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    15. 

  15. <td><h2>Safe Numerics</h2></td>
    16. 

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    17. 

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  20. <div class="section">
    21. 

  21. <div class="titlepage"><div><div><h3 class="title">
    22. 

  22. <a name="safe_numerics.eliminate_runtime_penalty.3"></a>Mixing Approaches</h3></div></div></div>
    23. 

  23. <p>For purposes of exposition, we've divided the discussion of how to
    24. 

  24.     eliminate runtime penalties by the different approaches available. A
    25. 

  25.     realistic program could likely include all techniques mentioned above.
    26. 

  26.     Consider the following:</p>
    27. 

  27. <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">stdexcept</span><span class="special">&gt;</span>
    28. 

  28. <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
    29. 

  29. 

  30. <span class="preprocessor">#include</span> <span class="special">&lt;</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">&gt;</span>
    31. 

  31. <span class="preprocessor">#include</span> <span class="special">&lt;</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">&gt;</span>
    32. 

  32. <span class="preprocessor">#include</span> <span class="special">&lt;</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">&gt;</span>
    33. 

  33. <span class="preprocessor">#include</span> <span class="special">&lt;</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">&gt;</span>
    34. 

  34. 

  35. <span class="preprocessor">#include</span> <span class="string">"safe_format.hpp"</span> <span class="comment">// prints out range and value of any type</span>
    36. 

  36. 

  37. <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>
    38. 

  38. 

  39. <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">&lt;</span>
    40. 

  40.     <span class="special">-</span><span class="number">24</span><span class="special">,</span>
    41. 

  41.     <span class="number">82</span><span class="special">,</span>
    42. 

  42.     <span class="identifier">automatic</span><span class="special">,</span>
    43. 

  43.     <span class="identifier">loose_trap_policy</span>
    44. 

  44. <span class="special">&gt;</span><span class="special">;</span>
    45. 

  45. 

  46. <span class="comment">// define variables used for input</span>
    47. 

  47. <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">&lt;</span>
    48. 

  48.     <span class="special">-</span><span class="number">24</span><span class="special">,</span>
    49. 

  49.     <span class="number">82</span><span class="special">,</span>
    50. 

  50.     <span class="identifier">automatic</span><span class="special">,</span> <span class="comment">// we don't need automatic in this case</span>
    51. 

  51.     <span class="identifier">loose_exception_policy</span> <span class="comment">// assignment of out of range value should throw</span>
    52. 

  52. <span class="special">&gt;</span><span class="special">;</span>
    53. 

  53. 

  54. <span class="comment">// function arguments can never be outside of limits</span>
    55. 

  55. <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">&amp;</span> <span class="identifier">x</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">safe_t</span> <span class="special">&amp;</span> <span class="identifier">y</span><span class="special">)</span><span class="special">{</span>
    56. 

  56.     <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>
    57. 

  57.     <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"z = "</span> <span class="special">&lt;&lt;</span> <span class="identifier">safe_format</span><span class="special">(</span><span class="identifier">z</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
    58. 

  58.     <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"(x + y) = "</span> <span class="special">&lt;&lt;</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">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
    59. 

  59.     <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"(x - y) = "</span> <span class="special">&lt;&lt;</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">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
    60. 

  60.     <span class="keyword">return</span> <span class="identifier">z</span><span class="special">;</span>
    61. 

  61. <span class="special">}</span>
    62. 

  62. 

  63. <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>
    64. 

  64.     <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"example 84:\n"</span><span class="special">;</span>
    65. 

  65.     <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>
    66. 

  66.     <span class="keyword">try</span><span class="special">{</span>
    67. 

  67.         <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"type in values in format x y:"</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">flush</span><span class="special">;</span>
    68. 

  68.         <span class="identifier">std</span><span class="special">::</span><span class="identifier">cin</span> <span class="special">&gt;&gt;</span> <span class="identifier">x</span> <span class="special">&gt;&gt;</span> <span class="identifier">y</span><span class="special">;</span> <span class="comment">// read varibles, maybe throw exception</span>
    69. 

  69.     <span class="special">}</span>
    70. 

  70.     <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">&amp;</span> <span class="identifier">e</span><span class="special">)</span><span class="special">{</span>
    71. 

  71.         <span class="comment">// none of the above should trap. Mark failure if they do</span>
    72. 

  72.         <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</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">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
    73. 

  73.         <span class="keyword">return</span> <span class="number">1</span><span class="special">;</span>
    74. 

  74.     <span class="special">}</span>
    75. 

  75.     <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"x"</span> <span class="special">&lt;&lt;</span> <span class="identifier">safe_format</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
    76. 

  76.     <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"y"</span> <span class="special">&lt;&lt;</span> <span class="identifier">safe_format</span><span class="special">(</span><span class="identifier">y</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
    77. 

  77.     <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</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">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
    78. 

  78.     <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
    79. 

  79. <span class="special">}</span>
    80. 

  80. </pre>
    81. 

  81. <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
    82. 

  82. <li class="listitem"><p>As before, we define a type <code class="computeroutput">safe_t</code> to reflect our
    83. 

  83.           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>
    84. 

  84.           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>
    85. 

  85.           exception policy to enforce elimination of runtime penalties.</p></li>
    86. 

  86. <li class="listitem"><p>The function <code class="computeroutput">f</code> accepts only arguments of type
    87. 

  87.           <code class="computeroutput">safe_t</code> so there is no need to check the input values.
    88. 

  88.           This performs the functionality of <span class="emphasis"><em><span class="bold"><strong>programming by contract</strong></span></em></span> with no
    89. 

  89.           runtime cost.</p></li>
    90. 

  90. <li class="listitem"><p>In addition, we define <code class="computeroutput">input_safe_t</code> to be used
    91. 

  91.           when reading variables from the program console. Clearly, these can
    92. 

  92.           only be checked at runtime so they use the throw_exception policy.
    93. 

  93.           When variables are read from the console they are checked for legal
    94. 

  94.           values. We need no ad hoc code to do this, as these types are
    95. 

  95.           guaranteed to contain legal values and will throw an exception when
    96. 

  96.           this guarantee is violated. In other words, we automatically get
    97. 

  97.           checking of input variables with no additional programming.</p></li>
    98. 

  98. <li class="listitem"><p>On calling of the function <code class="computeroutput">f</code>, arguments of type
    99. 

  99.           <code class="computeroutput">input_safe_t</code> are converted to values of type
    100. 

  100.           <code class="computeroutput">safe_t</code> . In this particular example, it can be
    101. 

  101.           determined at compile time that construction of an instance of a
    102. 

  102.           <code class="computeroutput">safe_t</code> from an <code class="computeroutput">input_safe_t</code> can never
    103. 

  103.           fail. Hence, no <code class="computeroutput">try/catch</code> block is necessary. The usage
    104. 

  104.           of the <a class="link" href="exception_policies.html#safe_numerics.exception_policies.loose_trap_policy"><code class="computeroutput">loose_trap_policy</code></a>
    105. 

  105.           policy for <code class="computeroutput">safe_t</code> types guarantees this to be true at
    106. 

  106.           compile time.</p></li>
    107. 

  107. </ul></div>
    108. 

  108. <p>Here is the output from the program when values 12 and 32
    109. 

  109.     are input from the console:</p>
    110. 

  110. <pre class="screen">example 84:
    111. 

  111. type in values in format x y:33 45
    112. 

  112. x&lt;signed char&gt;[-24,82] = 33
    113. 

  113. y&lt;signed char&gt;[-24,82] = 45
    114. 

  114. z = &lt;short&gt;[-48,164] = 78
    115. 

  115. (x + y) = &lt;short&gt;[-48,164] = 78
    116. 

  116. (x - y) = &lt;signed char&gt;[-106,106] = -12
    117. 

  117. &lt;short&gt;[-48,164] = 78
    118. 

  118. </pre>
    119. 

  119. </div>
    120. 

  120. <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
    121. 

  121. <td align="left"></td>
    122. 

  122. <td align="right"><div class="copyright-footer">Copyright &#169; 2012-2018 Robert Ramey<p><a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">Subject to Boost
    123. 

  123.       Software License</a></p>
    124. 

  124. </div></td>
    125. 

  125. </tr></table>
    126. 

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    127. 

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