safe_numerics/doc/boostbook/cpp.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE section PUBLIC "-//Boost//DTD BoostBook XML V1.1//EN"
"http://www.boost.org/tools/boostbook/dtd/boostbook.dtd">
<section id="safe_numerics.promotion_policies.cpp">
  <title>cpp&lt;int C, int S, int I, int L, int LL&gt;</title>

  <section>
    <title>Description</title>

    <para>This policy is used to promote safe types in arithmetic expressions
    according to the rules in the C++ standard. But rather than using the
    native C++ standard types supported by the compiler, it uses types whose
    length in number of bits is specified by the template parameters.</para>

    <para>This policy is useful for running test programs which use C++
    portable integer types but which are destined to run on an architecture
    which is different than the one on which the test program is being built
    and run. This can happen when developing code for embedded systems.
    Algorithms developed or borrowed from one architecture but destined for
    another can be tested on the desktop.</para>

    <para>Note that this policy is only applicable to safe types whose base
    type is a type fulfilling the type requirements of <link
    linkend="safe_numerics.integer">Integer</link>. </para>
  </section>

  <section>
    <title>Template Parameters</title>

    <informaltable>
      <tgroup cols="3">
        <colspec align="left" colwidth="1*"/>

        <colspec align="left" colwidth="1*"/>

        <colspec align="left" colwidth="6*"/>

        <thead>
          <row>
            <entry align="left">Parameter</entry>

            <entry align="left">Type</entry>

            <entry>Description</entry>
          </row>
        </thead>

        <tbody>
          <row>
            <entry><code>C</code></entry>

            <entry>int</entry>

            <entry>Number of bits in a char</entry>
          </row>

          <row>
            <entry><code>S</code></entry>

            <entry>int</entry>

            <entry>Number of bits in a short</entry>
          </row>

          <row>
            <entry><code>I</code></entry>

            <entry>int</entry>

            <entry>Number of bits in an integer</entry>
          </row>

          <row>
            <entry><code>L</code></entry>

            <entry>int</entry>

            <entry>Number of bits in a long</entry>
          </row>

          <row>
            <entry><code>LL</code></entry>

            <entry>int</entry>

            <entry>Number of bits in a long long</entry>
          </row>
        </tbody>
      </tgroup>
    </informaltable>
  </section>

  <section>
    <title>Model of</title>

    <para><link
    linkend="safe_numerics.promotion_policy">PromotionPolicy</link></para>
  </section>

  <section>
    <title>Example of Use</title>

    <para>Consider the following problem. One is developing software which
    uses a very small microprocessor and a very limited C compiler. The chip
    is so small, you can't print anything from the code, log, debug or
    anything else. One debugs this code by using the "burn" and "crash" method
    - you burn the chip (download the code), run the code, observe the
    results, make changes and try again. This is a crude method which is
    usually the one used. But it can be quite time consuming.</para>

    <para>Consider an alternative. Build and compile your code in testable
    modules. For each module write a test which exercises all the code and
    makes it work. Finally download your code into the chip and - voilà -
    working product. This sounds great, but there's one problem. Our target
    processor - in this case a PIC162550 from Microchip Technology is only an
    8 bit CPU. The compiler we use defines INT as 8 bits. This (and a few
    other problems), make our algorithm testing environment differ from our
    target environment. We can address this by defining INT as a safe integer
    with a range of 8 bits. By using a custom promotion policy, we can force
    the evaluation of C++ expressions in the test environment to be the same
    as that in the target environment. Also in our target environment, we can
    trap any overflows or other errors. So we can write and test our code on
    our desktop system and download the code to the target knowing that it
    just has to work. This is a huge time saver and confidence builder. The
    following code is taken from a real project which has used this
    method.</para>

    <programlisting>#include "../include/safe_integer.hpp"
#include "../include/cpp.hpp"

//////////////////////////////////////////////////////////////
// Stepper Motor Control

// emululate evironment for pic162550
// data widths used by the CCS compiler for pic 16xxx series

using pic16_promotion = boost::numeric::cpp&lt;
    8,  // char
    8,  // short - not used by pic 16xxxx
    8,  // int
    16, // long
    32  // long long
&gt;;

template &lt;typename T&gt; // T is char, int, etc data type
using safe_t = boost::numeric::safe&lt;
    T,
    pic16_promotion,
    boost::numeric::loose_trap_policy // use for compiling and running tests
&gt;;

using int8 = safe_t&lt;std::int8_t&gt;;
using int16 = safe_t&lt;std::int16_t&gt;;
using int32 = safe_t&lt;std::int32_t&gt;;
using uint8 = safe_t&lt;std::uint8_t&gt;;
using uint16 = safe_t&lt;std::uint16_t&gt;;
using uint32 = safe_t&lt;std::uint32_t&gt;;

////////////////////////////////////////////////////////////////
// Mock defines, functions etc which are in the "real application"

...

// return value in steps
/*
Use the formula:
    stopping dist = v **2 / a / 2
*/
uint16 get_stopping_distance(LEMPARAMETER velocity){
    int32 d;
    d = velocity;
    d *= velocity;
    d /= lem.acceleration;
    d /= 2;
    return d;
}

...</programlisting>

    <para>Note the usage of the compile time trap policy in order to detect at
    compile time any possible error conditions. As I write this, this is still
    being refined. Hopefully this will be available by the time you read
    this.</para>
  </section>

  <section>
    <title>Header</title>

    <para><code><ulink url="../../include/cpp.hpp"><code>#include
    &lt;boost/numeric/safe_numerics/cpp.hpp&gt; </code></ulink></code></para>
  </section>
</section>