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187 lines
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<title>cpp<int C, int S, int I, int L, int LL></title>
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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
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<table cellpadding="2" width="100%"><tr>
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<td valign="top"><img href="index.html" height="164px" src="pre-boost.jpg" alt="Library Documentation Index"></td>
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<td><h2>Safe Numerics</h2></td>
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</tr></table>
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<a accesskey="p" href="automatic_policy.html"><img src="images/prev.png" alt="Prev"></a><a accesskey="u" href="promotion_policies.html"><img src="images/up.png" alt="Up"></a><a accesskey="h" href="index.html"><img src="images/home.png" alt="Home"></a><a accesskey="n" href="exception_policies.html"><img src="images/next.png" alt="Next"></a>
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</div>
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<div class="section">
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<div class="titlepage"><div><div><h4 class="title">
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<a name="safe_numerics.cpp_policy"></a>cpp<int C, int S, int I, int L, int LL></h4></div></div></div>
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<div class="section">
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<div class="titlepage"><div><div><h5 class="title">
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<a name="idm291788402256"></a>Description</h5></div></div></div>
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<p>This policy is used to promote safe types in arithmetic expressions
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according the standard rules in the C++ standard. But rather than using
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the native C++ standard types supported by the compiler, it uses types
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whose length in number of bits is specified by the template
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parameters.</p>
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<p>This policy is useful for running test program which use C++
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portable integer types but which are destined to run on an architecture
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which is different than the one on which the test program is being built
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and run. This can happen when developing code for embedded systems.
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Algorithms developed or borrowed from one architecture but destined for
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another can be tested on the desk top.</p>
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</div>
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<div class="section">
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<div class="titlepage"><div><div><h5 class="title">
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<a name="idm291788400000"></a>Template Parameters</h5></div></div></div>
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<div class="informaltable"><table class="table">
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<colgroup>
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<col align="left">
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<col align="left">
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<col align="left">
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</colgroup>
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<thead><tr>
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<th align="left">Parameter</th>
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<th align="left">Type</th>
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<th align="left">Description</th>
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</tr></thead>
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<tbody>
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<tr>
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<td align="left"><code class="computeroutput">C</code></td>
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<td align="left">int</td>
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<td align="left">Number of bits in a char</td>
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</tr>
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<tr>
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<td align="left"><code class="computeroutput">S</code></td>
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<td align="left">int</td>
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<td align="left">Number of bits in a short</td>
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</tr>
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<tr>
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<td align="left"><code class="computeroutput">I</code></td>
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<td align="left">int</td>
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<td align="left">Number of bits in an integer</td>
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</tr>
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<tr>
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<td align="left"><code class="computeroutput">L</code></td>
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<td align="left">int</td>
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<td align="left">Number of bits in a long</td>
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</tr>
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<tr>
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<td align="left"><code class="computeroutput">LL</code></td>
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<td align="left">int</td>
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<td align="left">Number of bits in a long long</td>
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</tr>
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</tbody>
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</table></div>
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</div>
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<div class="section">
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<div class="titlepage"><div><div><h5 class="title">
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<a name="idm291788383056"></a>Model of</h5></div></div></div>
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<p><a class="link" href="promotion_policy.html" title="PromotionPolicy<PP>">PromotionPolicy</a></p>
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</div>
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<div class="section">
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<div class="titlepage"><div><div><h5 class="title">
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<a name="idm291788381344"></a>Header</h5></div></div></div>
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<p><code class="computeroutput"><a href="../../examples/example9.cpp" target="_top"><code class="computeroutput">#include
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<boost/safe_numerics/cpp.hpp> </code></a></code></p>
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</div>
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<div class="section">
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<div class="titlepage"><div><div><h5 class="title">
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<a name="idm291788378816"></a>Example of Use</h5></div></div></div>
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<p>Consider the following problem. One is developing software which
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uses a very small microprocessor and a very limited C compiler. The chip
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is so small, you can't print anything from the code, log, debug or
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anything else. One debugs this code by using the "burn" and "crash" method
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- you burn the chip (download the code) run the code, observe the results,
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make changes and try again. This is a crude method which is usually the
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one used. But it can be quite time consuming.</p>
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<p>Consider an alternative. Build and compile your code in testable
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modules. For each module write a test which exercises all the code and
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makes it work. Finally download your code into the chip and - voilà -
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working product. This sounds great, but there's one problem. Our target
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processor - in this case a PIC162550 from Microchip Technology is only an
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8 bit CPU. The compiler we use defines INT as 8 bits. This (and a few
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other problems), make our algorithm testing environment differ from our
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target environment. We can address this by defining INT as a safe integer
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with a range of 8 bits. By using a custom promotion policy, we can force
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the evaluation of C++ expressions test environment to be the same as that
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in the target environment. Also in our target environment, we can trap any
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overflows or other errors. So we can write and test our code on our
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desktop system and download the code to the target knowing that it just
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has to work. This is a huge time saver and confidence builder. The
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following code is taken from a real project which has used this
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method.</p>
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<pre class="programlisting"><span class="preprocessor">#include</span> <span class="string">"../include/safe_integer.hpp"</span>
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<span class="preprocessor">#include</span> <span class="string">"../include/cpp.hpp"</span>
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<span class="comment">//////////////////////////////////////////////////////////////</span>
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<span class="comment">// Stepper Motor Control</span>
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<span class="comment">// emululate evironment for pic162550</span>
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<span class="comment">// data widths used by the CCS compiler for pic 16xxx series</span>
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<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>
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<span class="number">8</span><span class="special">,</span> <span class="comment">// char</span>
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<span class="number">8</span><span class="special">,</span> <span class="comment">// short - not used by pic 16xxxx</span>
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<span class="number">8</span><span class="special">,</span> <span class="comment">// int</span>
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<span class="number">16</span><span class="special">,</span> <span class="comment">// long</span>
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<span class="number">32</span> <span class="comment">// long long</span>
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<span class="special">></span><span class="special">;</span>
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<span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span> <span class="comment">// T is char, int, etc data type</span>
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<span class="keyword">using</span> <span class="identifier">safe_t</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">safe</span><span class="special"><</span>
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<span class="identifier">T</span><span class="special">,</span>
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<span class="identifier">pic16_promotion</span><span class="special">,</span>
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<span class="identifier">boost</span><span class="special">::</span><span class="identifier">numeric</span><span class="special">::</span><span class="identifier">trap_exception</span> <span class="comment">// use for compiling and running tests</span>
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<span class="special">></span><span class="special">;</span>
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<span class="keyword">using</span> <span class="identifier">int8</span> <span class="special">=</span> <span class="identifier">safe_t</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">int8_t</span><span class="special">></span><span class="special">;</span>
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<span class="keyword">using</span> <span class="identifier">int16</span> <span class="special">=</span> <span class="identifier">safe_t</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">int16_t</span><span class="special">></span><span class="special">;</span>
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<span class="keyword">using</span> <span class="identifier">int32</span> <span class="special">=</span> <span class="identifier">safe_t</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">int32_t</span><span class="special">></span><span class="special">;</span>
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<span class="keyword">using</span> <span class="identifier">uint8</span> <span class="special">=</span> <span class="identifier">safe_t</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">uint8_t</span><span class="special">></span><span class="special">;</span>
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<span class="keyword">using</span> <span class="identifier">uint16</span> <span class="special">=</span> <span class="identifier">safe_t</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="special">;</span>
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<span class="keyword">using</span> <span class="identifier">uint32</span> <span class="special">=</span> <span class="identifier">safe_t</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">uint32_t</span><span class="special">></span><span class="special">;</span>
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<span class="comment">////////////////////////////////////////////////////////////////</span>
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<span class="comment">// Mock defines, functions etc which are in he "real application</span>
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<span class="special">...</span>
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<span class="comment">// return value in steps</span>
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<span class="comment">/*
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Use the formula:
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stopping dist = v **2 / a / 2
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*/</span>
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<span class="identifier">uint16</span> <span class="identifier">get_stopping_distance</span><span class="special">(</span><span class="identifier">LEMPARAMETER</span> <span class="identifier">velocity</span><span class="special">)</span><span class="special">{</span>
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<span class="identifier">int32</span> <span class="identifier">d</span><span class="special">;</span>
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<span class="identifier">d</span> <span class="special">=</span> <span class="identifier">velocity</span><span class="special">;</span>
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<span class="identifier">d</span> <span class="special">*=</span> <span class="identifier">velocity</span><span class="special">;</span>
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<span class="identifier">d</span> <span class="special">/=</span> <span class="identifier">lem</span><span class="special">.</span><span class="identifier">acceleration</span><span class="special">;</span>
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<span class="identifier">d</span> <span class="special">/=</span> <span class="number">2</span><span class="special">;</span>
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<span class="keyword">return</span> <span class="identifier">d</span><span class="special">;</span>
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<span class="special">}</span>
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<span class="special">...</span></pre>
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<p>Note the usage of the compile time trap policy in order to detect at
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compile time any possible error conditions. As I write this, this is still
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being refined. Hopefully this will be available by the time you read
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this.</p>
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</div>
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</div>
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="left"></td>
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<td align="right"><div class="copyright-footer">Copyright © 2012 Robert Ramey<p><a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">Subject to Boost
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Software License</a></p>
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</div></td>
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</tr></table>
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