1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>Examples</title> 5<link rel="stylesheet" href="../../math.css" type="text/css"> 6<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 7<link rel="home" href="../../index.html" title="Math Toolkit 2.12.0"> 8<link rel="up" href="../fp_facets.html" title="Facets for Floating-Point Infinities and NaNs"> 9<link rel="prev" href="reference.html" title="Reference"> 10<link rel="next" href="portability.html" title="Portability"> 11</head> 12<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 13<table cellpadding="2" width="100%"><tr> 14<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../boost.png"></td> 15<td align="center"><a href="../../../../../../index.html">Home</a></td> 16<td align="center"><a href="../../../../../../libs/libraries.htm">Libraries</a></td> 17<td align="center"><a 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class="phrase"><a name="math_toolkit.fp_facets.examples.simple_example_with_std_stringst"></a></span><a class="link" href="examples.html#math_toolkit.fp_facets.examples.simple_example_with_std_stringst">Simple 32 example with std::stringstreams</a> 33 </h6> 34<pre class="programlisting"><span class="identifier">locale</span> <span class="identifier">old_locale</span><span class="special">;</span> 35<span class="identifier">locale</span> <span class="identifier">tmp_locale</span><span class="special">(</span><span class="identifier">old_locale</span><span class="special">,</span> <span class="keyword">new</span> <span class="identifier">nonfinite_num_put</span><span class="special"><</span><span class="keyword">char</span><span class="special">>);</span> 36<span class="identifier">locale</span> <span class="identifier">new_locale</span><span class="special">(</span><span class="identifier">tmp_locale</span><span class="special">,</span> <span class="keyword">new</span> <span class="identifier">nonfinite_num_get</span><span class="special"><</span><span class="keyword">char</span><span class="special">>);</span> 37</pre> 38<pre class="programlisting"><span class="identifier">stringstream</span> <span class="identifier">ss</span><span class="special">;</span> 39<span class="identifier">ss</span><span class="special">.</span><span class="identifier">imbue</span><span class="special">(</span><span class="identifier">new_locale</span><span class="special">);</span> 40<span class="keyword">double</span> <span class="identifier">inf</span> <span class="special">=</span> <span class="identifier">numeric_limits</span><span class="special"><</span><span class="keyword">double</span><span class="special">>::</span><span class="identifier">infinity</span><span class="special">();</span> 41<span class="identifier">ss</span> <span class="special"><<</span> <span class="identifier">inf</span><span class="special">;</span> <span class="comment">// Write out.</span> 42<span class="identifier">BOOST_ASSERT</span><span class="special">(</span><span class="identifier">ss</span><span class="special">.</span><span class="identifier">str</span><span class="special">()</span> <span class="special">==</span> <span class="string">"inf"</span><span class="special">);</span> 43<span class="keyword">double</span> <span class="identifier">r</span><span class="special">;</span> 44<span class="identifier">ss</span> <span class="special">>></span> <span class="identifier">r</span><span class="special">;</span> <span class="comment">// Read back in.</span> 45<span class="identifier">BOOST_ASSERT</span><span class="special">(</span><span class="identifier">inf</span> <span class="special">==</span> <span class="identifier">r</span><span class="special">);</span> <span class="comment">// Confirms that the double values really are identical.</span> 46 47<span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"infinity output was "</span> <span class="special"><<</span> <span class="identifier">ss</span><span class="special">.</span><span class="identifier">str</span><span class="special">()</span> <span class="special"><<</span> <span class="identifier">endl</span><span class="special">;</span> 48<span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"infinity input was "</span> <span class="special"><<</span> <span class="identifier">r</span> <span class="special"><<</span> <span class="identifier">endl</span><span class="special">;</span> 49<span class="comment">// But the string representation of r displayed will be the native type</span> 50<span class="comment">// because, when it was constructed, cout had NOT been imbued</span> 51<span class="comment">// with the new locale containing the nonfinite_numput facet.</span> 52<span class="comment">// So the cout output will be "1.#INF on MS platforms</span> 53<span class="comment">// and may be "inf" or other string representation on other platforms.</span> 54</pre> 55<h6> 56<a name="math_toolkit.fp_facets.examples.h1"></a> 57 <span class="phrase"><a name="math_toolkit.fp_facets.examples.use_with_lexical_cast"></a></span><a class="link" href="examples.html#math_toolkit.fp_facets.examples.use_with_lexical_cast">Use with 58 lexical_cast</a> 59 </h6> 60<div class="note"><table border="0" summary="Note"> 61<tr> 62<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../../doc/src/images/note.png"></td> 63<th align="left">Note</th> 64</tr> 65<tr><td align="left" valign="top"><p> 66 From Boost 1.48, lexical_cast no longer uses stringstreams internally, 67 and is now able to handle infinities and NaNs natively on most platforms. 68 </p></td></tr> 69</table></div> 70<p> 71 Without using a new locale that contains the nonfinite facets, previous versions 72 of <code class="computeroutput"><span class="identifier">lexical_cast</span></code> using stringstream 73 were not portable (and often failed) if nonfinite values are found. 74 </p> 75<pre class="programlisting"><span class="identifier">locale</span> <span class="identifier">old_locale</span><span class="special">;</span> 76<span class="identifier">locale</span> <span class="identifier">tmp_locale</span><span class="special">(</span><span class="identifier">old_locale</span><span class="special">,</span> <span class="keyword">new</span> <span class="identifier">nonfinite_num_put</span><span class="special"><</span><span class="keyword">char</span><span class="special">>);</span> 77<span class="identifier">locale</span> <span class="identifier">new_locale</span><span class="special">(</span><span class="identifier">tmp_locale</span><span class="special">,</span> <span class="keyword">new</span> <span class="identifier">nonfinite_num_get</span><span class="special"><</span><span class="keyword">char</span><span class="special">>);</span> 78</pre> 79<p> 80 Although other examples imbue individual streams with the new locale, for 81 the streams constructed inside lexical_cast, it was necessary to assign to 82 a global locale. 83 </p> 84<pre class="programlisting"><span class="identifier">locale</span><span class="special">::</span><span class="identifier">global</span><span class="special">(</span><span class="identifier">new_locale</span><span class="special">);</span> 85</pre> 86<p> 87 <code class="computeroutput"><span class="identifier">lexical_cast</span></code> then works as 88 expected, even with infinity and NaNs. 89 </p> 90<pre class="programlisting"><span class="keyword">double</span> <span class="identifier">x</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">lexical_cast</span><span class="special"><</span><span class="keyword">double</span><span class="special">>(</span><span class="string">"inf"</span><span class="special">);</span> 91<span class="identifier">assert</span><span class="special">(</span><span class="identifier">x</span> <span class="special">==</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric</span><span class="special">:</span><span class="identifier">limits</span><span class="special"><</span><span class="keyword">double</span><span class="special">>::</span><span class="identifier">infinity</span><span class="special">());</span> 92 93<span class="identifier">string</span> <span class="identifier">s</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">lexical_cast</span><span class="special"><</span><span class="identifier">string</span><span class="special">>(</span><span class="identifier">numeric_limits</span><span class="special"><</span><span class="keyword">double</span><span class="special">>::</span><span class="identifier">infinity</span><span class="special">());</span> 94<span class="identifier">assert</span><span class="special">(</span><span class="identifier">s</span> <span class="special">==</span> <span class="string">"inf"</span><span class="special">);</span> 95</pre> 96<div class="warning"><table border="0" summary="Warning"> 97<tr> 98<td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../../../../../doc/src/images/warning.png"></td> 99<th align="left">Warning</th> 100</tr> 101<tr><td align="left" valign="top"><p> 102 If you use stringstream inside your functions, you may still need to use 103 a global locale to handle nonfinites correctly. Or you need to imbue your 104 stringstream with suitable get and put facets. 105 </p></td></tr> 106</table></div> 107<div class="warning"><table border="0" summary="Warning"> 108<tr> 109<td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../../../../../doc/src/images/warning.png"></td> 110<th align="left">Warning</th> 111</tr> 112<tr><td align="left" valign="top"><p> 113 You should be aware that the C++ specification does not explicitly require 114 that input from decimal digits strings converts with rounding to the nearest 115 representable floating-point binary value. (In contrast, decimal digits 116 read by the compiler, for example by an assignment like <code class="computeroutput"><span class="keyword">double</span> 117 <span class="identifier">d</span> <span class="special">=</span> 118 <span class="number">1.234567890123456789</span></code>, are guaranteed 119 to assign the nearest representable value to double d). This implies that, 120 no matter how many decimal digits you provide, there is a potential uncertainty 121 of 1 least significant bit in the resulting binary value. 122 </p></td></tr> 123</table></div> 124<p> 125 See <a href="http://en.wikipedia.org/wiki/Floating_point#Representable_numbers.2C_conversion_and_rounding" target="_top">conversion 126 and rounding</a> for more information on <span class="emphasis"><em>nearest representable</em></span> 127 and <span class="emphasis"><em>rounding</em></span> and <a href="http://www.exploringbinary.com/" target="_top">Exploring 128 Binary</a> for much detail on input and round-tripping difficulties. 129 </p> 130<p> 131 Most iostream libraries do in fact achieve the desirable <span class="emphasis"><em>nearest 132 representable floating-point binary value</em></span> for all values of input. 133 However one popular STL library does not quite achieve this for 64-bit doubles. 134 See <a href="http://connect.microsoft.com/VisualStudio/feedback/details/98770/decimal-digit-string-input-to-double-may-be-1-bit-wrong" target="_top">Decimal 135 digit string input to double may be 1 bit wrong</a> for the bizarre full 136 details. 137 </p> 138<p> 139 If you are expecting to 'round-trip' <code class="computeroutput"><span class="identifier">lexical_cast</span></code> 140 or <code class="computeroutput"><span class="identifier">serialization</span></code>, for example 141 archiving and loading, and want to be <span class="bold"><strong>absolutely certain 142 that you will always get an exactly identical double value binary pattern</strong></span>, 143 you should use the suggested 'workaround' below that is believed to work 144 on all platforms. 145 </p> 146<p> 147 You should output using all potentially significant decimal digits, by setting 148 stream precision to <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special"><</span><span class="keyword">double</span><span class="special">>::</span><span class="identifier">max_digits10</span></code>, (or for the appropriate floating-point 149 type, if not double) and crucially, <span class="bold"><strong>require <code class="computeroutput"><span class="identifier">scientific</span></code> format</strong></span>, not <code class="computeroutput"><span class="identifier">fixed</span></code> or automatic (default), for example: 150 </p> 151<pre class="programlisting"><span class="keyword">double</span> <span class="identifier">output_value</span> <span class="special">=</span> <span class="identifier">any</span> <span class="identifier">value</span><span class="special">;</span> 152<span class="identifier">std</span><span class="special">::</span><span class="identifier">stringstream</span> <span class="identifier">s</span><span class="special">;</span> 153<span class="identifier">s</span> <span class="special"><<</span> <span class="identifier">setprecison</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special"><</span><span class="keyword">double</span><span class="special">>::</span><span class="identifier">max_digits10</span><span class="special">)</span> <span class="special"><<</span> <span class="identifier">scientific</span> <span class="special"><<</span> <span class="identifier">output_value</span><span class="special">;</span> 154<span class="identifier">s</span> <span class="special">>></span> <span class="identifier">input_value</span><span class="special">;</span> 155</pre> 156<h5> 157<a name="math_toolkit.fp_facets.examples.h2"></a> 158 <span class="phrase"><a name="math_toolkit.fp_facets.examples.use_with_serialization_archives"></a></span><a class="link" href="examples.html#math_toolkit.fp_facets.examples.use_with_serialization_archives">Use 159 with serialization archives</a> 160 </h5> 161<p> 162 It is vital that the same locale is used when an archive is saved and when 163 it is loaded. Otherwise, loading the archive may fail. By default, archives 164 are saved and loaded with a classic C locale with a <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">archive</span><span class="special">::</span><span class="identifier">codecvt_null</span></code> 165 facet added. Normally you do not have to worry about that. 166 </p> 167<p> 168 The constructors for the archive classes, as a side-effect, imbue the stream 169 with such a locale. However, if you want to use the facets <code class="computeroutput"><span class="identifier">nonfinite_num_put</span></code> and <code class="computeroutput"><span class="identifier">nonfinite_num_get</span></code> 170 with archives, then you have to manage the locale manually. That is done 171 by calling the archive constructor with the flag <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">archive</span><span class="special">::</span><span class="identifier">no_codecvt</span></code>, 172 thereby ensuring that the archive constructor will <span class="bold"><strong>not 173 imbue the stream with a new locale</strong></span>. 174 </p> 175<p> 176 The following code shows how to use <code class="computeroutput"><span class="identifier">nonfinite_num_put</span></code> 177 with a <code class="computeroutput"><span class="identifier">text_oarchive</span></code>. 178 </p> 179<pre class="programlisting"><span class="identifier">locale</span> <span class="identifier">default_locale</span><span class="special">(</span><span class="identifier">locale</span><span class="special">::</span><span class="identifier">classic</span><span class="special">(),</span> <span class="keyword">new</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">archive</span><span class="special">::</span><span class="identifier">codecvt_null</span><span class="special"><</span><span class="keyword">char</span><span class="special">>);</span> 180<span class="identifier">locale</span> <span class="identifier">my_locale</span><span class="special">(</span><span class="identifier">default_locale</span><span class="special">,</span> <span class="keyword">new</span> <span class="identifier">nonfinite_num_put</span><span class="special"><</span><span class="keyword">char</span><span class="special">>);</span> 181 182<span class="identifier">ofstream</span> <span class="identifier">ofs</span><span class="special">(</span><span class="string">"test.txt"</span><span class="special">);</span> 183<span class="identifier">ofs</span><span class="special">.</span><span class="identifier">imbue</span><span class="special">(</span><span class="identifier">my_locale</span><span class="special">);</span> 184 185<span class="identifier">boost</span><span class="special">::</span><span class="identifier">archive</span><span class="special">::</span><span class="identifier">text_oarchive</span> <span class="identifier">oa</span><span class="special">(</span><span class="identifier">ofs</span><span class="special">,</span> <span class="identifier">no_codecvt</span><span class="special">);</span> 186 187<span class="keyword">double</span> <span class="identifier">x</span> <span class="special">=</span> <span class="identifier">numeric_limits</span><span class="special"><</span><span class="keyword">double</span><span class="special">>::</span><span class="identifier">infinity</span><span class="special">();</span> 188<span class="identifier">oa</span> <span class="special">&</span> <span class="identifier">x</span><span class="special">;</span> 189</pre> 190<p> 191 The same method works with <code class="computeroutput"><span class="identifier">nonfinite_num_get</span></code> 192 and <code class="computeroutput"><span class="identifier">text_iarchive</span></code>. 193 </p> 194<p> 195 If you use the <code class="computeroutput"><span class="identifier">nonfinite_num_put</span></code> 196 with <code class="computeroutput"><span class="identifier">trap_infinity</span></code> and/or 197 <code class="computeroutput"><span class="identifier">trap_nan</span></code> flag with a serialization 198 archive, then you must set the exception mask of the stream. Serialization 199 archives do not check the stream state. 200 </p> 201<h6> 202<a name="math_toolkit.fp_facets.examples.h3"></a> 203 <span class="phrase"><a name="math_toolkit.fp_facets.examples.other_examples"></a></span><a class="link" href="examples.html#math_toolkit.fp_facets.examples.other_examples">Other 204 examples</a> 205 </h6> 206<p> 207 <a href="../../../../example/nonfinite_facet_simple.cpp" target="_top">nonfinite_facet_simple.cpp</a> 208 give some more simple demonstrations of the difference between using classic 209 C locale and constructing a C99 infinity and NaN compliant locale for input 210 and output. 211 </p> 212<p> 213 See <a href="../../../../example/nonfinite_facet_sstream.cpp" target="_top">nonfinite_facet_sstream.cpp</a> 214 for this example of use with <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">stringstream</span></code>s. 215 </p> 216<p> 217 For an example of how to enforce the MSVC 'legacy' "1.#INF" and 218 "1.#QNAN" representations of infinity and NaNs, for input and output, 219 see <a href="../../../../example/nonfinite_legacy.cpp" target="_top">nonfinite_legacy.cpp</a>. 220 </p> 221<p> 222 Treatment of signaling NaN is demonstrated at <a href="../../../../example/nonfinite_signaling_NaN.cpp" target="_top">../../example/nonfinite_signaling_NaN.cpp</a> 223 </p> 224<p> 225 Example <a href="../../../../example/nonfinite_loopback_ok.cpp" target="_top">../../example/nonfinite_loopback_ok.cpp</a> 226 shows loopback works OK. 227 </p> 228<p> 229 Example <a href="../../../../example/nonfinite_num_facet.cpp" target="_top">../../example/nonfinite_num_facet.cpp</a> 230 shows output and re-input of various finite and nonfinite values. 231 </p> 232<p> 233 A simple example of trapping nonfinite output is at <a href="../../../../example/nonfinite_num_facet_trap.cpp" target="_top">nonfinite_num_facet_trap.cpp</a>. 234 </p> 235<p> 236 A very basic example of using Boost.Archive is at <a href="../../../../example/nonfinite_serialization_archives.cpp" target="_top">../../example/nonfinite_serialization_archives.cpp</a>. 237 </p> 238<p> 239 A full demonstration of serialization by Francois Mauger is at <a href="../../../../example/nonfinite_num_facet_serialization.cpp" target="_top">../../example/nonfinite_num_facet_serialization.cpp</a> 240 </p> 241</div> 242<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 243<td align="left"></td> 244<td align="right"><div class="copyright-footer">Copyright © 2006-2019 Nikhar 245 Agrawal, Anton Bikineev, Paul A. Bristow, Marco Guazzone, Christopher Kormanyos, 246 Hubert Holin, Bruno Lalande, John Maddock, Jeremy Murphy, Matthew Pulver, Johan 247 Råde, Gautam Sewani, Benjamin Sobotta, Nicholas Thompson, Thijs van den Berg, 248 Daryle Walker and Xiaogang Zhang<p> 249 Distributed under the Boost Software License, Version 1.0. (See accompanying 250 file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>) 251 </p> 252</div></td> 253</tr></table> 254<hr> 255<div class="spirit-nav"> 256<a accesskey="p" href="reference.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../fp_facets.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="portability.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> 257</div> 258</body> 259</html> 260
