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| <div class="section" title="Type Requirements and User-defined-types support"> |
| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="boost_numericconversion.type_requirements_and_user_defined_types_support"></a><a class="link" href="type_requirements_and_user_defined_types_support.html" title="Type Requirements and User-defined-types support">Type |
| Requirements and User-defined-types support</a> |
| </h2></div></div></div> |
| <div class="toc"><dl> |
| <dt><span class="section"><a href="type_requirements_and_user_defined_types_support.html#boost_numericconversion.type_requirements_and_user_defined_types_support.type_requirements">Type |
| Requirements</a></span></dt> |
| <dt><span class="section"><a href="type_requirements_and_user_defined_types_support.html#boost_numericconversion.type_requirements_and_user_defined_types_support.udt_s_special_semantics">UDT's |
| special semantics</a></span></dt> |
| <dt><span class="section"><a href="type_requirements_and_user_defined_types_support.html#boost_numericconversion.type_requirements_and_user_defined_types_support.special_policies">Special |
| Policies</a></span></dt> |
| </dl></div> |
| <div class="section" title="Type Requirements"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="boost_numericconversion.type_requirements_and_user_defined_types_support.type_requirements"></a><a class="link" href="type_requirements_and_user_defined_types_support.html#boost_numericconversion.type_requirements_and_user_defined_types_support.type_requirements" title="Type Requirements">Type |
| Requirements</a> |
| </h3></div></div></div> |
| <p> |
| Both arithmetic (built-in) and user-defined numeric types require proper |
| specialization of <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special"><></span></code> |
| (that is, with (in-class) integral constants). |
| </p> |
| <p> |
| The library uses <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">is_specialized</span></code> to detect whether the type |
| is builtin or user defined, and <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">is_integer</span></code>, <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">numeric_limits</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">is_signed</span></code> to detect whether the type is |
| integer or floating point; and whether it is signed/unsigned. |
| </p> |
| <p> |
| The default <code class="computeroutput"><span class="identifier">Float2IntRounder</span></code> |
| policies uses unqualified calls to functions <code class="computeroutput"><span class="identifier">floor</span><span class="special">()</span></code> and <code class="computeroutput"><span class="identifier">ceil</span><span class="special">()</span></code>; but the standard functions are introduced |
| in scope by a using directive: |
| </p> |
| <pre class="programlisting"><span class="keyword">using</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">floor</span> <span class="special">;</span> <span class="keyword">return</span> <span class="identifier">floor</span><span class="special">(</span><span class="identifier">s</span><span class="special">);</span> |
| </pre> |
| <p> |
| Therefore, for builtin arithmetic types, the std functions will be used. |
| User defined types should provide overloaded versions of these functions |
| in order to use the default rounder policies. If these overloads are defined |
| within a user namespace argument dependent lookup (ADL) should find them, |
| but if your compiler has a weak ADL you might need to put these functions |
| some place else or write your own rounder policy. |
| </p> |
| <p> |
| The default <code class="computeroutput"><span class="identifier">Trunc</span><span class="special"><></span></code> |
| rounder policy needs to determine if the source value is positive or not, |
| and for this it evaluates the expression <code class="computeroutput"><span class="identifier">s</span> |
| <span class="special"><</span> <span class="keyword">static_cast</span><span class="special"><</span><span class="identifier">S</span><span class="special">>(</span><span class="number">0</span><span class="special">)</span></code>. Therefore, |
| user defined types require a visible <code class="computeroutput"><span class="keyword">operator</span><span class="special"><</span></code> in order to use the <code class="computeroutput"><span class="identifier">Trunc</span><span class="special"><></span></code> policy (the default). |
| </p> |
| </div> |
| <div class="section" title="UDT's special semantics"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="boost_numericconversion.type_requirements_and_user_defined_types_support.udt_s_special_semantics"></a><a class="link" href="type_requirements_and_user_defined_types_support.html#boost_numericconversion.type_requirements_and_user_defined_types_support.udt_s_special_semantics" title="UDT's special semantics">UDT's |
| special semantics</a> |
| </h3></div></div></div> |
| <a name="boost_numericconversion.type_requirements_and_user_defined_types_support.udt_s_special_semantics.conversion_traits"></a><h5> |
| <a name="id564459"></a> |
| <a class="link" href="type_requirements_and_user_defined_types_support.html#boost_numericconversion.type_requirements_and_user_defined_types_support.udt_s_special_semantics.conversion_traits">Conversion |
| Traits</a> |
| </h5> |
| <p> |
| If a User Defined Type is involved in a conversion, it is <span class="emphasis"><em>assumed</em></span> |
| that the UDT has <a class="link" href="definitions.html#boost_numericconversion.definitions.range_and_precision" title="Range and Precision">wider |
| range</a> than any built-in type, and consequently the values of some |
| <code class="computeroutput"><span class="identifier">converter_traits</span><span class="special"><></span></code> |
| members are hardwired regardless of the reality. The following table summarizes |
| this: |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" type="disc"> |
| <li class="listitem"> |
| <code class="computeroutput"><span class="identifier">Target</span><span class="special">=</span></code><span class="emphasis"><em>UDT</em></span> |
| and <code class="computeroutput"><span class="identifier">Source</span><span class="special">=</span></code><span class="emphasis"><em>built-in</em></span><div class="itemizedlist"><ul class="itemizedlist" type="circle"> |
| <li class="listitem"><code class="computeroutput"><span class="identifier">subranged</span><span class="special">=</span><span class="keyword">false</span></code></li> |
| <li class="listitem"><code class="computeroutput"><span class="identifier">supertype</span><span class="special">=</span><span class="identifier">Target</span></code></li> |
| <li class="listitem"><code class="computeroutput"><span class="identifier">subtype</span><span class="special">=</span><span class="identifier">Source</span></code></li> |
| </ul></div> |
| </li> |
| <li class="listitem"> |
| <code class="computeroutput"><span class="identifier">Target</span><span class="special">=</span></code><span class="emphasis"><em>built-in</em></span> |
| and <code class="computeroutput"><span class="identifier">Source</span><span class="special">=</span></code><span class="emphasis"><em>UDT</em></span><div class="itemizedlist"><ul class="itemizedlist" type="circle"> |
| <li class="listitem"><code class="computeroutput"><span class="identifier">subranged</span><span class="special">=</span><span class="keyword">true</span></code></li> |
| <li class="listitem"><code class="computeroutput"><span class="identifier">supertype</span><span class="special">=</span><span class="identifier">Source</span></code></li> |
| <li class="listitem"><code class="computeroutput"><span class="identifier">subtype</span><span class="special">=</span><span class="identifier">Target</span></code></li> |
| </ul></div> |
| </li> |
| <li class="listitem"> |
| <code class="computeroutput"><span class="identifier">Target</span><span class="special">=</span></code><span class="emphasis"><em>UDT</em></span> |
| and <code class="computeroutput"><span class="identifier">Source</span><span class="special">=</span></code><span class="emphasis"><em>UDT</em></span><div class="itemizedlist"><ul class="itemizedlist" type="circle"> |
| <li class="listitem"><code class="computeroutput"><span class="identifier">subranged</span><span class="special">=</span><span class="keyword">false</span></code></li> |
| <li class="listitem"><code class="computeroutput"><span class="identifier">supertype</span><span class="special">=</span><span class="identifier">Target</span></code></li> |
| <li class="listitem"><code class="computeroutput"><span class="identifier">subtype</span><span class="special">=</span><span class="identifier">Source</span></code></li> |
| </ul></div> |
| </li> |
| </ul></div> |
| <p> |
| The <code class="computeroutput"><span class="identifier">Traits</span></code> member <code class="computeroutput"><span class="identifier">udt_mixture</span></code> can be used to detect whether |
| a UDT is involved and to infer the validity of the other members as shown |
| above. |
| </p> |
| <a name="boost_numericconversion.type_requirements_and_user_defined_types_support.udt_s_special_semantics.range_checking"></a><h5> |
| <a name="id564867"></a> |
| <a class="link" href="type_requirements_and_user_defined_types_support.html#boost_numericconversion.type_requirements_and_user_defined_types_support.udt_s_special_semantics.range_checking">Range |
| Checking</a> |
| </h5> |
| <p> |
| Because User Defined Numeric Types might have peculiar ranges (such as an |
| unbounded range), this library does not attempt to supply a meaningful range |
| checking logic when UDTs are involved in a conversion. Therefore, if either |
| Target or Source are not built-in types, the bundled range checking of the |
| <code class="computeroutput"><span class="identifier">converter</span><span class="special"><></span></code> |
| function object is automatically disabled. However, it is possible to supply |
| a user-defined range-checker. See <a class="link" href="type_requirements_and_user_defined_types_support.html#boost_numericconversion.type_requirements_and_user_defined_types_support.special_policies" title="Special Policies">Special |
| Policies</a> |
| </p> |
| </div> |
| <div class="section" title="Special Policies"> |
| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="boost_numericconversion.type_requirements_and_user_defined_types_support.special_policies"></a><a class="link" href="type_requirements_and_user_defined_types_support.html#boost_numericconversion.type_requirements_and_user_defined_types_support.special_policies" title="Special Policies">Special |
| Policies</a> |
| </h3></div></div></div> |
| <p> |
| There are two components of the <code class="computeroutput"><span class="identifier">converter</span><span class="special"><></span></code> class that might require special |
| behavior if User Defined Numeric Types are involved: the Range Checking and |
| the Raw Conversion. |
| </p> |
| <p> |
| When both Target and Source are built-in types, the converter class uses |
| an internal range checking logic which is optimized and customized for the |
| combined properties of the types. |
| </p> |
| <p> |
| However, this internal logic is disabled when either type is User Defined. |
| In this case, the user can specify an <span class="emphasis"><em>external</em></span> range |
| checking policy which will be used in place of the internal code. See <a class="link" href="numeric_converter_policy_classes.html#boost_numericconversion.numeric_converter_policy_classes.policy_userrangechecker" title="Policy UserRangeChecker">UserRangeChecker</a> |
| policy for details. |
| </p> |
| <p> |
| The converter class performs the actual conversion using a Raw Converter |
| policy. The default raw converter simply performs a <code class="computeroutput"><span class="keyword">static_cast</span><span class="special"><</span><span class="identifier">Target</span><span class="special">>(</span><span class="identifier">source</span><span class="special">)</span></code>. |
| </p> |
| <p> |
| However, if the a UDT is involved, the <code class="computeroutput"><span class="keyword">static_cast</span></code> |
| might not work. In this case, the user can implement and pass a different |
| raw converter policy. See <a class="link" href="numeric_converter_policy_classes.html#boost_numericconversion.numeric_converter_policy_classes.policy_rawconverter" title="Policy RawConverter">RawConverter</a> |
| policy for details |
| </p> |
| </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> |
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