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100 lines
4.2 KiB
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<html>
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<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
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<title>Mixing Data Types Can Create Subtle Errors</title>
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<link rel="home" href="../index.html" title="Safe Numerics">
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<link rel="prev" href="4.html" title="Implicit Conversions Change Data Values">
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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="4.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>
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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.10"></a>Mixing Data Types Can Create Subtle Errors</h3></div></div></div>
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<p>C++ contains signed and unsigned integer types. In spite of their
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names, they function differently which often produces surprising results
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for some operands. Program errors from this behavior can be exceedingly
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difficult to find. This has lead to recommendations of various ad hoc
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"rules" to avoid these problems. It's not always easy to apply these
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"rules" to existing code without creating even more bugs. Here is a
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typical example of this problem:</p>
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<pre class="programlisting">#include <iostream>
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#include <cstdint>
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#include "../include/safe_integer.hpp"
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#include "../include/cpp.hpp"
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using namespace std;
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using namespace boost::numeric;
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void f(const unsigned int & x, const int8_t & y){
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cout << x * y << endl;
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}
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void safe_f(
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const safe<unsigned int> & x,
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const safe<int8_t> & y
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){
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cout << x * y << endl;
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}
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int main(){
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cout << "example 10: ";
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cout << "mixing types produces surprising results" << endl;
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try {
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std::cout << "Not using safe numerics" << std::endl;
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// problem: arithmetic operations can yield incorrect results.
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f(100, 100); // works as expected
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f(100, -100); // wrong result - unnoticed
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}
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catch(std::exception){
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// never arrive here
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std::cout << "error detected!" << std::endl;
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}
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try {
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// solution: use safe types
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std::cout << "Using safe numerics" << std::endl;
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safe_f(100, 100); // works as expected
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safe_f(100, -100); // throw error
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}
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catch(const exception & e){
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cout << "detected error:" << e.what() << endl;;
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}
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return 0;
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}
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</pre>
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<p>Here
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is the output of the above program:</p>
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<pre class="programlisting">example 10: mixing types produces surprising results
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Not using safe numerics
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10000
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4294957296
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Using safe numerics
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10000
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detected error:converted negative value to unsigned
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</pre>
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<p>This solution is simple, Just replace instances of the
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<code class="computeroutput">int</code>with <code class="computeroutput">safe<int></code>.</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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</tr></table>
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<hr>
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