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| 24 | </div> | |
| 25 | <div class="section"> | |
| 26 | <div class="titlepage"><div><div><h3 class="title"> | |
| 27 | <a name="boost_multiprecision.tut.input_output"></a><a class="link" href="input_output.html" title="Input Output">Input Output</a> | |
| 28 | </h3></div></div></div> | |
| 29 | <h5> | |
| 30 | <a name="boost_multiprecision.tut.input_output.h0"></a> | |
| 31 | <span class="phrase"><a name="boost_multiprecision.tut.input_output.loopback_testing"></a></span><a class="link" href="input_output.html#boost_multiprecision.tut.input_output.loopback_testing">Loopback | |
| 32 | testing</a> | |
| 33 | </h5> | |
| 34 | <p> | |
| 35 | <span class="emphasis"><em>Loopback</em></span> or <span class="emphasis"><em>round-tripping</em></span> refers | |
| 36 | to writing out a value as a decimal digit string using <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">iostream</span></code>, | |
| 37 | usually to a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">stringstream</span></code>, and then reading the string | |
| 38 | back in to another value, and confirming that the two values are identical. | |
| 39 | A trivial example using <code class="computeroutput"><span class="keyword">float</span></code> | |
| 40 | is: | |
| 41 | </p> | |
| 42 | <pre class="programlisting"><span class="keyword">float</span> <span class="identifier">write</span><span class="special">;</span> <span class="comment">// Value to round-trip.</span> | |
| 43 | <span class="identifier">std</span><span class="special">::</span><span class="identifier">stringstream</span> <span class="identifier">ss</span><span class="special">;</span> <span class="comment">// Read and write std::stringstream.</span> | |
| 44 | <span class="identifier">ss</span><span class="special">.</span><span class="identifier">precision</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="identifier">T</span><span class="special">>::</span><span class="identifier">max_digits10</span><span class="special">);</span> <span class="comment">// Ensure all potentially significant bits are output.</span> | |
| 45 | <span class="identifier">ss</span><span class="special">.</span><span class="identifier">flags</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">ios_base</span><span class="special">::</span><span class="identifier">fmtflags</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">ios_base</span><span class="special">::</span><span class="identifier">scientific</span><span class="special">));</span> <span class="comment">// Use scientific format.</span> | |
| 46 | <span class="identifier">ss</span> <span class="special"><<</span> <span class="identifier">write</span><span class="special">;</span> <span class="comment">// Output to string.</span> | |
| 47 | <span class="keyword">float</span> <span class="identifier">read</span><span class="special">;</span> <span class="comment">// Expected.</span> | |
| 48 | <span class="identifier">ss</span> <span class="special">>></span> <span class="identifier">read</span><span class="special">;</span> <span class="comment">// Read decimal digits string from stringstream.</span> | |
| 49 | <span class="identifier">BOOST_CHECK_EQUAL</span><span class="special">(</span><span class="identifier">write</span><span class="special">,</span> <span class="identifier">read</span><span class="special">);</span> <span class="comment">// Should be the same.</span> | |
| 50 | </pre> | |
| 51 | <p> | |
| 52 | and this can be run in a loop for all possible values of a 32-bit float. | |
| 53 | For other floating-point types <code class="computeroutput"><span class="identifier">T</span></code>, | |
| 54 | including built-in <code class="computeroutput"><span class="keyword">double</span></code>, it | |
| 55 | takes far too long to test all values, so a reasonable test strategy is to | |
| 56 | use a large number of random values. | |
| 57 | </p> | |
| 58 | <pre class="programlisting"><span class="identifier">T</span> <span class="identifier">write</span><span class="special">;</span> | |
| 59 | <span class="identifier">std</span><span class="special">::</span><span class="identifier">stringstream</span> <span class="identifier">ss</span><span class="special">;</span> | |
| 60 | <span class="identifier">ss</span><span class="special">.</span><span class="identifier">precision</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="identifier">T</span><span class="special">>::</span><span class="identifier">max_digits10</span><span class="special">);</span> <span class="comment">// Ensure all potentially significant bits are output.</span> | |
| 61 | <span class="identifier">ss</span><span class="special">.</span><span class="identifier">flags</span><span class="special">(</span><span class="identifier">f</span><span class="special">);</span> <span class="comment">// Changed from default iostream format flags if desired.</span> | |
| 62 | <span class="identifier">ss</span> <span class="special"><<</span> <span class="identifier">write</span><span class="special">;</span> <span class="comment">// Output to stringstream.</span> | |
| 63 | ||
| 64 | <span class="identifier">T</span> <span class="identifier">read</span><span class="special">;</span> | |
| 65 | <span class="identifier">ss</span> <span class="special">>></span> <span class="identifier">read</span><span class="special">;</span> <span class="comment">// Get read using operator>> from stringstream.</span> | |
| 66 | <span class="identifier">BOOST_CHECK_EQUAL</span><span class="special">(</span><span class="identifier">read</span><span class="special">,</span> <span class="identifier">write</span><span class="special">);</span> | |
| 67 | ||
| 68 | <span class="identifier">read</span> <span class="special">=</span> <span class="keyword">static_cast</span><span class="special"><</span><span class="identifier">T</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="comment">// Get read by converting from decimal digits string representation of write.</span> | |
| 69 | <span class="identifier">BOOST_CHECK_EQUAL</span><span class="special">(</span><span class="identifier">read</span><span class="special">,</span> <span class="identifier">write</span><span class="special">);</span> | |
| 70 | ||
| 71 | <span class="identifier">read</span> <span class="special">=</span> <span class="keyword">static_cast</span><span class="special"><</span><span class="identifier">T</span><span class="special">>(</span><span class="identifier">write</span><span class="special">.</span><span class="identifier">str</span><span class="special">(</span><span class="number">0</span><span class="special">,</span> <span class="identifier">f</span><span class="special">));</span> <span class="comment">// Get read using format specified when written.</span> | |
| 72 | <span class="identifier">BOOST_CHECK_EQUAL</span><span class="special">(</span><span class="identifier">read</span><span class="special">,</span> <span class="identifier">write</span><span class="special">);</span> | |
| 73 | </pre> | |
| 74 | <p> | |
| 75 | The test at <a href="../../../../test/test_cpp_bin_float_io.cpp" target="_top">test_cpp_bin_float_io.cpp</a> | |
| 76 | allows any floating-point type to be <span class="emphasis"><em>round_tripped</em></span> using | |
| 77 | a wide range of fairly random values. It also includes tests compared a collection | |
| 78 | of <a href="../../../../test/string_data.ipp" target="_top">stringdata</a> test cases | |
| 79 | in a file. | |
| 80 | </p> | |
| 81 | <h5> | |
| 82 | <a name="boost_multiprecision.tut.input_output.h1"></a> | |
| 83 | <span class="phrase"><a name="boost_multiprecision.tut.input_output.comparing_with_output_using_buil"></a></span><a class="link" href="input_output.html#boost_multiprecision.tut.input_output.comparing_with_output_using_buil">Comparing | |
| 84 | with output using Built-in types</a> | |
| 85 | </h5> | |
| 86 | <p> | |
| 87 | One can make some comparisons with the output of | |
| 88 | </p> | |
| 89 | <pre class="programlisting"><span class="special"><</span><span class="identifier">number</span><span class="special"><</span><span class="identifier">cpp_bin_float</span><span class="special"><</span><span class="number">53</span><span class="special">,</span> <span class="identifier">digit_count_2</span><span class="special">></span> <span class="special">></span> | |
| 90 | </pre> | |
| 91 | <p> | |
| 92 | which has the same number of significant bits (53) as 64-bit double precision | |
| 93 | floating-point. | |
| 94 | </p> | |
| 95 | <p> | |
| 96 | However, although most outputs are identical, there are differences on some | |
| 97 | platforms caused by the implementation-dependent behaviours allowed by the | |
| 98 | C99 specification <a href="http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1124.pdf" target="_top">C99 | |
| 99 | ISO/IEC 9899:TC2</a>, incorporated by C++. | |
| 100 | </p> | |
| 101 | <div class="blockquote"><blockquote class="blockquote"><p> | |
| 102 | <span class="emphasis"><em>"For e, E, f, F, g, and G conversions, if the number of | |
| 103 | significant decimal digits is at most DECIMAL_DIG, then the result should | |
| 104 | be correctly rounded. If the number of significant decimal digits is more | |
| 105 | than DECIMAL_DIG but the source value is exactly representable with DECIMAL_DIG | |
| 106 | digits, then the result should be an exact representation with trailing | |
| 107 | zeros. Otherwise, the source value is bounded by two adjacent decimal strings | |
| 108 | L < U, both having DECIMAL_DIG significant digits; the value of the | |
| 109 | resultant decimal string D should satisfy L<= D <= U, with the extra | |
| 110 | stipulation that the error should have a correct sign for the current rounding | |
| 111 | direction."</em></span> | |
| 112 | </p></blockquote></div> | |
| 113 | <p> | |
| 114 | So not only is correct rounding for the full number of digits not required, | |
| 115 | but even if the <span class="bold"><strong>optional</strong></span> recommended practice | |
| 116 | is followed, then the value of these last few digits is unspecified as long | |
| 117 | as the value is within certain bounds. | |
| 118 | </p> | |
| 119 | <div class="note"><table border="0" summary="Note"> | |
| 120 | <tr> | |
| 121 | <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../../doc/src/images/note.png"></td> | |
| 122 | <th align="left">Note</th> | |
| 123 | </tr> | |
| 124 | <tr><td align="left" valign="top"><p> | |
| 125 | Do not expect the output from different platforms to be <span class="bold"><strong>identical</strong></span>, | |
| 126 | but <code class="computeroutput"><span class="identifier">cpp_dec_float</span></code>, <code class="computeroutput"><span class="identifier">cpp_bin_float</span></code> (and other backends) outputs | |
| 127 | should be correctly rounded to the number of digits requested by the set | |
| 128 | precision and format. | |
| 129 | </p></td></tr> | |
| 130 | </table></div> | |
| 131 | <h5> | |
| 132 | <a name="boost_multiprecision.tut.input_output.h2"></a> | |
| 133 | <span class="phrase"><a name="boost_multiprecision.tut.input_output.macro_boost_mp_min_exponent_digi"></a></span><a class="link" href="input_output.html#boost_multiprecision.tut.input_output.macro_boost_mp_min_exponent_digi">Macro | |
| 134 | BOOST_MP_MIN_EXPONENT_DIGITS</a> | |
| 135 | </h5> | |
| 136 | <p> | |
| 137 | <a href="http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1124.pdf" target="_top">C99 | |
| 138 | Standard</a> for format specifiers, 7.19.6 Formatted input/output functions | |
| 139 | requires: | |
| 140 | </p> | |
| 141 | <p> | |
| 142 | "The exponent always contains at least two digits, and only as many | |
| 143 | more digits as necessary to represent the exponent." | |
| 144 | </p> | |
| 145 | <p> | |
| 146 | So to conform to the C99 standard (incorporated by C++) | |
| 147 | </p> | |
| 148 | <pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">BOOST_MP_MIN_EXPONENT_DIGITS</span> <span class="number">2</span> | |
| 149 | </pre> | |
| 150 | <p> | |
| 151 | Confusingly, Microsoft (and MinGW) do not conform to this standard and provide | |
| 152 | <span class="bold"><strong>at least three digits</strong></span>, for example <code class="computeroutput"><span class="number">1e+001</span></code>. So if you want the output to match | |
| 153 | that from built-in floating-point types on compilers that use Microsofts | |
| 154 | runtime then use: | |
| 155 | </p> | |
| 156 | <pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">BOOST_MP_MIN_EXPONENT_DIGITS</span> <span class="number">3</span> | |
| 157 | </pre> | |
| 158 | <p> | |
| 159 | Also useful to get the minimum exponent field width is | |
| 160 | </p> | |
| 161 | <pre class="programlisting"><span class="preprocessor">#define</span> <span class="identifier">BOOST_MP_MIN_EXPONENT_DIGITS</span> <span class="number">1</span> | |
| 162 | </pre> | |
| 163 | <p> | |
| 164 | producing a compact output like <code class="computeroutput"><span class="number">2e+4</span></code>, | |
| 165 | useful when conserving space is important. | |
| 166 | </p> | |
| 167 | <p> | |
| 168 | Larger values are also supported, for example, value 4 for <code class="computeroutput"><span class="number">2e+0004</span></code> which may be useful to ensure that | |
| 169 | columns line up. | |
| 170 | </p> | |
| 171 | </div> | |
| 172 | <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> | |
| 173 | <td align="left"></td> | |
| 174 | <td align="right"><div class="copyright-footer">Copyright © 2002-2013 John Maddock and Christopher Kormanyos<p> | |
| 175 | Distributed under the Boost Software License, Version 1.0. (See accompanying | |
| 176 | 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>) | |
| 177 | </p> | |
| 178 | </div></td> | |
| 179 | </tr></table> | |
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