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a) made trap_exception work b) updated manual and examples to show how to use library to eliminate runtime penalty c) added in safe_literal d) made corrections of various types
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89 lines
4.8 KiB
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<title>Eliminating Runtime Penalty</title>
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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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<a accesskey="p" href="7.html"><img src="../images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.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="../8.html"><img src="../images/next.png" alt="Next"></a>
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<div class="section">
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="safe_numerics.tutorial.8"></a>Eliminating Runtime Penalty</h2></div></div></div>
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<div class="toc"><dl>
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<dt><span class="section"><a href="../8.html">Using Automatic Type Promotion</a></span></dt>
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<dt><span class="section"><a href="../8.html">Using safe_range</a></span></dt>
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<dt><span class="section"><a href="../8.html"></a></span></dt>
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</dl></div>
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<p>Up until now, we've focused on detecting when incorrect results are
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produced and handling these occurances either by throwing an exception or
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invoking some designated function. We've achieved our goal of enforcing
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arithmetically correct behavior - but at what cost. For many C++ programers
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any runtime penalty is unacceptable. Whether or not one agrees with this
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tradeoff, its a fact that many C++ programmers feel this way. So the
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question arises as to how we alter our program to minimize or eliminate any
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runtime penalty.</p>
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<p>The first step is to determine what parts of a program might invoke
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exceptions. The following program is similar to previous examples but uses a
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special exception policy: <a class="link" href=""><code class="computeroutput">trap_exception</code></a>.</p>
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<pre class="programlisting">#include <iostream>
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#include "../include/safe_integer.hpp"
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#include "../include/exception.hpp" // include exception policies
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#include "safe_format.hpp" // prints out range and value of any type
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using safe_t = boost::numeric::safe<
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int,
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boost::numeric::native,
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boost::numeric::trap_exception // note use of "trap_exception" policy!
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>;
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safe_t f(const safe_t & x, const safe_t & y){
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// each statement below will fail to compile !
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safe_t z = x + y;
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std::cout << "(x + y)" << safe_format(x + y) << std::endl;
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std::cout << "(x - y)" << safe_format(x - y) << std::endl;
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return z;
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}
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int main(int argc, const char * argv[]){
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std::cout << "example 81:\n";
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safe_t x(INT_MAX); // will fail to compile
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safe_t y(2); // will fail to compile
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std::cout << "x" << safe_format(x) << std::endl;
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std::cout << "y" << safe_format(y) << std::endl;
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std::cout << "z" << safe_format(f(x, y)) << std::endl;
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return 0;
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}
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</pre>
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<p>Now,
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any expression which <span class="emphasis"><em><span class="bold"><strong>MIGHT</strong></span></em></span> fail at runtime is flagged with a
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compile time error. There is no longer any need for <code class="computeroutput">try/catch
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blocks</code>. Since this program does not compile, the <span class="bold"><strong>library absolutely <span class="bold"><strong>guarantees that no
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arithmetic expression</strong></span> will yield incorrect results</strong></span> .
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This is our original goal. Now all we need to do is make the program work.
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There are a couple of ways to do this.</p>
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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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