123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100101102103104105106107108109110111112113114115116117118119120121122123124125126127128129130131132133134135136137138139140141142143144145146147148149150151152153154155156157158159160161162163164165166167168169170171172173174175176177178179180181182183184185186187188189190191192193194195196197198199200201202203204205206207208209210211212 |
- <html>
- <head>
- <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
- <title>Improved numeric_cast<></title>
- <link rel="stylesheet" href="../boostbook.css" type="text/css">
- <meta name="generator" content="DocBook XSL Stylesheets V1.79.1">
- <link rel="home" href="../index.html" title="Chapter 1. Boost.NumericConversion">
- <link rel="up" href="../index.html" title="Chapter 1. Boost.NumericConversion">
- <link rel="prev" href="numeric_converter_policy_classes.html" title="Numeric Converter Policy Classes">
- <link rel="next" href="../numeric_conversion/history_and_acknowledgments.html" title="History and Acknowledgments">
- </head>
- <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
- <table cellpadding="2" width="100%"><tr>
- <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../boost.png"></td>
- <td align="center"><a href="../../../../../../index.html">Home</a></td>
- <td align="center"><a href="../../../../../../libs/libraries.htm">Libraries</a></td>
- <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td>
- <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
- <td align="center"><a href="../../../../../../more/index.htm">More</a></td>
- </tr></table>
- <hr>
- <div class="spirit-nav">
- <a accesskey="p" href="numeric_converter_policy_classes.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="../numeric_conversion/history_and_acknowledgments.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
- </div>
- <div class="section">
- <div class="titlepage"><div><div><h2 class="title" style="clear: both">
- <a name="boost_numericconversion.improved_numeric_cast__"></a><a class="link" href="improved_numeric_cast__.html" title="Improved numeric_cast<>">Improved
- numeric_cast<></a>
- </h2></div></div></div>
- <div class="toc"><dl class="toc">
- <dt><span class="section"><a href="improved_numeric_cast__.html#boost_numericconversion.improved_numeric_cast__.introduction">Introduction</a></span></dt>
- <dt><span class="section"><a href="improved_numeric_cast__.html#boost_numericconversion.improved_numeric_cast__.numeric_cast">numeric_cast</a></span></dt>
- <dt><span class="section"><a href="improved_numeric_cast__.html#boost_numericconversion.improved_numeric_cast__.numeric_cast_traits">numeric_cast_traits</a></span></dt>
- <dt><span class="section"><a href="improved_numeric_cast__.html#boost_numericconversion.improved_numeric_cast__.examples">Examples</a></span></dt>
- </dl></div>
- <div class="section">
- <div class="titlepage"><div><div><h3 class="title">
- <a name="boost_numericconversion.improved_numeric_cast__.introduction"></a><a class="link" href="improved_numeric_cast__.html#boost_numericconversion.improved_numeric_cast__.introduction" title="Introduction">Introduction</a>
- </h3></div></div></div>
- <p>
- The lack of preservation of range makes conversions between numeric types
- error prone. This is true for both implicit conversions and explicit conversions
- (through <code class="computeroutput"><span class="keyword">static_cast</span></code>). <a class="link" href="improved_numeric_cast__.html#boost_numericconversion.improved_numeric_cast__.numeric_cast" title="numeric_cast"><code class="computeroutput"><span class="identifier">numeric_cast</span></code></a> detects loss of range
- when a numeric type is converted, and throws an exception if the range cannot
- be preserved.
- </p>
- <p>
- There are several situations where conversions are unsafe:
- </p>
- <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
- <li class="listitem">
- Conversions from an integral type with a wider range than the target
- integral type.
- </li>
- <li class="listitem">
- Conversions from unsigned to signed (and vice versa) integral types.
- </li>
- <li class="listitem">
- Conversions from floating point types to integral types.
- </li>
- </ul></div>
- <p>
- The C++ Standard does not specify the behavior when a numeric type is assigned
- a value that cannot be represented by the type, except for unsigned integral
- types [3.9.1.4], which must obey the laws of arithmetic modulo 2n (this implies
- that the result will be reduced modulo the number that is one greater than
- the largest value that can be represented). The fact that the behavior for
- overflow is undefined for all conversions (except the aforementioned unsigned
- to unsigned) makes any code that may produce positive or negative overflows
- exposed to portability issues.
- </p>
- <p>
- By default <code class="computeroutput"><span class="identifier">numeric_cast</span></code> adheres
- to the rules for implicit conversions mandated by the C++ Standard, such
- as truncating floating point types when converting to integral types. The
- implementation must guarantee that for a conversion to a type that can hold
- all possible values of the source type, there will be no runtime overhead.
- </p>
- </div>
- <div class="section">
- <div class="titlepage"><div><div><h3 class="title">
- <a name="boost_numericconversion.improved_numeric_cast__.numeric_cast"></a><a class="link" href="improved_numeric_cast__.html#boost_numericconversion.improved_numeric_cast__.numeric_cast" title="numeric_cast">numeric_cast</a>
- </h3></div></div></div>
- <pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Target</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Source</span><span class="special">></span> <span class="keyword">inline</span>
- <span class="identifier">Target</span> <span class="identifier">numeric_cast</span><span class="special">(</span> <span class="identifier">Source</span> <span class="identifier">arg</span> <span class="special">)</span>
- <span class="special">{</span>
- <span class="keyword">typedef</span> <span class="identifier">conversion_traits</span><span class="special"><</span><span class="identifier">Target</span><span class="special">,</span> <span class="identifier">Source</span><span class="special">></span> <span class="identifier">conv_traits</span><span class="special">;</span>
- <span class="keyword">typedef</span> <span class="identifier">numeric_cast_traits</span><span class="special"><</span><span class="identifier">Target</span><span class="special">,</span> <span class="identifier">Source</span><span class="special">></span> <span class="identifier">cast_traits</span><span class="special">;</span>
- <span class="keyword">typedef</span> <span class="identifier">converter</span>
- <span class="special"><</span>
- <span class="identifier">Target</span><span class="special">,</span>
- <span class="identifier">Source</span><span class="special">,</span>
- <span class="identifier">conv_traits</span><span class="special">,</span>
- <span class="keyword">typename</span> <span class="identifier">cast_traits</span><span class="special">::</span><span class="identifier">overflow_policy</span><span class="special">,</span>
- <span class="keyword">typename</span> <span class="identifier">cast_traits</span><span class="special">::</span><span class="identifier">rounding_policy</span><span class="special">,</span>
- <span class="identifier">raw_converter</span><span class="special"><</span><span class="identifier">conv_traits</span><span class="special">>,</span>
- <span class="keyword">typename</span> <span class="identifier">cast_traits</span><span class="special">::</span><span class="identifier">range_checking_policy</span>
- <span class="special">></span> <span class="identifier">converter</span><span class="special">;</span>
- <span class="keyword">return</span> <span class="identifier">converter</span><span class="special">::</span><span class="identifier">convert</span><span class="special">(</span><span class="identifier">arg</span><span class="special">);</span>
- <span class="special">}</span>
- </pre>
- <p>
- <code class="computeroutput"><span class="identifier">numeric_cast</span></code> returns the
- result of converting a value of type Source to a value of type Target. If
- out-of-range is detected, an overflow policy is executed whose default behavior
- is to throw an an exception (see <a class="link" href="numeric_converter_policy_classes.html#numeric_conversion_bad_numeric_cast">bad_numeric_cast</a>,
- <a class="link" href="numeric_converter_policy_classes.html#numeric_conversion_negative_overflow">negative_overflow</a>
- and <a class="link" href="numeric_converter_policy_classes.html#numeric_conversion_possitive_overflow">positive_overflow</a>
- ).
- </p>
- </div>
- <div class="section">
- <div class="titlepage"><div><div><h3 class="title">
- <a name="boost_numericconversion.improved_numeric_cast__.numeric_cast_traits"></a><a class="link" href="improved_numeric_cast__.html#boost_numericconversion.improved_numeric_cast__.numeric_cast_traits" title="numeric_cast_traits">numeric_cast_traits</a>
- </h3></div></div></div>
- <pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Target</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Source</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">EnableIf</span> <span class="special">=</span> <span class="keyword">void</span><span class="special">></span>
- <span class="keyword">struct</span> <span class="identifier">numeric_cast_traits</span>
- <span class="special">{</span>
- <span class="keyword">typedef</span> <span class="identifier">def_overflow_handler</span> <span class="identifier">overflow_policy</span><span class="special">;</span>
- <span class="keyword">typedef</span> <span class="identifier">UseInternalRangeChecker</span> <span class="identifier">range_checking_policy</span><span class="special">;</span>
- <span class="keyword">typedef</span> <span class="identifier">Trunc</span><span class="special"><</span><span class="identifier">Source</span><span class="special">></span> <span class="identifier">rounding_policy</span><span class="special">;</span>
- <span class="special">};</span>
- </pre>
- <p>
- The behavior of <code class="computeroutput"><span class="identifier">numeric_cast</span></code>
- may be tailored for custom numeric types through the specialization of <code class="computeroutput"><span class="identifier">numeric_cast_traits</span></code>. (see <a class="link" href="type_requirements_and_user_defined_types_support.html" title="Type Requirements and User-defined-types support">User
- Defined Types</a> for details. )
- </p>
- </div>
- <div class="section">
- <div class="titlepage"><div><div><h3 class="title">
- <a name="boost_numericconversion.improved_numeric_cast__.examples"></a><a class="link" href="improved_numeric_cast__.html#boost_numericconversion.improved_numeric_cast__.examples" title="Examples">Examples</a>
- </h3></div></div></div>
- <p>
- The following example performs some typical conversions between numeric types:
- </p>
- <div class="orderedlist"><ol class="orderedlist" type="1">
- <li class="listitem">
- include <boost/numeric/conversion/cast.hpp>
- </li>
- <li class="listitem">
- include <iostream>
- </li>
- </ol></div>
- <pre class="programlisting"><span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
- <span class="special">{</span>
- <span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">numeric_cast</span><span class="special">;</span>
- <span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">numeric</span><span class="special">::</span><span class="identifier">bad_numeric_cast</span><span class="special">;</span>
- <span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">numeric</span><span class="special">::</span><span class="identifier">positive_overflow</span><span class="special">;</span>
- <span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">numeric</span><span class="special">::</span><span class="identifier">negative_overflow</span><span class="special">;</span>
- <span class="keyword">try</span>
- <span class="special">{</span>
- <span class="keyword">int</span> <span class="identifier">i</span><span class="special">=</span><span class="number">42</span><span class="special">;</span>
- <span class="keyword">short</span> <span class="identifier">s</span><span class="special">=</span><span class="identifier">numeric_cast</span><span class="special"><</span><span class="keyword">short</span><span class="special">>(</span><span class="identifier">i</span><span class="special">);</span> <span class="comment">// This conversion succeeds (is in range)</span>
- <span class="special">}</span>
- <span class="keyword">catch</span><span class="special">(</span><span class="identifier">negative_overflow</span><span class="special">&</span> <span class="identifier">e</span><span class="special">)</span> <span class="special">{</span>
- <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">e</span><span class="special">.</span><span class="identifier">what</span><span class="special">();</span>
- <span class="special">}</span>
- <span class="keyword">catch</span><span class="special">(</span><span class="identifier">positive_overflow</span><span class="special">&</span> <span class="identifier">e</span><span class="special">)</span> <span class="special">{</span>
- <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">e</span><span class="special">.</span><span class="identifier">what</span><span class="special">();</span>
- <span class="special">}</span>
- <span class="keyword">try</span>
- <span class="special">{</span>
- <span class="keyword">float</span> <span class="identifier">f</span><span class="special">=-</span><span class="number">42.1234</span><span class="special">;</span>
- <span class="comment">// This will cause a boost::numeric::negative_overflow exception to be thrown</span>
- <span class="keyword">unsigned</span> <span class="keyword">int</span> <span class="identifier">i</span><span class="special">=</span><span class="identifier">numeric_cast</span><span class="special"><</span><span class="keyword">unsigned</span> <span class="keyword">int</span><span class="special">>(</span><span class="identifier">f</span><span class="special">);</span>
- <span class="special">}</span>
- <span class="keyword">catch</span><span class="special">(</span><span class="identifier">bad_numeric_cast</span><span class="special">&</span> <span class="identifier">e</span><span class="special">)</span> <span class="special">{</span>
- <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">e</span><span class="special">.</span><span class="identifier">what</span><span class="special">();</span>
- <span class="special">}</span>
- <span class="keyword">double</span> <span class="identifier">d</span><span class="special">=</span> <span class="identifier">f</span> <span class="special">+</span> <span class="identifier">numeric_cast</span><span class="special"><</span><span class="keyword">double</span><span class="special">>(</span><span class="number">123</span><span class="special">);</span> <span class="comment">// int -> double</span>
- <span class="keyword">unsigned</span> <span class="keyword">long</span> <span class="identifier">l</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="keyword">unsigned</span> <span class="keyword">long</span><span class="special">>::</span><span class="identifier">max</span><span class="special">();</span>
- <span class="keyword">try</span>
- <span class="special">{</span>
- <span class="comment">// This will cause a boost::numeric::positive_overflow exception to be thrown</span>
- <span class="comment">// NOTE: *operations* on unsigned integral types cannot cause overflow</span>
- <span class="comment">// but *conversions* to a signed type ARE range checked by numeric_cast.</span>
- <span class="keyword">unsigned</span> <span class="keyword">char</span> <span class="identifier">c</span><span class="special">=</span><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">l</span><span class="special">);</span>
- <span class="special">}</span>
- <span class="keyword">catch</span><span class="special">(</span><span class="identifier">positive_overflow</span><span class="special">&</span> <span class="identifier">e</span><span class="special">)</span> <span class="special">{</span>
- <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">e</span><span class="special">.</span><span class="identifier">what</span><span class="special">();</span>
- <span class="special">}</span>
- <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
- <span class="special">}</span>
- </pre>
- </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 © 2004-2007 Fernando
- Luis Cacciola Carballal<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>
- </div></td>
- </tr></table>
- <hr>
- <div class="spirit-nav">
- <a accesskey="p" href="numeric_converter_policy_classes.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="../numeric_conversion/history_and_acknowledgments.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
- </div>
- </body>
- </html>
|