INVALID</span></a><span class="br0">)</span> <a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;invalid is raised<span class="es1">\n</span>&quot;</span><span class="sy4">;</span>
<span class="kw1">if</span><span class="br0">(</span>e <span class="sy3">&amp;</span> <a href="cpp-numeric-fenv-FE_exceptions.html"><span class="kw1466">FE_UNDERFLOW</span></a><span class="br0">)</span> <a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;underflow is raised<span class="es1">\n</span>&quot;</span><span class="sy4">;</span>
<span class="kw1">if</span><span class="br0">(</span>e <span class="sy3">&amp;</span> <a href="cpp-numeric-fenv-FE_exceptions.html"><span class="kw1465">FE_OVERFLOW</span></a><span class="br0">)</span> <a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;overflow is raised<span class="es1">\n</span>&quot;</span><span class="sy4">;</span>
<span class="kw4">int</span> r <span class="sy1">=</span> <a href="cpp-numeric-fenv-feround.html"><span class="kw1455">std::<span class="me2">fegetround</span></span></a><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
<span class="kw1">switch</span><span class="br0">(</span>r<span class="br0">)</span>
<span class="br0">{</span>
<span class="kw1">case</span> <a href="cpp-numeric-fenv-FE_round.html"><span class="kw1467">FE_DOWNWARD</span></a><span class="sy4">:</span> <a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;rounding down<span class="es1">\n</span>&quot;</span><span class="sy4">;</span> <span class="kw1">break</span><span class="sy4">;</span>
<span class="kw1">case</span> <a href="cpp-numeric-fenv-FE_round.html"><span class="kw1468">FE_TONEAREST</span></a><span class="sy4">:</span> <a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;rounding to nearest<span class="es1">\n</span>&quot;</span><span class="sy4">;</span> <span class="kw1">break</span><span class="sy4">;</span>
<span class="kw1">case</span> <a href="cpp-numeric-fenv-FE_round.html"><span class="kw1469">FE_TOWARDZERO</span></a><span class="sy4">:</span> <a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;rounding to zero<span class="es1">\n</span>&quot;</span><span class="sy4">;</span> <span class="kw1">break</span><span class="sy4">;</span>
<span class="kw1">case</span> <a href="cpp-numeric-fenv-FE_round.html"><span class="kw1470">FE_UPWARD</span></a><span class="sy4">:</span> <a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;rounding up<span class="es1">\n</span>&quot;</span><span class="sy4">;</span> <span class="kw1">break</span><span class="sy4">;</span>
<span class="br0">}</span>
<span class="br0">}</span>
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;On startup: <span class="es1">\n</span>&quot;</span><span class="sy4">;</span>
show_env<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
<a href="cpp-numeric-fenv-feraiseexcept.html"><span class="kw1452">std::<span class="me2">feraiseexcept</span></span></a><span class="br0">(</span><a href="cpp-numeric-fenv-FE_exceptions.html"><span class="kw1466">FE_UNDERFLOW</span></a> <span class="sy3">|</span> <a href="cpp-numeric-fenv-FE_exceptions.html"><span class="kw1465">FE_OVERFLOW</span></a><span class="br0">)</span><span class="sy4">;</span>
<a href="cpp-numeric-fenv-feround.html"><span class="kw1456">std::<span class="me2">fesetround</span></span></a><span class="br0">(</span><a href="cpp-numeric-fenv-FE_round.html"><span class="kw1470">FE_UPWARD</span></a><span class="br0">)</span><span class="sy4">;</span>
<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;<span class="es1">\n</span>Before restoration: <span class="es1">\n</span>&quot;</span><span class="sy4">;</span>
show_env<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
<a href="cpp-numeric-fenv-feenv.html"><span class="kw1458">std::<span class="me2">fesetenv</span></span></a><span class="br0">(</span>FE_DFL_ENV<span class="br0">)</span><span class="sy4">;</span>
<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;<span class="es1">\n</span>After reset to default: <span class="es1">\n</span>&quot;</span><span class="sy4">;</span>
show_env<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div>
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<p>&#36755;&#20986;&#65306;</p>
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<div class="text source-text">
<pre class="de1">On startup:
rounding to nearest
Before restoration:
underflow is raised
overflow is raised
rounding up
After reset to default:
rounding to nearest</pre></div>
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<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">&#21442;&#38405;</span></h3>
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<div><a href="cpp-numeric-fenv-feenv.html"><span class="t-lines"><span>fegetenv</span><span>fesetenv</span></span></a></div>
<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div>
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<td>&#20445;&#23384;&#25110;&#24674;&#22797;&#24403;&#21069;&#28014;&#28857;&#29615;&#22659;<br />
<span class="t-mark">(&#20989;&#25968;)</span></td>
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<div><a href="cpp-numeric-fenv-feupdateenv.html"><span class="t-lines"><span>feupdateenv</span></span></a></div>
<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div>
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<td>&#24674;&#22797;&#28014;&#28857;&#29615;&#22659;&#24182;&#24341;&#21457;&#20043;&#21069;&#24341;&#21457;&#30340;&#24322;&#24120;<br />
<span class="t-mark">(&#20989;&#25968;)</span></td>
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