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<div class="titlepage"><div><div><h3 class="title">
<a name="math_toolkit.fp_facets.rationale"></a><a class="link" href="rationale.html" title="Design Rationale">Design Rationale</a>
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<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
The flags are implemented as a const data member of the facet. Facets
are reference counted, and locales can share facets. Therefore changing
the flags of a facet would have effects that are hard to predict. An
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>.
Then one could safely modify the flags, and one could define manipulators
that do so. However, for that to work with dynamically linked libraries,
a <code class="computeroutput"><span class="special">.</span><span class="identifier">cpp</span></code>
file would have to be added to the library. It was judged be more desirable
to have a header-only library, than to have mutable flags and manipulators.
</li>
<li class="listitem">
The facet <code class="computeroutput"><span class="identifier">nonfinite_num_put</span></code>
throws an exception when the <code class="computeroutput"><span class="identifier">trap_infinity</span></code>
or <code class="computeroutput"><span class="identifier">trap_nan</span></code> flag is set
and an attempt is made to format infinity or NaN. It would be better
if the facet set the <code class="computeroutput"><span class="identifier">fail</span> <span class="identifier">bit</span></code> of the stream. However, facets
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
state.
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<td align="right"><div class="copyright-footer">Copyright © 2006-2021 Nikhar Agrawal, Anton Bikineev, Matthew Borland,
Paul A. Bristow, Marco Guazzone, Christopher Kormanyos, Hubert Holin, Bruno
Lalande, John Maddock, Evan Miller, Jeremy Murphy, Matthew Pulver, Johan Råde,
Gautam Sewani, Benjamin Sobotta, Nicholas Thompson, Thijs van den Berg, Daryle
Walker and Xiaogang Zhang<p>
Distributed under the Boost Software License, Version 1.0. (See accompanying
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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