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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="boost_lexical_cast.frequently_asked_questions"></a><a class="link" href="frequently_asked_questions.html" title="Frequently Asked Questions">Frequently
Asked Questions</a>
</h2></div></div></div>
<div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem">
<span class="bold"><strong>Question:</strong></span> Why does <code class="computeroutput"><span class="identifier">lexical_cast</span><span class="special"><</span><span class="identifier">int8_t</span><span class="special">>(</span><span class="string">"127"</span><span class="special">)</span></code> throw <code class="computeroutput"><span class="identifier">bad_lexical_cast</span></code>?
<div class="itemizedlist"><ul class="itemizedlist" type="circle"><li class="listitem">
<span class="bold"><strong>Answer:</strong></span> The type <code class="computeroutput"><span class="identifier">int8_t</span></code>
is a <code class="computeroutput"><span class="keyword">typedef</span></code> to <code class="computeroutput"><span class="keyword">char</span></code> or <code class="computeroutput"><span class="keyword">signed</span>
<span class="keyword">char</span></code>. Lexical conversion to
these types is simply reading a byte from source but since the source
has more than one byte, the exception is thrown. Please use other
integer types such as <code class="computeroutput"><span class="keyword">int</span></code>
or <code class="computeroutput"><span class="keyword">short</span> <span class="keyword">int</span></code>.
If bounds checking is important, you can also call <a href="../../../libs/numeric/conversion/doc/html/boost_numericconversion/improved_numeric_cast__.html" target="_top"><code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">numeric_cast</span></code></a>: <code class="computeroutput"><span class="identifier">numeric_cast</span><span class="special"><</span><span class="identifier">int8_t</span><span class="special">>(</span><span class="identifier">lexical_cast</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(</span><span class="string">"127"</span><span class="special">));</span></code>
</li></ul></div>
</li></ul></div>
<pre class="programlisting"></pre>
<div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem">
<span class="bold"><strong>Question:</strong></span> Why does <code class="computeroutput"><span class="identifier">lexical_cast</span><span class="special"><</span><span class="keyword">unsigned</span> <span class="keyword">char</span><span class="special">>(</span><span class="string">"127"</span><span class="special">)</span></code>
throw <code class="computeroutput"><span class="identifier">bad_lexical_cast</span></code>?
<div class="itemizedlist"><ul class="itemizedlist" type="circle"><li class="listitem">
<span class="bold"><strong>Answer:</strong></span> Lexical conversion to any
char type is simply reading a byte from source. But since the source
has more than one byte, the exception is thrown. Please use other
integer types such as <code class="computeroutput"><span class="keyword">int</span></code>
or <code class="computeroutput"><span class="keyword">short</span> <span class="keyword">int</span></code>.
If bounds checking is important, you can also call <a href="../../../libs/numeric/conversion/doc/html/boost_numericconversion/improved_numeric_cast__.html" target="_top"><code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">numeric_cast</span></code></a>: <code class="computeroutput"><span class="identifier">numeric_cast</span><span class="special"><</span><span class="keyword">unsigned</span> <span class="keyword">char</span><span class="special">>(</span><span class="identifier">lexical_cast</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(</span><span class="string">"127"</span><span class="special">));</span></code>
</li></ul></div>
</li></ul></div>
<pre class="programlisting"></pre>
<div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem">
<span class="bold"><strong>Question:</strong></span> What does <code class="computeroutput"><span class="identifier">lexical_cast</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">></span></code> of an <code class="computeroutput"><span class="identifier">int8_t</span></code>
or <code class="computeroutput"><span class="identifier">uint8_t</span></code> not do what
I expect?
<div class="itemizedlist"><ul class="itemizedlist" type="circle"><li class="listitem">
<span class="bold"><strong>Answer:</strong></span> As above, note that int8_t
and uint8_t are actually chars and are formatted as such. To avoid
this, cast to an integer type first: <code class="computeroutput"><span class="identifier">lexical_cast</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">>(</span><span class="keyword">static_cast</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(</span><span class="identifier">n</span><span class="special">));</span></code>
</li></ul></div>
</li></ul></div>
<pre class="programlisting"></pre>
<div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem">
<span class="bold"><strong>Question:</strong></span> The implementation always resets
the <code class="computeroutput"><span class="identifier">ios_base</span><span class="special">::</span><span class="identifier">skipws</span></code> flag of an underlying stream object.
It breaks my <code class="computeroutput"><span class="keyword">operator</span><span class="special">>></span></code>
that works only in presence of this flag. Can you remove code that resets
the flag?
<div class="itemizedlist"><ul class="itemizedlist" type="circle"><li class="listitem">
<span class="bold"><strong>Answer:</strong></span> May be in a future version.
There is no requirement in <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n1973.html" target="_top">Lexical
Conversion Library Proposal for TR2, N1973 by Kevlin Henney and Beman
Dawes</a> to reset the flag but remember that <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n1973.html" target="_top">Lexical
Conversion Library Proposal for TR2, N1973</a> is not yet accepted
by the committee. By the way, it's a great opportunity to make your
<code class="computeroutput"><span class="keyword">operator</span><span class="special">>></span></code>
conform to the standard. Read a good C++ book, study <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">sentry</span></code> and <a href="../../../libs/io/doc/ios_state.html" target="_top"><code class="computeroutput"><span class="identifier">ios_state_saver</span></code></a>.
</li></ul></div>
</li></ul></div>
<pre class="programlisting"></pre>
<div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem">
<span class="bold"><strong>Question:</strong></span> Why <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">lexical_cast</span><span class="special"><</span><span class="keyword">unsigned</span> <span class="keyword">int</span><span class="special">>(</span><span class="string">"-1"</span><span class="special">);</span></code>
does not throw, but outputs 4294967295?
<div class="itemizedlist"><ul class="itemizedlist" type="circle"><li class="listitem">
<span class="bold"><strong>Answer:</strong></span> <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">lexical_cast</span></code>
has the behavior of <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">stringstream</span></code>,
which uses <code class="computeroutput"><span class="identifier">num_get</span></code>
functions of <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">locale</span></code>
to convert numbers. If we look at the Programming languages — C++,
we'll see, that <code class="computeroutput"><span class="identifier">num_get</span></code>
uses the rules of <code class="computeroutput"><span class="identifier">scanf</span></code>
for conversions. And in the C99 standard for unsigned input value
minus sign is optional, so if a negative number is read, no errors
will arise and the result will be the two's complement.
</li></ul></div>
</li></ul></div>
<pre class="programlisting"></pre>
<div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem">
<span class="bold"><strong>Question:</strong></span> Why <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">lexical_cast</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(</span><span class="identifier">L</span><span class="char">'A'</span><span class="special">);</span></code> outputs
65 and <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">lexical_cast</span><span class="special"><</span><span class="keyword">wchar_t</span><span class="special">>(</span><span class="identifier">L</span><span class="string">"65"</span><span class="special">);</span></code> does not throw?
<div class="itemizedlist"><ul class="itemizedlist" type="circle"><li class="listitem">
<span class="bold"><strong>Answer:</strong></span> If you are using an old
version of Visual Studio or compile code with /Zc:wchar_t- flag,
<code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">lexical_cast</span></code> sees single <code class="computeroutput"><span class="keyword">wchar_t</span></code> character as <code class="computeroutput"><span class="keyword">unsigned</span> <span class="keyword">short</span></code>.
It is not a <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">lexical_cast</span></code>
mistake, but a limitation of compiler options that you use.
</li></ul></div>
</li></ul></div>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright © 2000-2005 Kevlin Henney<br>Copyright © 2006-2010 Alexander Nasonov<br>Copyright © 2011 Antony Polukhin<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>)
</p>
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