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  26. <div class="titlepage"><div><div><h3 class="title">
    27. 

  27. <a name="boost_multiprecision.ref.internals"></a><a class="link" href="internals.html" title="Internal Support Code">Internal Support
    28. 

  28.       Code</a>
    29. 

  29. </h3></div></div></div>
    30. 

  30. <p>
    31. 

  31.         There are some traits classes which authors of new backends should be aware
    32. 

  32.         of:
    33. 

  33.       </p>
    34. 

  34. <pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">multiprecision</span><span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">detail</span><span class="special">{</span>
    35. 

  35. 

  36. <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">From</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">To</span><span class="special">&gt;</span>
    37. 

  37. <span class="keyword">struct</span> <span class="identifier">is_explicitly_convertible</span><span class="special">;</span>
    38. 

  38. 

  39. <span class="special">}}}</span>
    40. 

  40. </pre>
    41. 

  41. <p>
    42. 

  42.         Inherits from <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">integral_constant</span><span class="special">&lt;</span><span class="keyword">bool</span><span class="special">,</span><span class="keyword">true</span><span class="special">&gt;</span></code> if type <code class="computeroutput"><span class="identifier">From</span></code>
    43. 

  43.         has an explicit conversion from <code class="computeroutput"><span class="identifier">To</span></code>.
    44. 

  44.       </p>
    45. 

  45. <p>
    46. 

  46.         For compilers that support C++11 SFINAE-expressions this trait should "just
    47. 

  47.         work". Otherwise it inherits from <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">is_convertible</span><span class="special">&lt;</span><span class="identifier">From</span><span class="special">,</span> <span class="identifier">To</span><span class="special">&gt;::</span><span class="identifier">type</span></code>, and will need to be specialised for
    48. 

  48.         Backends that have constructors marked as <code class="computeroutput"><span class="keyword">explicit</span></code>.
    49. 

  49.       </p>
    50. 

  50. <pre class="programlisting"><span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">From</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">To</span><span class="special">&gt;</span>
    51. 

  51. <span class="keyword">struct</span> <span class="identifier">is_lossy_conversion</span>
    52. 

  52. <span class="special">{</span>
    53. 

  53.    <span class="keyword">static</span> <span class="keyword">const</span> <span class="keyword">bool</span> <span class="identifier">value</span> <span class="special">=</span> <span class="identifier">see</span> <span class="identifier">below</span><span class="special">;</span>
    54. 

  54. <span class="special">};</span>
    55. 

  55. </pre>
    56. 

  56. <p>
    57. 

  57.         Member <code class="computeroutput"><span class="identifier">value</span></code> is true if the
    58. 

  58.         conversion from <code class="computeroutput"><span class="identifier">From</span></code> to
    59. 

  59.         <code class="computeroutput"><span class="identifier">To</span></code> would result in a loss
    60. 

  60.         of precision, and <code class="computeroutput"><span class="keyword">false</span></code> otherwise.
    61. 

  61.       </p>
    62. 

  62. <p>
    63. 

  63.         The default version of this trait simply checks whether the <span class="emphasis"><em>kind</em></span>
    64. 

  64.         of conversion (for example from a floating-point to an integer type) is inherently
    65. 

  65.         lossy. Note that if either of the types <code class="computeroutput"><span class="identifier">From</span></code>
    66. 

  66.         or <code class="computeroutput"><span class="identifier">To</span></code> are of an unknown number
    67. 

  67.         category (because <code class="computeroutput"><span class="identifier">number_category</span></code>
    68. 

  68.         is not specialised for that type) then this trait will be <code class="computeroutput"><span class="keyword">true</span></code>.
    69. 

  69.       </p>
    70. 

  70. <pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">From</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">To</span><span class="special">&gt;</span>
    71. 

  71. <span class="keyword">struct</span> <span class="identifier">is_restricted_conversion</span>
    72. 

  72. <span class="special">{</span>
    73. 

  73.    <span class="keyword">static</span> <span class="keyword">const</span> <span class="keyword">bool</span> <span class="identifier">value</span> <span class="special">=</span> <span class="identifier">see</span> <span class="identifier">below</span><span class="special">;</span>
    74. 

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

  75. </pre>
    76. 

  76. <p>
    77. 

  77.         Member <code class="computeroutput"><span class="identifier">value</span></code> is <code class="computeroutput"><span class="keyword">true</span></code> if <code class="computeroutput"><span class="identifier">From</span></code>
    78. 

  78.         is only explicitly convertible to <code class="computeroutput"><span class="identifier">To</span></code>
    79. 

  79.         and not implicitly convertible, or if <code class="computeroutput"><span class="identifier">is_lossy_conversion</span><span class="special">&lt;</span><span class="identifier">From</span><span class="special">,</span> <span class="identifier">To</span><span class="special">&gt;::</span><span class="identifier">value</span></code> is <code class="computeroutput"><span class="keyword">true</span></code>.
    80. 

  80.         Otherwise <code class="computeroutput"><span class="keyword">false</span></code>.
    81. 

  81.       </p>
    82. 

  82. <p>
    83. 

  83.         Note that while this trait is the ultimate arbiter of which constructors
    84. 

  84.         are marked as <code class="computeroutput"><span class="keyword">explicit</span></code> in class
    85. 

  85.         <code class="computeroutput"><span class="identifier">number</span></code>, authors of backend
    86. 

  86.         types should generally specialise one of the traits above, rather than this
    87. 

  87.         one directly.
    88. 

  88.       </p>
    89. 

  89. <pre class="programlisting"><span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">&gt;</span>
    90. 

  90. <span class="identifier">is_signed_number</span><span class="special">;</span>
    91. 

  91. <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">&gt;</span>
    92. 

  92. <span class="identifier">is_unsigned_number</span><span class="special">;</span>
    93. 

  93. </pre>
    94. 

  94. <p>
    95. 

  95.         These two traits inherit from either <code class="computeroutput"><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">true_</span></code>
    96. 

  96.         or <code class="computeroutput"><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">false_</span></code>, by default types are assumed to
    97. 

  97.         be signed unless <code class="computeroutput"><span class="identifier">is_unsigned_number</span></code>
    98. 

  98.         is specialized for that type.
    99. 

  99.       </p>
    100. 

  100. </div>
    101. 

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    102. 

  102. <td align="left"></td>
    103. 

  103. <td align="right"><div class="copyright-footer">Copyright &#169; 2002-2019 John Maddock
    104. 

  104.       and Christopher Kormanyos<p>
    105. 

  105.         Distributed under the Boost Software License, Version 1.0. (See accompanying
    106. 

  106.         file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
    107. 

  107.       </p>
    108. 

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    110. 

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