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<div class="section">
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="boost_convert.performance"></a><a class="link" href="performance.html" title="Performance">Performance</a>
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</h2></div></div></div>
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<div class="toc"><dl class="toc">
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<dt><span class="section"><a href="performance.html#boost_convert.performance.converters_compared">Converters
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Compared</a></span></dt>
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<dt><span class="section"><a href="performance/boost_convert_overhead.html">Boost.Convert
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Overhead</a></span></dt>
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<dt><span class="section"><a href="performance/the_bigger_picture.html">The Bigger
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Picture</a></span></dt>
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</dl></div>
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<div class="section">
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<div class="titlepage"><div><div><h3 class="title">
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<a name="boost_convert.performance.converters_compared"></a><a class="link" href="performance.html#boost_convert.performance.converters_compared" title="Converters Compared">Converters
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Compared</a>
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</h3></div></div></div>
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<p>
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The performance of <span class="emphasis"><em>Boost.Convert</em></span> depends entirely on
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the performance of the converter deployed. A few converters have been tested
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for string conversions to basic types and to a user-defined type.
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</p>
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<p>
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In turn, the performance of every particular converter depends on the platform,
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the compiler used and the particular implementation of the underlying conversion
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component (<code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">stream</span></code>, <code class="computeroutput"><span class="identifier">printf</span></code>,
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<span class="emphasis"><em>Boost.Spirit</em></span>, etc.). Consequently, the results below
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are only an approximate indication of <span class="emphasis"><em>relative</em></span> performance
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of the mentioned converters on the tested platforms.
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</p>
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<p>
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When compiled with gcc-5.4.0 on 64-bit Ubuntu 16.04, tests produced the following
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results:
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</p>
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<pre class="programlisting"><span class="identifier">str</span><span class="special">-</span><span class="identifier">to</span><span class="special">-</span><span class="keyword">int</span><span class="special">:</span> <span class="identifier">spirit</span><span class="special">/</span><span class="identifier">strtol</span><span class="special">/</span><span class="identifier">lcast</span><span class="special">/</span><span class="identifier">scanf</span><span class="special">/</span><span class="identifier">stream</span><span class="special">=</span> <span class="number">0.27</span><span class="special">/</span> <span class="number">0.35</span><span class="special">/</span> <span class="number">0.92</span><span class="special">/</span> <span class="number">2.11</span><span class="special">/</span> <span class="number">2.09</span> <span class="identifier">seconds</span><span class="special">.</span>
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<span class="identifier">str</span><span class="special">-</span><span class="identifier">to</span><span class="special">-</span><span class="identifier">lng</span><span class="special">:</span> <span class="identifier">spirit</span><span class="special">/</span><span class="identifier">strtol</span><span class="special">/</span><span class="identifier">lcast</span><span class="special">/</span><span class="identifier">scanf</span><span class="special">/</span><span class="identifier">stream</span><span class="special">=</span> <span class="number">0.69</span><span class="special">/</span> <span class="number">0.31</span><span class="special">/</span> <span class="number">1.28</span><span class="special">/</span> <span class="number">2.07</span><span class="special">/</span> <span class="number">2.50</span> <span class="identifier">seconds</span><span class="special">.</span>
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<span class="identifier">str</span><span class="special">-</span><span class="identifier">to</span><span class="special">-</span><span class="identifier">dbl</span><span class="special">:</span> <span class="identifier">spirit</span><span class="special">/</span><span class="identifier">strtol</span><span class="special">/</span><span class="identifier">lcast</span><span class="special">/</span><span class="identifier">scanf</span><span class="special">/</span><span class="identifier">stream</span><span class="special">=</span> <span class="number">0.73</span><span class="special">/</span> <span class="number">1.06</span><span class="special">/</span> <span class="number">7.95</span><span class="special">/</span> <span class="number">2.87</span><span class="special">/</span> <span class="number">5.10</span> <span class="identifier">seconds</span><span class="special">.</span>
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<span class="keyword">int</span><span class="special">-</span><span class="identifier">to</span><span class="special">-</span><span class="identifier">str</span><span class="special">:</span> <span class="identifier">spirit</span><span class="special">/</span><span class="identifier">strtol</span><span class="special">/</span><span class="identifier">lcast</span><span class="special">/</span><span class="identifier">prntf</span><span class="special">/</span><span class="identifier">stream</span><span class="special">=</span> <span class="number">1.96</span><span class="special">/</span> <span class="number">1.39</span><span class="special">/</span> <span class="number">2.52</span><span class="special">/</span> <span class="number">3.49</span><span class="special">/</span> <span class="number">2.58</span> <span class="identifier">seconds</span><span class="special">.</span>
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<span class="identifier">lng</span><span class="special">-</span><span class="identifier">to</span><span class="special">-</span><span class="identifier">str</span><span class="special">:</span> <span class="identifier">spirit</span><span class="special">/</span><span class="identifier">strtol</span><span class="special">/</span><span class="identifier">lcast</span><span class="special">/</span><span class="identifier">prntf</span><span class="special">/</span><span class="identifier">stream</span><span class="special">=</span> <span class="number">2.45</span><span class="special">/</span> <span class="number">1.51</span><span class="special">/</span> <span class="number">2.32</span><span class="special">/</span> <span class="number">3.30</span><span class="special">/</span> <span class="number">2.63</span> <span class="identifier">seconds</span><span class="special">.</span>
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<span class="identifier">dbl</span><span class="special">-</span><span class="identifier">to</span><span class="special">-</span><span class="identifier">str</span><span class="special">:</span> <span class="identifier">spirit</span><span class="special">/</span><span class="identifier">strtol</span><span class="special">/</span><span class="identifier">lcast</span><span class="special">/</span><span class="identifier">prntf</span><span class="special">/</span><span class="identifier">stream</span><span class="special">=</span> <span class="number">6.62</span><span class="special">/</span> <span class="number">4.46</span><span class="special">/</span> <span class="number">28.69</span><span class="special">/</span> <span class="number">20.60</span><span class="special">/</span> <span class="number">14.16</span> <span class="identifier">seconds</span><span class="special">.</span>
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</pre>
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<p>
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Based on the results, all things considered, I tend to conclude that there
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is no clear winner:
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</p>
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<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
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<li class="listitem">
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the <span class="emphasis"><em>Spirit.Qi</em></span>-based converter was the fastest for
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string to basic (<code class="computeroutput"><span class="keyword">int</span></code>, <code class="computeroutput"><span class="keyword">double</span></code>) conversions. So, it might be a
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good candidate for the tasks predominantly doing that kind of conversions
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(with <span class="emphasis"><em>Spirit.Qi</em></span> conversion-related limitations in
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mind); <span class="emphasis"><em>Spirit.Karma</em></span>'s <span class="emphasis"><em>to-string</em></span>
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performance did not seem as impressive;
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</li>
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<li class="listitem">
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the <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">iostream</span></code>-based converter was comparatively
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slow. Still, given its maturity, availability and formatting support,
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it might be an option to consider if conversion performance is not your
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primary concern;
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</li>
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<li class="listitem">
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the <code class="computeroutput"><span class="identifier">strtol</span></code>-inspired converter
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was reasonably fast and with formatting support might be an attractive
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all-rounder. It should be noted that it is nowhere as mature as <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">cnv</span><span class="special">::</span><span class="identifier">lexical_cast</span></code> or <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">cnv</span><span class="special">::</span><span class="identifier">stream</span></code>.
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So, bugs are to be expected.
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</li>
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</ul></div>
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<p>
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For user-defined types <code class="computeroutput"><span class="identifier">cnv</span><span class="special">::</span><span class="identifier">lexical_cast</span></code>,
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<code class="computeroutput"><span class="identifier">cnv</span><span class="special">::</span><span class="identifier">cstream</span></code> and <code class="computeroutput"><span class="identifier">cnv</span><span class="special">::</span><span class="identifier">strtol</span></code>
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were tested with the following results:
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</p>
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<pre class="programlisting"><span class="identifier">str</span><span class="special">-</span><span class="identifier">to</span><span class="special">-</span><span class="identifier">user</span><span class="special">-</span><span class="identifier">type</span><span class="special">:</span> <span class="identifier">lcast</span><span class="special">/</span><span class="identifier">stream</span><span class="special">/</span><span class="identifier">strtol</span><span class="special">=</span><span class="number">0.36</span><span class="special">/</span><span class="number">0.18</span><span class="special">/</span><span class="number">0.07</span> <span class="identifier">seconds</span><span class="special">.</span>
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<span class="identifier">user</span><span class="special">-</span><span class="identifier">type</span><span class="special">-</span><span class="identifier">to</span><span class="special">-</span><span class="identifier">str</span><span class="special">:</span> <span class="identifier">lcast</span><span class="special">/</span><span class="identifier">stream</span><span class="special">/</span><span class="identifier">strtol</span><span class="special">=</span><span class="number">0.58</span><span class="special">/</span><span class="number">0.09</span><span class="special">/</span><span class="number">0.06</span> <span class="identifier">seconds</span><span class="special">.</span>
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</pre>
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<p>
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To provide <span class="emphasis"><em>string-to-user-type</em></span> and <span class="emphasis"><em>user-type-to-string</em></span>
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conversions the first two deploy the same standard <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">iostream</span></code>
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library. However, <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">cnv</span><span class="special">::</span><span class="identifier">cstream</span></code>
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considerably outperforms <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">lexical_cast</span></code>
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in these tests. The results reflect different underlying designs. Namely,
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the standard <span class="emphasis"><em>Boost.Convert</em></span> deployment pattern is to
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create a converter or converters once and then re-use them. <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">lexical_cast</span></code>, on the other hand, creates
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and then destroys a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">stream</span></code> instance every time the function
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is called and the <a href="http://www.boost.org/doc/libs/1_55_0/doc/html/boost_lexical_cast/performance.html" target="_top"><code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">lexical_cast</span></code> performance</a> table
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indicates that the "std::stringstream <span class="emphasis"><em>with construction</em></span>"
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operation is considerably more expensive compared to "std::stringstream
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<span class="emphasis"><em>without construction</em></span>".
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</p>
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<p>
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<code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">cnv</span><span class="special">::</span><span class="identifier">strtol</span></code>
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support for user types has been implemented similarly but without the <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">stream</span></code>-related
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overhead. That resulted in the best out-of-three performance results.
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</p>
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<p>
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Based on the performance data, I tend to conclude that, given type-safety
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and benefits provided by the <span class="emphasis"><em>Boost.Convert</em></span> framework,
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it (with appropriate converters) should probably be the first choice for
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conversion-related tasks.
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</p>
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</div>
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</div>
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="left"></td>
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<td align="right"><div class="copyright-footer">Copyright © 2009-2016 Vladimir Batov<p>
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Distributed under the Boost Software License, Version 1.0. See copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>.
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</p>
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