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<title>Using safe_range and safe_literal</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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<div class="titlepage"><div><div><h3 class="title">
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<a name="safe_numerics.eliminate_runtime_penalty.2"></a>Using <a class="link" href="safe_range.html" title="safe_signed_range<MIN, MAX, PP, EP> and safe_unsigned_range<MIN, MAX, PP, EP>">safe_range</a>
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and <a class="link" href="safe_literal.html" title="safe_signed_literal<Value, PP , EP> and safe_unsigned_literal<Value, PP, EP>">safe_literal</a>
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</h3></div></div></div>
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<p>When trying to avoid arithmetic errors of the above type,
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programmers will select data types which are wide enough to hold values
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large enough to be certain that results won't overflow, but are not so
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large as to make the program needlessly inefficient. In the example below,
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we presume we know that the values we want to work with fall in the range
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[-24,82]. So we "know" the program will always result in a correct result.
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But since we trust no one, and since the program could change and the
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expressions be replaced with other ones, we'll still use the <a class="link" href="exception_policies.html#safe_numerics.exception_policies.loose_trap_policy"><code class="computeroutput">loose_trap_policy</code></a>
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exception policy to verify at compile time that what we "know" to be true
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is in fact true.</p>
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<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span>
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<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">safe_numerics</span><span class="special">/</span><span class="identifier">safe_integer_range</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
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<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">safe_numerics</span><span class="special">/</span><span class="identifier">safe_integer_literal</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
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<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">safe_numerics</span><span class="special">/</span><span class="identifier">exception</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
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<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">safe_numerics</span><span class="special">/</span><span class="identifier">native</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
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<span class="preprocessor">#include</span> <span class="string">"safe_format.hpp"</span> <span class="comment">// prints out range and value of any type</span>
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<span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">safe_numerics</span><span class="special">;</span>
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<span class="comment">// create a type for holding small integers in a specific range</span>
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<span class="keyword">using</span> <span class="identifier">safe_t</span> <span class="special">=</span> <span class="identifier">safe_signed_range</span><span class="special"><</span>
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<span class="special">-</span><span class="number">24</span><span class="special">,</span>
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<span class="number">82</span><span class="special">,</span>
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<span class="identifier">native</span><span class="special">,</span> <span class="comment">// C++ type promotion rules work OK for this example</span>
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<span class="identifier">loose_trap_policy</span> <span class="comment">// catch problems at compile time</span>
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<span class="special">></span><span class="special">;</span>
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<span class="comment">// create a type to hold one specific value</span>
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<span class="keyword">template</span><span class="special"><</span><span class="keyword">int</span> <span class="identifier">I</span><span class="special">></span>
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<span class="keyword">using</span> <span class="identifier">const_safe_t</span> <span class="special">=</span> <span class="identifier">safe_signed_literal</span><span class="special"><</span><span class="identifier">I</span><span class="special">,</span> <span class="identifier">native</span><span class="special">,</span> <span class="identifier">loose_trap_policy</span><span class="special">></span><span class="special">;</span>
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<span class="comment">// We "know" that C++ type promotion rules will work such that</span>
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<span class="comment">// addition will never overflow. If we change the program to break this,</span>
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<span class="comment">// the usage of the loose_trap_policy promotion policy will prevent compilation.</span>
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<span class="keyword">int</span> <span class="identifier">main</span><span class="special">(</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">const</span> <span class="keyword">char</span> <span class="special">*</span><span class="special">[</span><span class="special">]</span><span class="special">)</span><span class="special">{</span>
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<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"example 83:\n"</span><span class="special">;</span>
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<span class="keyword">constexpr</span> <span class="keyword">const</span> <span class="identifier">const_safe_t</span><span class="special"><</span><span class="number">10</span><span class="special">></span> <span class="identifier">x</span><span class="special">;</span>
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<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"x = "</span> <span class="special"><<</span> <span class="identifier">safe_format</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
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<span class="keyword">constexpr</span> <span class="keyword">const</span> <span class="identifier">const_safe_t</span><span class="special"><</span><span class="number">67</span><span class="special">></span> <span class="identifier">y</span><span class="special">;</span>
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<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"y = "</span> <span class="special"><<</span> <span class="identifier">safe_format</span><span class="special">(</span><span class="identifier">y</span><span class="special">)</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
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<span class="keyword">const</span> <span class="identifier">safe_t</span> <span class="identifier">z</span> <span class="special">=</span> <span class="identifier">x</span> <span class="special">+</span> <span class="identifier">y</span><span class="special">;</span>
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<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"x + y = "</span> <span class="special"><<</span> <span class="identifier">safe_format</span><span class="special">(</span><span class="identifier">x</span> <span class="special">+</span> <span class="identifier">y</span><span class="special">)</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
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<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="string">"z = "</span> <span class="special"><<</span> <span class="identifier">safe_format</span><span class="special">(</span><span class="identifier">z</span><span class="special">)</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
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<span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
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<span class="special">}</span>
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</pre>
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<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
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<li class="listitem"><p><code class="computeroutput"><code class="computeroutput">safe_signed_range</code></code> defines a type
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which is limited to the indicated range. Out of range assignments
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will be detected at compile time if possible (as in this case) or at
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run time if necessary.</p></li>
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<li class="listitem"><p>A safe range could be defined with the same minimum and
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maximum value effectively restricting the type to holding one
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specific value. This is what <code class="computeroutput">safe_signed_literal</code>
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does.</p></li>
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<li class="listitem"><p>Defining constants with <code class="computeroutput">safe_signed_literal</code>
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enables the library to correctly anticipate the correct range of the
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results of arithmetic expressions at compile time.</p></li>
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<li class="listitem"><p>The usage of <code class="computeroutput"><a class="link" href="exception_policies.html#safe_numerics.exception_policies.loose_trap_policy"><code class="computeroutput">loose_trap_policy</code></a></code>
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will mean that any assignment to z which could be outside its legal
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range will result in a compile time error.</p></li>
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<li class="listitem"><p>All safe integer operations are implemented as constant
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expressions. The usage of <code class="computeroutput">constexpr</code> will guarantee that
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<code class="computeroutput">z</code> will be available at compile time for any subsequent
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use.</p></li>
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<li class="listitem"><p>So if this program compiles, it's guaranteed to return a valid
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result.</p></li>
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</ul></div>
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<p>The output uses a custom output manipulator,
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<code class="computeroutput">safe_format</code>, for safe types to display the underlying type
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and its range as well as current value. This program produces the
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following run time output.</p>
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<pre class="screen">example 83:
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x = <signed char>[10,10] = 10
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y = <signed char>[67,67] = 67
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x + y = <int>[77,77] = 77
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z = <signed char>[-24,82] = 77</pre>
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<p>Take note of the various variable types:</p>
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<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
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<li class="listitem"><p><code class="computeroutput">x</code> and <code class="computeroutput">y</code> are safe types with fixed
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ranges which encompass one single value. They can hold only that
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value which they have been assigned at compile time.</p></li>
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<li class="listitem"><p><code class="computeroutput">The sum x + y can also be determined at compile
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time.</code></p></li>
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<li class="listitem"><p>The type of z is defined so that It can hold only values in
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the closed range -24,82. We can assign the sum of x + y because it
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is in the range that <code class="computeroutput">z</code> is guaranteed to hold. If the
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sum could not be be guaranteed to fall in the range of
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<code class="computeroutput">z</code>, we would get a compile time error due to the fact we
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are using the <code class="computeroutput">loose_trap_policy</code> exception
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policy.</p></li>
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</ul></div>
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<p>All this information regarding the range and values of
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variables has been determined at compile time. There is no runtime
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overhead. The usage of safe types does not alter the calculations or
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results in anyway. So <code class="computeroutput">safe_t</code> and <code class="computeroutput">const_safe_t</code>
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could be redefined to <code class="computeroutput">int</code> and <code class="computeroutput">const int</code>
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respectively and the program would operate identically - although it might
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We could compile the program for another machine - as is common when
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building embedded systems and know (assuming the target machine
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architecture was the same as our native one) that no erroneous results
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would ever be produced.</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 © 2012-2018 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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