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- <a name="boost_test.test_output.test_tools_support_for_logging.log_floating_points"></a><a class="link" href="log_floating_points.html" title="Logging floating point type numbers">Logging
- floating point type numbers</a>
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- <p>
- It may appear that floating-point numbers are displayed by the <span class="emphasis"><em>Unit
- Test Framework</em></span> with an excessive number of decimal digits. However
- the number of digits shown is chosen to avoid apparently nonsensical displays
- like <code class="computeroutput"><span class="special">[</span><span class="number">1.00000</span>
- <span class="special">!=</span> <span class="number">1.00000</span><span class="special">]</span></code> when comparing exactly unity against a
- value which is increased by just one least significant binary digit using
- the default precision for float of just 6 decimal digits, given by <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">float</span><span class="special">>::</span><span class="identifier">digits10</span></code>. The function used for the number
- of decimal digits displayed is that proposed for a future C++ Standard,
- <a href="http://www2.open-std.org/JTC1/SC22/WG21/docs/papers/2005/n1822.pdf" target="_top">A
- Proposal to add a max significant decimal digits value</a>, to be called
- <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special">::</span><span class="identifier">max_digits10</span><span class="special">();</span></code>.
- For 32-bit floats, 9 decimal digits are needed to ensure a single bit change
- produces a different decimal digit string.
- </p>
- <p>
- So a much more helpful display using 9 decimal digits is thus: <code class="computeroutput"><span class="special">[</span><span class="number">1.00000000</span> <span class="special">!=</span> <span class="number">1.00000012</span><span class="special">]</span></code> showing that the two values are in fact
- different.
- </p>
- <p>
- For <span class="bold"><strong>IEEE754</strong></span> 32-bit float values - 9 decimal
- digits are shown. For 64-bit <span class="bold"><strong>IEEE754</strong></span> double
- - 17 decimal digits. For <span class="bold"><strong>IEEE754</strong></span> extended
- long double using 80-bit - 21 decimal digits. For <span class="bold"><strong>IEEE754</strong></span>
- quadruple long double 128-bit, and SPARC extended long double 128-bit -
- 36 decimal digits. For floating-point types, a convenient formula to calculate
- <code class="computeroutput"><span class="identifier">max_digits10</span></code> is: <code class="computeroutput"><span class="number">2</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">FPT</span><span class="special">>::</span><span class="identifier">digits</span>
- <span class="special">*</span> <span class="number">3010</span><span class="special">/</span><span class="number">10000</span></code>;
- </p>
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- <th align="left">Note</th>
- </tr>
- <tr><td align="left" valign="top"><p>
- Note that a user defined floating point type UDFPT must define <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special"><</span><span class="identifier">UDFPT</span><span class="special">>::</span><span class="identifier">is_specialized</span> <span class="special">=</span>
- <span class="keyword">true</span></code> and provide an appropriate
- value for <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special"><</span><span class="identifier">UDFPT</span><span class="special">>::</span><span class="identifier">digits</span></code>, the number of bits used for
- the significand or mantissa. For example, for the SPARC extended long
- double 128, 113 bits are used for the significand (one of which is implicit).
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