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24 | </div> | |
25 | <div class="section"> | |
26 | <div class="titlepage"><div><div><h3 class="title"> | |
27 | <a name="math_toolkit.fp_facets.examples"></a><a class="link" href="examples.html" title="Examples">Examples</a> | |
28 | </h3></div></div></div> | |
29 | <h6> | |
30 | <a name="math_toolkit.fp_facets.examples.h0"></a> | |
31 | <span 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">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">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 necesary 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 infinty 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 © 2006-2010, 2012-2014 Nikhar Agrawal, | |
245 | Anton Bikineev, Paul A. Bristow, Marco Guazzone, Christopher Kormanyos, Hubert | |
246 | Holin, Bruno Lalande, John Maddock, Jeremy Murphy, Johan Råde, Gautam Sewani, | |
247 | Benjamin Sobotta, Thijs van den Berg, Daryle Walker and Xiaogang Zhang<p> | |
248 | Distributed under the Boost Software License, Version 1.0. (See accompanying | |
249 | 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>) | |
250 | </p> | |
251 | </div></td> | |
252 | </tr></table> | |
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254 | <div class="spirit-nav"> | |
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