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99 lines
4.8 KiB
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<title>Implicit conversions change data values</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="3.html"><img src="../images/prev.png" alt="Prev"></a><a accesskey="u" href="../tutorial.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="5.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><h3 class="title">
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<a name="safe_numerics.tutorial.4"></a>Implicit conversions change data values</h3></div></div></div>
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<p>A simple assignment or arithmetic expression will generally convert
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all the terms to the same type. Sometimes this can silently change values.
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For example, when a signed data variable contains a negative type,
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assigning to a unsigned type will be permitted by any C/C++ compiler but
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will be treated as large unsigned value. Most modern compilers will emit a
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compile time warning when this conversion is performed. The user may then
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decide to change some data types or apply a <code class="computeroutput">static_cast</code>. This
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is less than satisfactory for two reasons:</p>
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<div class="itemizedlist"><ul class="itemizedlist" type="disc">
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<li class="listitem"><p>It may be unwieldy to change all the types to signed or
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unsigned.</p></li>
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<li class="listitem"><p>Littering one's program with <code class="computeroutput">static_cast</code><code class="computeroutput">
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</code>makes it more difficult to read.</p></li>
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<li class="listitem"><p>We may believe that our signed type will never contain a
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negative value. If we ignore the any compiler warnings or use a
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<code class="computeroutput">static_cast</code> to suppress them, we'll fail to detect a
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program error when it is committed. This is aways a risk with
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casts.</p></li>
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</ul></div>
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<p>This solution is simple, Just replace instances of the <code class="computeroutput">int
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</code>with <code class="computeroutput">safe<int></code>.</p>
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<pre class="programlisting">#include <cassert>
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#include <exception>
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#include <iostream>
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#include "../include/safe_integer.hpp"
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void detected_msg(bool detected){
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std::cout << (detected ? "error detected!" : "error NOT detected! ") << std::endl;
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}
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int main(int argc, const char * argv[]){
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std::cout << "example 4: ";
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std::cout << "implicit conversions change data values" << std::endl;
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std::cout << "Not using safe numerics" << std::endl;
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try{
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int x = -1000;
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// the following silently produces an incorrect result
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char y = x;
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detected_msg(false);
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}
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catch(std::exception){
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assert(false); // never arrive here
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}
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// solution: replace int with safe<int> and char with safe<char>
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std::cout << "Using safe numerics" << std::endl;
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try{
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using namespace boost::numeric;
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safe<int> x = -1000;
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// throws exception when conversion change data value
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safe<char> y1(x);
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safe<char> y3 = x;
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safe<char> y = {x};
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y = x;
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assert(false); // never arrive here
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}
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catch(std::exception & e){
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std::cout << e.what() << std::endl;
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detected_msg(true);
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}
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return 0;
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}
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</pre>
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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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