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- <title>PromotionPolicy<PP></title>
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- <div class="titlepage"><div><div><h3 class="title">
- <a name="safe_numerics.promotion_policy"></a>PromotionPolicy<PP></h3></div></div></div>
- <div class="toc"><dl class="toc">
- <dt><span class="section"><a href="promotion_policy.html#idm130203778896">Description</a></span></dt>
- <dt><span class="section"><a href="promotion_policy.html#idm130203768304">Notation</a></span></dt>
- <dt><span class="section"><a href="promotion_policy.html#idm130203759120">Valid Expressions</a></span></dt>
- <dt><span class="section"><a href="promotion_policy.html#idm130203734208">Models</a></span></dt>
- <dt><span class="section"><a href="promotion_policy.html#idm130203639168">Header</a></span></dt>
- </dl></div>
- <div class="section">
- <div class="titlepage"><div><div><h4 class="title">
- <a name="idm130203778896"></a>Description</h4></div></div></div>
- <p>In C++, arithmetic operations result in types which may or may not
- be the same as the constituent types. A promotion policy determines the
- type of the result of an arithmetic operation. For example, in the
- following code</p>
- <pre class="programlisting"><span class="keyword">int</span> <span class="identifier">x</span><span class="special">;</span>
- <span class="keyword">char</span> <span class="identifier">y</span><span class="special">;</span>
- <span class="keyword">auto</span> <span class="identifier">z</span> <span class="special">=</span> <span class="identifier">x</span> <span class="special">+</span> <span class="identifier">y</span></pre>
- <p>the type of <code class="computeroutput">z</code> will be an
- <code class="computeroutput">int</code>. This is a consequence for the standard rules for type
- promotion for C/C++ arithmetic. A key feature of library permits one to
- specify his own type promotion rules via a PromotionPolicy class.</p>
- </div>
- <div class="section">
- <div class="titlepage"><div><div><h4 class="title">
- <a name="idm130203768304"></a>Notation</h4></div></div></div>
- <div class="informaltable"><table class="table">
- <colgroup>
- <col align="left">
- <col align="left">
- </colgroup>
- <tbody>
- <tr>
- <td align="left"><code class="computeroutput">PP</code></td>
- <td align="left">A type that full fills the requirements of a
- PromotionPollicy</td>
- </tr>
- <tr>
- <td align="left"><code class="computeroutput">T, U</code></td>
- <td align="left">A type that is a model of the Numeric concept</td>
- </tr>
- <tr>
- <td align="left"><code class="computeroutput">R</code></td>
- <td align="left">An object of type modeling Numeric which can be used to
- construct a SafeNumeric type.</td>
- </tr>
- </tbody>
- </table></div>
- </div>
- <div class="section">
- <div class="titlepage"><div><div><h4 class="title">
- <a name="idm130203759120"></a>Valid Expressions</h4></div></div></div>
- <p>Any operations which result in integers which cannot be represented
- as some Numeric type will throw an exception.These expressions return a
- type which can be used as the basis create a SafeNumeric type.</p>
- <div class="informaltable"><table class="table">
- <colgroup>
- <col align="left">
- <col align="left">
- </colgroup>
- <thead><tr>
- <th align="left">Expression</th>
- <th align="left">Return Value</th>
- </tr></thead>
- <tbody>
- <tr>
- <td align="left"><code class="computeroutput">PP::addition_result<T,
- U>::type</code></td>
- <td align="left">unspecified Numeric type</td>
- </tr>
- <tr>
- <td align="left"><code class="computeroutput">PP::subtraction_result<T,
- U>::type</code></td>
- <td align="left">unspecified Numeric type</td>
- </tr>
- <tr>
- <td align="left"><code class="computeroutput">PP::multiplication_result<T,
- U>::type</code></td>
- <td align="left">unspecified Numeric type</td>
- </tr>
- <tr>
- <td align="left"><code class="computeroutput">PP::division_result<T,
- U>::type</code></td>
- <td align="left">unspecified Numeric type</td>
- </tr>
- <tr>
- <td align="left"><code class="computeroutput">PP::modulus_result<T, U>::type</code></td>
- <td align="left">unspecified Numeric type</td>
- </tr>
- <tr>
- <td align="left"><code class="computeroutput">PP::comparison_result<T,
- U>::type</code></td>
- <td align="left">bool</td>
- </tr>
- <tr>
- <td align="left"><code class="computeroutput">PP::left_shift_result<T,
- U>::type</code></td>
- <td align="left">unspecified Numeric type</td>
- </tr>
- <tr>
- <td align="left"><code class="computeroutput">PP::right_shift_result<T,
- u>::type</code></td>
- <td align="left">unspecified Numeric type</td>
- </tr>
- <tr>
- <td align="left"><code class="computeroutput">PP::bitwise_or_result<T,
- U>::type</code></td>
- <td align="left">unspecified Numeric type</td>
- </tr>
- <tr>
- <td align="left"><code class="computeroutput">PP::bitwise_and_result<T,
- U>::type</code></td>
- <td align="left">unspecified Numeric type</td>
- </tr>
- <tr>
- <td align="left"><code class="computeroutput">PP::bitwise_xor_result<T,
- U>::type</code></td>
- <td align="left">unspecified Numeric type</td>
- </tr>
- </tbody>
- </table></div>
- </div>
- <div class="section">
- <div class="titlepage"><div><div><h4 class="title">
- <a name="idm130203734208"></a>Models</h4></div></div></div>
- <p>The library contains a number of pre-made promotion policies:</p>
- <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
- <li class="listitem">
- <p><a name="safe_numerics.promotion_policy.models.native"></a><code class="computeroutput">boost::numeric::native</code></p>
- <p>Use the normal C/C++ expression type promotion rules.
- </p>
- <pre class="programlisting"><span class="keyword">int</span> <span class="identifier">x</span><span class="special">;</span>
- <span class="keyword">char</span> <span class="identifier">y</span><span class="special">;</span>
- <span class="keyword">auto</span> <span class="identifier">z</span> <span class="special">=</span> <span class="identifier">x</span> <span class="special">+</span> <span class="identifier">y</span><span class="special">;</span> <span class="comment">// could result in overflow</span>
- <span class="identifier">safe</span><span class="special"><</span><span class="keyword">int</span><span class="special">,</span> <span class="identifier">native</span><span class="special">></span> <span class="identifier">sx</span><span class="special">;</span>
- <span class="keyword">auto</span> <span class="identifier">sz</span> <span class="special">=</span> <span class="identifier">sx</span> <span class="special">+</span> <span class="identifier">y</span><span class="special">;</span> <span class="comment">// standard C++ code which detects errors</span></pre>
- <p>Type sz will be a <a class="link" href="safe_numeric_concept.html" title="SafeNumeric<T>">SafeNumeric</a> type
- based on <code class="computeroutput">int</code>. If the result exceeds the maximum value
- that can be stored in an <code class="computeroutput">int</code>, an error is
- detected.</p>
- <p>The <code class="computeroutput">native</code> policy is documented in <a class="link" href="promotion_policies/native.html" title="native">Promotion Policies -
- native</a>.</p>
- </li>
- <li class="listitem">
- <p><a name="safe_numerics.promotion_policy.models.automatic"></a><code class="computeroutput">boost::numeric::automatic</code></p>
- <p>Use optimizing expression type promotion rules. These rules
- replace the normal C/C++ type promotion rules with other rules which
- are designed to result in more efficient computations. Expression
- types are promoted to the smallest type which can be guaranteed to
- hold the result without overflow. If there is no such type, the result
- will be checked for overflow. Consider the following
- example:</p>
- <pre class="programlisting"><span class="keyword">int</span> <span class="identifier">x</span><span class="special">;</span>
- <span class="keyword">char</span> <span class="identifier">y</span><span class="special">;</span>
- <span class="keyword">auto</span> <span class="identifier">z</span> <span class="special">=</span> <span class="identifier">x</span> <span class="special">+</span> <span class="identifier">y</span><span class="special">;</span> <span class="comment">// could result in overflow</span>
- <span class="identifier">safe</span><span class="special"><</span><span class="keyword">int</span><span class="special">,</span> <span class="identifier">automatic</span><span class="special">></span> <span class="identifier">sx</span><span class="special">;</span>
- <span class="keyword">auto</span> <span class="identifier">sz</span> <span class="special">=</span> <span class="identifier">sx</span> <span class="special">+</span> <span class="identifier">y</span><span class="special">;</span>
- <span class="comment">// sz is a safe type based on long</span>
- <span class="comment">// hence sz is guaranteed not to overflow.</span>
- <span class="identifier">safe_unsigned_range</span><span class="special"><</span><span class="number">1</span><span class="special">,</span> <span class="number">4</span><span class="special">></span> <span class="identifier">a</span><span class="special">;</span>
- <span class="identifier">safe_unsigned_range</span><span class="special"><</span><span class="number">2</span><span class="special">,</span> <span class="number">4</span><span class="special">></span> <span class="identifier">b</span><span class="special">;</span>
- <span class="keyword">auto</span> <span class="identifier">c</span> <span class="special">=</span> <span class="identifier">a</span> <span class="special">+</span> <span class="identifier">b</span><span class="special">;</span> <span class="comment">// c will be a safe type with a range [3,8] and cannot overflow</span>
- </pre>
- <p>Type sz will be a <a class="link" href="safe_numeric_concept.html" title="SafeNumeric<T>">SafeNumeric</a> type
- which is guaranteed to hold he result of x + y. In this case that will
- be a long int (or perhaps a long long) depending upon the compiler and
- machine architecture. In this case, there will be no need for any
- special checking on the result and there can be no overflow.</p>
- <p>Type of c will be a signed character as that type can be
- guaranteed to hold the sum so no overflow checking is done.</p>
- <p>This policy is documented in <a class="link" href="promotion_policies/automatic.html" title="automatic">Promotion
- Policies - automatic</a></p>
- </li>
- <li class="listitem">
- <p>boost::numeric::cpp</p>
- <p>Use expression type promotion rules to emulate another
- processor. When this policy is used, C++ type for safe integers
- follows the rules that a compiler on the target processor would use.
- This permits one to test code destined for a one processor on the
- another one. One situation there this can be very, very useful is when
- testing code destined for a micro controller which doesn't have the
- logging, debugging, input/output facilities of a
- desktop.</p>
- <pre class="programlisting"><span class="comment">// specify a promotion policy to support proper emulation of </span>
- <span class="comment">// PIC 18f2520 types on the desktop</span>
- <span class="keyword">using</span> <span class="identifier">pic16_promotion</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">numeric</span><span class="special">::</span><span class="identifier">cpp</span><span class="special"><</span>
- <span class="number">8</span><span class="special">,</span> <span class="comment">// char 8 bits</span>
- <span class="number">16</span><span class="special">,</span> <span class="comment">// short 16 bits</span>
- <span class="number">16</span><span class="special">,</span> <span class="comment">// int 16 bits</span>
- <span class="number">16</span><span class="special">,</span> <span class="comment">// long 16 bits</span>
- <span class="number">32</span> <span class="comment">// long long 32 bits</span>
- <span class="special">></span><span class="special">;</span>
- <span class="special">...</span>
- <span class="identifier">safe</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">uint16_t</span><span class="special">,</span> <span class="identifier">pic16_promotion</span><span class="special">></span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">y</span><span class="special">;</span>
- <span class="special">...</span>
- <span class="identifier">x</span> <span class="special">+</span> <span class="identifier">y</span><span class="special">;</span> <span class="comment">// detect possible overflow on the pic.</span></pre>
- <p>For a complete example see <a class="link" href="safety_critical_embedded_controller.html" title="Safety Critical Embedded Controller">Safety
- Critical Embedded Controller</a>.</p>
- </li>
- </ul></div>
- </div>
- <div class="section">
- <div class="titlepage"><div><div><h4 class="title">
- <a name="idm130203639168"></a>Header</h4></div></div></div>
- <p><a href="../../include/concept/promotion_policy.hpp" target="_top"><code class="computeroutput">#include
- <boost/numeric/safe_numerics/concepts/promotion_policy.hpp>
- </code></a></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 © 2012-2018 Robert Ramey<p><a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">Subject to Boost
- Software License</a></p>
- </div></td>
- </tr></table>
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