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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.3"></a>Mixing Approaches</h3></div></div></div>
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<p>For purposes of exposition, we've divided the discussion of how to
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eliminate runtime penalties by the different approaches available. A
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realistic program could likely include all techniques mentioned above.
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Consider the following:</p>
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<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">stdexcept</span><span class="special">></span>
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<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</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_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">automatic</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="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="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">automatic</span><span class="special">,</span>
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<span class="identifier">loose_trap_policy</span>
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<span class="special">></span><span class="special">;</span>
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<span class="comment">// define variables used for input</span>
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<span class="keyword">using</span> <span class="identifier">input_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">automatic</span><span class="special">,</span> <span class="comment">// we don't need automatic in this case</span>
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<span class="identifier">loose_exception_policy</span> <span class="comment">// assignment of out of range value should throw</span>
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<span class="special">></span><span class="special">;</span>
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<span class="comment">// function arguments can never be outside of limits</span>
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<span class="keyword">auto</span> <span class="identifier">f</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">safe_t</span> <span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">safe_t</span> <span class="special">&</span> <span class="identifier">y</span><span class="special">)</span><span class="special">{</span>
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<span class="keyword">auto</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> <span class="comment">// we know that this cannot fail</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="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">"(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="keyword">return</span> <span class="identifier">z</span><span class="special">;</span>
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<span class="special">}</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="identifier">argc</span><span class="special">,</span> <span class="keyword">const</span> <span class="keyword">char</span> <span class="special">*</span> <span class="identifier">argv</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 84:\n"</span><span class="special">;</span>
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<span class="identifier">input_safe_t</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="keyword">try</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">"type in values in format x y:"</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">flush</span><span class="special">;</span>
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<span class="identifier">std</span><span class="special">::</span><span class="identifier">cin</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="comment">// read varibles, maybe throw exception</span>
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<span class="special">}</span>
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<span class="keyword">catch</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">exception</span> <span class="special">&</span> <span class="identifier">e</span><span class="special">)</span><span class="special">{</span>
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<span class="comment">// none of the above should trap. Mark failure if they do</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="identifier">e</span><span class="special">.</span><span class="identifier">what</span><span class="special">(</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">1</span><span class="special">;</span>
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<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="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="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">safe_format</span><span class="special">(</span><span class="identifier">f</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="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>As before, we define a type <code class="computeroutput">safe_t</code> to reflect our
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view of legal values for this program. This uses the <a class="link" href="promotion_policies/automatic.html" title="automatic"><code class="computeroutput">automatic</code></a>
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type promotion policy as well as 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 enforce elimination of runtime penalties.</p></li>
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<li class="listitem"><p>The function <code class="computeroutput">f</code> accepts only arguments of type
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<code class="computeroutput">safe_t</code> so there is no need to check the input values.
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This performs the functionality of <span class="emphasis"><em><span class="bold"><strong>programming by contract</strong></span></em></span> with no
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runtime cost.</p></li>
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<li class="listitem"><p>In addition, we define <code class="computeroutput">input_safe_t</code> to be used
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when reading variables from the program console. Clearly, these can
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only be checked at runtime so they use the throw_exception policy.
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When variables are read from the console they are checked for legal
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values. We need no ad hoc code to do this, as these types are
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guaranteed to contain legal values and will throw an exception when
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this guarantee is violated. In other words, we automatically get
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checking of input variables with no additional programming.</p></li>
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<li class="listitem"><p>On calling of the function <code class="computeroutput">f</code>, arguments of type
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<code class="computeroutput">input_safe_t</code> are converted to values of type
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<code class="computeroutput">safe_t</code> . In this particular example, it can be
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determined at compile time that construction of an instance of a
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<code class="computeroutput">safe_t</code> from an <code class="computeroutput">input_safe_t</code> can never
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fail. Hence, no <code class="computeroutput">try/catch</code> block is necessary. The usage
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of 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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policy for <code class="computeroutput">safe_t</code> types guarantees this to be true at
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compile time.</p></li>
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</ul></div>
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<p>Here is the output from the program when values 12 and 32
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are input from the console:</p>
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<pre class="screen">example 84:
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type in values in format x y:33 45
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x<signed char>[-24,82] = 33
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y<signed char>[-24,82] = 45
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z = <short>[-48,164] = 78
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(x + y) = <short>[-48,164] = 78
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(x - y) = <signed char>[-106,106] = -12
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<short>[-48,164] = 78
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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 © 2012-2018 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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