70 lines
5.3 KiB
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70 lines
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<div class="titlepage"><div><div><h3 class="title">
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<a name="math_toolkit.fp_facets.rationale"></a><a class="link" href="rationale.html" title="Design Rationale">Design Rationale</a>
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</h3></div></div></div>
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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 flags are implemented as a const data member of the facet. Facets
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are reference counted, and locales can share facets. Therefore changing
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the flags of a facet would have effects that are hard to predict. An
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alternative design would be to implement the flags using <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">ios_base</span><span class="special">::</span><span class="identifier">xalloc</span></code> and <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">ios_base</span><span class="special">::</span><span class="identifier">iword</span></code>.
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Then one could safely modify the flags, and one could define manipulators
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that do so. However, for that to work with dynamically linked libraries,
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a <code class="computeroutput"><span class="special">.</span><span class="identifier">cpp</span></code>
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file would have to be added to the library. It was judged be more desirable
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to have a header-only library, than to have mutable flags and manipulators.
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</li>
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<li class="listitem">
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The facet <code class="computeroutput"><span class="identifier">nonfinite_num_put</span></code>
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throws an exception when the <code class="computeroutput"><span class="identifier">trap_infinity</span></code>
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or <code class="computeroutput"><span class="identifier">trap_nan</span></code> flag is set
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and an attempt is made to format infinity or NaN. It would be better
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if the facet set the <code class="computeroutput"><span class="identifier">fail</span> <span class="identifier">bit</span></code> of the stream. However, facets
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derived from <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">num_put</span></code> do not have access to the stream
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state.
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</li>
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</ul></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 © 2006-2019 Nikhar
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Agrawal, Anton Bikineev, Paul A. Bristow, Marco Guazzone, Christopher Kormanyos,
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Hubert Holin, Bruno Lalande, John Maddock, Jeremy Murphy, Matthew Pulver, Johan
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Råde, Gautam Sewani, Benjamin Sobotta, Nicholas Thompson, Thijs van den Berg,
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Daryle Walker and Xiaogang Zhang<p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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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>)
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</p>
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