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  4. <title>native</title>
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  12. <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
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  13. <table cellpadding="2" width="100%"><tr>
    14. 

  14. <td valign="top"><img href="index.html" height="164px" src="pre-boost.jpg" alt="Library Documentation Index"></td>
    15. 

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

  16. </tr></table>
    17. 

  17. <div class="spirit-nav">
    18. 

  18. <a accesskey="p" href="../promotion_policies.html"><img src="../images/prev.png" alt="Prev"></a><a accesskey="u" href="../promotion_policies.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="automatic.html"><img src="../images/next.png" alt="Next"></a>
    19. 

  19. </div>
    20. 

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

  21. <div class="titlepage"><div><div><h4 class="title">
    22. 

  22. <a name="safe_numerics.promotion_policies.native"></a>native</h4></div></div></div>
    23. 

  23. <div class="section">
    24. 

  24. <div class="titlepage"><div><div><h5 class="title">
    25. 

  25. <a name="idm130202851568"></a>Description</h5></div></div></div>
    26. 

  26. <p>This type contains the functions to return a safe type corresponding
    27. 

  27.     to the C++ type resulting from a given arithmetic operation.</p>
    28. 

  28. <p>Usage of this policy with safe types will produce the exact same
    29. 

  29.     arithmetic results that using normal unsafe integer types will. Hence this
    30. 

  30.     policy is suitable as a drop-in replacement for these unsafe types. Its
    31. 

  31.     main function is to trap incorrect arithmetic results when using C++ for
    32. 

  32.     integer arithmetic.</p>
    33. 

  33. </div>
    34. 

  34. <div class="section">
    35. 

  35. <div class="titlepage"><div><div><h5 class="title">
    36. 

  36. <a name="idm130202849536"></a>Model of</h5></div></div></div>
    37. 

  37. <p><a class="link" href="promotion_policy.html" title="PromotionPolicy&lt;PP&gt;">PromotionPolicy</a></p>
    38. 

  38. <p>As an example of how this works consider C++ rules from section 5 of
    39. 

  39.     the standard - "usual arithmetic conversions".</p>
    40. 

  40. <pre class="programlisting"><span class="keyword">void</span> <span class="keyword">int</span> <span class="identifier">f</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">x</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">y</span><span class="special">)</span><span class="special">{</span>
    41. 

  41.     <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">// z will be of type "int"</span>
    42. 

  42.     <span class="keyword">return</span> <span class="identifier">z</span><span class="special">;</span>
    43. 

  43. <span class="special">}</span></pre>
    44. 

  44. <p>According to these rules, z will be of type int. Depending on the
    45. 

  45.     values of x and y, z may or may not contain the correct arithmetic result
    46. 

  46.     of the operation x + y.</p>
    47. 

  47. <pre class="programlisting"><span class="keyword">using</span> <span class="identifier">safe_int</span> <span class="special">=</span> <span class="identifier">safe</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">,</span> <span class="identifier">native</span><span class="special">&gt;</span><span class="special">;</span>
    48. 

  48. <span class="keyword">void</span> <span class="keyword">int</span> <span class="identifier">f</span><span class="special">(</span><span class="identifier">safe_int</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">safe_int</span> <span class="identifier">y</span><span class="special">)</span><span class="special">{</span>
    49. 

  49.     <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">// z will be of type "safe_int"</span>
    50. 

  50.     <span class="keyword">return</span> <span class="identifier">z</span><span class="special">;</span>
    51. 

  51. <span class="special">}</span></pre>
    52. 

  52. </div>
    53. 

  53. <div class="section">
    54. 

  54. <div class="titlepage"><div><div><h5 class="title">
    55. 

  55. <a name="idm130202813168"></a>Example of use</h5></div></div></div>
    56. 

  56. <p>The following example illustrates the <code class="computeroutput">native</code> type being
    57. 

  57.     passed as a template parameter for the type <code class="computeroutput">safe&lt;int&gt;</code>.
    58. 

  58.     This example is slightly contrived in that <code class="computeroutput">safe&lt;int&gt;</code>
    59. 

  59.     has <code class="computeroutput">native</code> as its default promotion parameter so explicitly
    60. 

  60.     using <code class="computeroutput">native</code> is not necessary.</p>
    61. 

  61. <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">cassert</span><span class="special">&gt;</span>
    62. 

  62. <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">numeric</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> 
    63. 

  63. <span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">numeric</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">&gt;</span> 
    64. 

  64. <span class="keyword">int</span> <span class="identifier">main</span><span class="special">(</span><span class="special">)</span><span class="special">{</span>
    65. 

  65.     <span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">numeric</span><span class="special">;</span>
    66. 

  66.     <span class="comment">// use native promotion policy where C++ standard arithmetic </span>
    67. 

  67.     <span class="comment">// might lead to incorrect results</span>
    68. 

  68.     <span class="keyword">using</span> <span class="identifier">safe_int8</span> <span class="special">=</span> <span class="identifier">safe</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">int8_t</span><span class="special">,</span> <span class="identifier">native</span><span class="special">&gt;</span><span class="special">;</span>
    69. 

  69.     <span class="keyword">try</span><span class="special">{</span>
    70. 

  70.         <span class="identifier">safe_int8</span> <span class="identifier">x</span> <span class="special">=</span> <span class="number">127</span><span class="special">;</span>
    71. 

  71.         <span class="identifier">safe_int8</span> <span class="identifier">y</span> <span class="special">=</span> <span class="number">2</span><span class="special">;</span>
    72. 

  72.         <span class="identifier">safe_int8</span> <span class="identifier">z</span><span class="special">;</span>
    73. 

  73.         <span class="comment">// rather than producing an invalid result an exception is thrown</span>
    74. 

  74.         <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>
    75. 

  75.         <span class="identifier">assert</span><span class="special">(</span><span class="keyword">false</span><span class="special">)</span><span class="special">;</span> <span class="comment">// never arrive here</span>
    76. 

  76.     <span class="special">}</span>
    77. 

  77.     <span class="keyword">catch</span><span class="special">(</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>
    78. 

  78.         <span class="comment">// which we can catch here</span>
    79. 

  79.         <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>
    80. 

  80.     <span class="special">}</span>
    81. 

  81. 

  82.     <span class="comment">// When result is an int, C++ promotion rules guarentee </span>
    83. 

  83.     <span class="comment">// that there will be no incorrect result.</span>
    84. 

  84.     <span class="comment">// In such cases, there is no runtime overhead from using safe types.</span>
    85. 

  85.     <span class="identifier">safe_int8</span> <span class="identifier">x</span> <span class="special">=</span> <span class="number">127</span><span class="special">;</span>
    86. 

  86.     <span class="identifier">safe_int8</span> <span class="identifier">y</span> <span class="special">=</span> <span class="number">2</span><span class="special">;</span>
    87. 

  87.     <span class="identifier">safe</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">,</span> <span class="identifier">native</span><span class="special">&gt;</span> <span class="identifier">z</span><span class="special">;</span> <span class="comment">// z can now hold the result of the addition of any two 8 bit numbers</span>
    88. 

  88.     <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">// is guaranteed correct without any runtime overhead or exception.</span>
    89. 

  89. 

  90.     <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
    91. 

  91. <span class="special">}</span></pre>
    92. 

  92. </div>
    93. 

  93. <div class="section">
    94. 

  94. <div class="titlepage"><div><div><h5 class="title">
    95. 

  95. <a name="idm130202727600"></a>Notes</h5></div></div></div>
    96. 

  96. <p>See Chapter 5, Expressions, C++ Standard</p>
    97. 

  97. </div>
    98. 

  98. <div class="section">
    99. 

  99. <div class="titlepage"><div><div><h5 class="title">
    100. 

  100. <a name="idm130202726384"></a>Header</h5></div></div></div>
    101. 

  101. <p><code class="computeroutput"><a href="../../include/native.hpp" target="_top"><code class="computeroutput">#include
    102. 

  102.     &lt;boost/numeric/safe_numerics/native.hpp&gt;
    103. 

  103.     </code></a></code></p>
    104. 

  104. </div>
    105. 

  105. </div>
    106. 

  106. <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
    107. 

  107. <td align="left"></td>
    108. 

  108. <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
    109. 

  109.       Software License</a></p>
    110. 

  110. </div></td>
    111. 

  111. </tr></table>
    112. 

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    113. 

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    114. 

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    115. 

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    118.