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tti_detail_has_template_metafunction.html

tti_detail_has_template_metafunction.html 26 KB
Riwayat Mentahan

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  4. <title>Using the has_template_(xxx) metafunction</title>
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    26. 

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

  27. <a name="the_type_traits_introspection_library.tti_detail_has_template.tti_detail_has_template_metafunction"></a><a class="link" href="tti_detail_has_template_metafunction.html" title="Using the has_template_(xxx) metafunction">Using
    28. 

  28.       the has_template_(xxx) metafunction</a>
    29. 

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

  30. <h5>
    31. 

  31. <a name="the_type_traits_introspection_library.tti_detail_has_template.tti_detail_has_template_metafunction.h0"></a>
    32. 

  32.         <span class="phrase"><a name="the_type_traits_introspection_library.tti_detail_has_template.tti_detail_has_template_metafunction.generating_the_metafunction"></a></span><a class="link" href="tti_detail_has_template_metafunction.html#the_type_traits_introspection_library.tti_detail_has_template.tti_detail_has_template_metafunction.generating_the_metafunction">Generating
    33. 

  33.         the metafunction</a>
    34. 

  34.       </h5>
    35. 

  35. <p>
    36. 

  36.         You generate the metafunction by invoking the macro with the name of an inner
    37. 

  37.         class template:
    38. 

  38.       </p>
    39. 

  39. <pre class="programlisting"><span class="comment">// `template type parameters` form</span>
    40. 

  40. 

  41. <span class="identifier">BOOST_TTI_HAS_TEMPLATE</span><span class="special">(</span><span class="identifier">AClassTemplate</span><span class="special">,</span><span class="identifier">BOOST_PP_NIL</span><span class="special">)</span> <span class="comment">// non-variadic macro</span>
    42. 

  42. <span class="identifier">BOOST_TTI_HAS_TEMPLATE</span><span class="special">(</span><span class="identifier">AClassTemplate</span><span class="special">)</span>              <span class="comment">// variadic macro</span>
    43. 

  43. 

  44. <span class="comment">// `specific parameters` form</span>
    45. 

  45. 

  46. <span class="identifier">BOOST_TTI_HAS_TEMPLATE</span><span class="special">(</span><span class="identifier">AClassTemplate</span><span class="special">,(</span><span class="number">2</span><span class="special">,(</span><span class="keyword">class</span><span class="special">,</span><span class="keyword">int</span><span class="special">)))</span> <span class="comment">// non-variadic macro</span>
    47. 

  47. <span class="identifier">BOOST_TTI_HAS_TEMPLATE</span><span class="special">(</span><span class="identifier">AClassTemplate</span><span class="special">,</span><span class="keyword">class</span><span class="special">,</span><span class="keyword">int</span><span class="special">)</span>       <span class="comment">// variadic macro</span>
    48. 

  48. </pre>
    49. 

  49. <p>
    50. 

  50.         generates a metafunction called 'has_template_AClassTemplate' in the current
    51. 

  51.         scope.
    52. 

  52.       </p>
    53. 

  53. <p>
    54. 

  54.         If you want to introspect the same class template name using both the <code class="computeroutput"><span class="keyword">template</span> <span class="identifier">type</span>
    55. 

  55.         <span class="identifier">parameters</span></code> form and the <code class="computeroutput"><span class="identifier">specific</span> <span class="identifier">parameters</span></code>
    56. 

  56.         form you will have the problem that you will be generating a metafunction
    57. 

  57.         of the same name and violating the C++ ODR rule. In this particular case
    58. 

  58.         you can use the alternate BOOST_TTI_TRAIT_HAS_TEMPLATE macro to name the
    59. 

  59.         particular metafunction which will be generated.
    60. 

  60.       </p>
    61. 

  61. <h5>
    62. 

  62. <a name="the_type_traits_introspection_library.tti_detail_has_template.tti_detail_has_template_metafunction.h1"></a>
    63. 

  63.         <span class="phrase"><a name="the_type_traits_introspection_library.tti_detail_has_template.tti_detail_has_template_metafunction.invoking_the_metafunction"></a></span><a class="link" href="tti_detail_has_template_metafunction.html#the_type_traits_introspection_library.tti_detail_has_template.tti_detail_has_template_metafunction.invoking_the_metafunction">Invoking
    64. 

  64.         the metafunction</a>
    65. 

  65.       </h5>
    66. 

  66. <p>
    67. 

  67.         You invoke the metafunction by instantiating the template with an enclosing
    68. 

  68.         type to introspect. A return value called 'value' is a compile time bool
    69. 

  69.         constant.
    70. 

  70.       </p>
    71. 

  71. <pre class="programlisting"><span class="identifier">has_template_AType</span><span class="special">&lt;</span><span class="identifier">Enclosing_Type</span><span class="special">&gt;::</span><span class="identifier">value</span>
    72. 

  72. </pre>
    73. 

  73. <h5>
    74. 

  74. <a name="the_type_traits_introspection_library.tti_detail_has_template.tti_detail_has_template_metafunction.h2"></a>
    75. 

  75.         <span class="phrase"><a name="the_type_traits_introspection_library.tti_detail_has_template.tti_detail_has_template_metafunction.examples"></a></span><a class="link" href="tti_detail_has_template_metafunction.html#the_type_traits_introspection_library.tti_detail_has_template.tti_detail_has_template_metafunction.examples">Examples</a>
    76. 

  76.       </h5>
    77. 

  77. <p>
    78. 

  78.         First we generate metafunctions for various inner class template names:
    79. 

  79.       </p>
    80. 

  80. <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">tti</span><span class="special">/</span><span class="identifier">has_template</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
    81. 

  81. 

  82. <span class="comment">// Using variadic macro, `template type parameters`</span>
    83. 

  83. 

  84. <span class="identifier">BOOST_TTI_HAS_TEMPLATE</span><span class="special">(</span><span class="identifier">Template1</span><span class="special">)</span>
    85. 

  85. <span class="identifier">BOOST_TTI_HAS_TEMPLATE</span><span class="special">(</span><span class="identifier">Template2</span><span class="special">)</span>
    86. 

  86. <span class="identifier">BOOST_TTI_HAS_TEMPLATE</span><span class="special">(</span><span class="identifier">Template3</span><span class="special">)</span>
    87. 

  87. <span class="identifier">BOOST_TTI_HAS_TEMPLATE</span><span class="special">(</span><span class="identifier">Template4</span><span class="special">)</span>
    88. 

  88. <span class="identifier">BOOST_TTI_HAS_TEMPLATE</span><span class="special">(</span><span class="identifier">Template5</span><span class="special">)</span>
    89. 

  89. 

  90. <span class="comment">// or using non-variadic macro, `template type parameters`</span>
    91. 

  91. 

  92. <span class="identifier">BOOST_TTI_HAS_TEMPLATE</span><span class="special">(</span><span class="identifier">Template1</span><span class="special">,</span><span class="identifier">BOOST_PP_NIL</span><span class="special">)</span>
    93. 

  93. <span class="identifier">BOOST_TTI_HAS_TEMPLATE</span><span class="special">(</span><span class="identifier">Template2</span><span class="special">,</span><span class="identifier">BOOST_PP_NIL</span><span class="special">)</span>
    94. 

  94. <span class="identifier">BOOST_TTI_HAS_TEMPLATE</span><span class="special">(</span><span class="identifier">Template3</span><span class="special">,</span><span class="identifier">BOOST_PP_NIL</span><span class="special">)</span>
    95. 

  95. <span class="identifier">BOOST_TTI_HAS_TEMPLATE</span><span class="special">(</span><span class="identifier">Template4</span><span class="special">,</span><span class="identifier">BOOST_PP_NIL</span><span class="special">)</span>
    96. 

  96. <span class="identifier">BOOST_TTI_HAS_TEMPLATE</span><span class="special">(</span><span class="identifier">Template5</span><span class="special">,</span><span class="identifier">BOOST_PP_NIL</span><span class="special">)</span>
    97. 

  97. 

  98. <span class="comment">// Using variadic macro, `specific parameters`</span>
    99. 

  99. 

  100. <span class="identifier">BOOST_TTI_HAS_TEMPLATE</span><span class="special">(</span><span class="identifier">Template6</span><span class="special">,</span><span class="keyword">class</span><span class="special">,</span><span class="keyword">int</span><span class="special">)</span>
    101. 

  101. <span class="identifier">BOOST_TTI_HAS_TEMPLATE</span><span class="special">(</span><span class="identifier">Template7</span><span class="special">,</span><span class="keyword">typename</span><span class="special">,</span><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span><span class="special">,</span><span class="keyword">class</span><span class="special">&gt;</span> <span class="keyword">struct</span><span class="special">,</span><span class="keyword">long</span><span class="special">)</span>
    102. 

  102. <span class="identifier">BOOST_TTI_HAS_TEMPLATE</span><span class="special">(</span><span class="identifier">Template8</span><span class="special">,</span><span class="keyword">double</span><span class="special">,</span><span class="keyword">typename</span><span class="special">)</span>
    103. 

  103. <span class="identifier">BOOST_TTI_HAS_TEMPLATE</span><span class="special">(</span><span class="identifier">Template9</span><span class="special">,</span><span class="keyword">typename</span><span class="special">,</span><span class="keyword">class</span><span class="special">,</span><span class="keyword">typename</span><span class="special">,</span><span class="keyword">class</span><span class="special">,</span><span class="keyword">typename</span><span class="special">,</span><span class="keyword">short</span><span class="special">)</span>
    104. 

  104. 

  105. <span class="comment">// or using non-variadic macro, `specific parameters`</span>
    106. 

  106. 

  107. <span class="identifier">BOOST_TTI_HAS_TEMPLATE</span><span class="special">(</span><span class="identifier">Template6</span><span class="special">,(</span><span class="number">2</span><span class="special">,(</span><span class="keyword">class</span><span class="special">,</span><span class="keyword">int</span><span class="special">)))</span>
    108. 

  108. <span class="identifier">BOOST_TTI_HAS_TEMPLATE</span><span class="special">(</span><span class="identifier">Template7</span><span class="special">,(</span><span class="number">4</span><span class="special">,(</span><span class="keyword">typename</span><span class="special">,</span><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span><span class="special">,</span><span class="keyword">class</span><span class="special">&gt;</span> <span class="keyword">struct</span><span class="special">,</span><span class="keyword">long</span><span class="special">)))</span>
    109. 

  109. <span class="identifier">BOOST_TTI_HAS_TEMPLATE</span><span class="special">(</span><span class="identifier">Template8</span><span class="special">,(</span><span class="number">2</span><span class="special">,(</span><span class="keyword">double</span><span class="special">,</span><span class="keyword">typename</span><span class="special">)))</span>
    110. 

  110. <span class="identifier">BOOST_TTI_HAS_TEMPLATE</span><span class="special">(</span><span class="identifier">Template9</span><span class="special">,(</span><span class="number">6</span><span class="special">,(</span><span class="keyword">typename</span><span class="special">,</span><span class="keyword">class</span><span class="special">,</span><span class="keyword">typename</span><span class="special">,</span><span class="keyword">class</span><span class="special">,</span><span class="keyword">typename</span><span class="special">,</span><span class="keyword">short</span><span class="special">)))</span>
    111. 

  111. </pre>
    112. 

  112. <p>
    113. 

  113.         Next let us create some user-defined types we want to introspect.
    114. 

  114.       </p>
    115. 

  115. <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">Top</span>
    116. 

  116.   <span class="special">{</span>
    117. 

  117.   <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">X</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">Template1</span> <span class="special">{</span> <span class="special">};</span>
    118. 

  118.   <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">A</span><span class="special">,</span><span class="keyword">typename</span> <span class="identifier">B</span><span class="special">,</span><span class="keyword">typename</span> <span class="identifier">C</span><span class="special">&gt;</span> <span class="keyword">class</span> <span class="identifier">Template2</span> <span class="special">{</span> <span class="special">};</span>
    119. 

  119.   <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">A</span><span class="special">,</span><span class="keyword">typename</span> <span class="identifier">B</span><span class="special">,</span><span class="keyword">typename</span> <span class="identifier">C</span><span class="special">,</span><span class="keyword">int</span> <span class="identifier">D</span><span class="special">&gt;</span> <span class="keyword">class</span> <span class="identifier">Template3</span> <span class="special">{</span> <span class="special">};</span>
    120. 

  120.   <span class="special">};</span>
    121. 

  121. <span class="keyword">struct</span> <span class="identifier">Top2</span>
    122. 

  122.   <span class="special">{</span>
    123. 

  123.   <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">A</span><span class="special">,</span><span class="keyword">typename</span> <span class="identifier">B</span><span class="special">,</span><span class="keyword">typename</span> <span class="identifier">C</span><span class="special">,</span><span class="keyword">class</span> <span class="identifier">D</span><span class="special">&gt;</span> <span class="keyword">class</span> <span class="identifier">Template3</span> <span class="special">{</span> <span class="special">};</span>
    124. 

  124.   <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">X</span><span class="special">,</span><span class="keyword">typename</span> <span class="identifier">Y</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">Template4</span> <span class="special">{</span> <span class="special">};</span>
    125. 

  125.   <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">A</span><span class="special">,</span><span class="keyword">class</span> <span class="identifier">B</span><span class="special">,</span><span class="keyword">typename</span> <span class="identifier">C</span><span class="special">,</span><span class="keyword">class</span> <span class="identifier">D</span><span class="special">,</span><span class="keyword">typename</span> <span class="identifier">E</span><span class="special">&gt;</span> <span class="keyword">class</span> <span class="identifier">Template5</span> <span class="special">{</span> <span class="special">};</span>
    126. 

  126.   <span class="special">};</span>
    127. 

  127. <span class="keyword">struct</span> <span class="identifier">Top3</span>
    128. 

  128.   <span class="special">{</span>
    129. 

  129.   <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">X</span><span class="special">,</span><span class="keyword">int</span> <span class="identifier">Y</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">Template6</span> <span class="special">{</span> <span class="special">};</span>
    130. 

  130.   <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">A</span><span class="special">,</span><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span><span class="special">,</span><span class="keyword">class</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">B</span><span class="special">,</span><span class="keyword">long</span> <span class="identifier">C</span><span class="special">&gt;</span> <span class="keyword">class</span> <span class="identifier">Template7</span> <span class="special">{</span> <span class="special">};</span>
    131. 

  131.   <span class="special">};</span>
    132. 

  132. <span class="keyword">struct</span> <span class="identifier">Top4</span>
    133. 

  133.   <span class="special">{</span>
    134. 

  134.   <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">double</span> <span class="identifier">X</span><span class="special">,</span><span class="keyword">typename</span> <span class="identifier">Y</span><span class="special">&gt;</span> <span class="keyword">struct</span> <span class="identifier">Template8</span> <span class="special">{</span> <span class="special">};</span>
    135. 

  135.   <span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">A</span><span class="special">,</span><span class="keyword">class</span> <span class="identifier">B</span><span class="special">,</span><span class="keyword">typename</span> <span class="identifier">C</span><span class="special">,</span><span class="keyword">class</span> <span class="identifier">D</span><span class="special">,</span><span class="keyword">typename</span> <span class="identifier">E</span><span class="special">,</span><span class="keyword">short</span> <span class="identifier">F</span><span class="special">&gt;</span> <span class="keyword">class</span> <span class="identifier">Template9</span> <span class="special">{</span> <span class="special">};</span>
    136. 

  136.   <span class="special">};</span>
    137. 

  137. </pre>
    138. 

  138. <p>
    139. 

  139.         Finally we invoke our metafunction and return our value. This all happens
    140. 

  140.         at compile time, and can be used by programmers doing compile time template
    141. 

  141.         metaprogramming.
    142. 

  142.       </p>
    143. 

  143. <pre class="programlisting"><span class="identifier">has_template_Template1</span><span class="special">&lt;</span><span class="identifier">Top</span><span class="special">&gt;::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// true</span>
    144. 

  144. <span class="identifier">has_template_Template1</span><span class="special">&lt;</span><span class="identifier">Top2</span><span class="special">&gt;::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// false</span>
    145. 

  145. 

  146. <span class="identifier">has_template_Template2</span><span class="special">&lt;</span><span class="identifier">Top</span><span class="special">&gt;::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// true</span>
    147. 

  147. <span class="identifier">has_template_Template2</span><span class="special">&lt;</span><span class="identifier">Top2</span><span class="special">&gt;::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// false</span>
    148. 

  148. 

  149. <span class="identifier">has_template_Template3</span><span class="special">&lt;</span><span class="identifier">Top</span><span class="special">&gt;::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// false, not all typename/class template parameters</span>
    150. 

  150. <span class="identifier">has_template_Template3</span><span class="special">&lt;</span><span class="identifier">Top2</span><span class="special">&gt;::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// true</span>
    151. 

  151. 

  152. <span class="identifier">has_template_Template4</span><span class="special">&lt;</span><span class="identifier">Top</span><span class="special">&gt;::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// false</span>
    153. 

  153. <span class="identifier">has_template_Template4</span><span class="special">&lt;</span><span class="identifier">Top2</span><span class="special">&gt;::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// true</span>
    154. 

  154. 

  155. <span class="identifier">has_template_Template5</span><span class="special">&lt;</span><span class="identifier">Top</span><span class="special">&gt;::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// false</span>
    156. 

  156. <span class="identifier">has_template_Template5</span><span class="special">&lt;</span><span class="identifier">Top2</span><span class="special">&gt;::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// true</span>
    157. 

  157. 

  158. <span class="identifier">has_template_Template6</span><span class="special">&lt;</span><span class="identifier">Top3</span><span class="special">&gt;::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// true</span>
    159. 

  159. <span class="identifier">has_template_Template6</span><span class="special">&lt;</span><span class="identifier">Top4</span><span class="special">&gt;::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// false</span>
    160. 

  160. 

  161. <span class="identifier">has_template_Template7</span><span class="special">&lt;</span><span class="identifier">Top3</span><span class="special">&gt;::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// true</span>
    162. 

  162. <span class="identifier">has_template_Template7</span><span class="special">&lt;</span><span class="identifier">Top4</span><span class="special">&gt;::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// false</span>
    163. 

  163. 

  164. <span class="identifier">has_template_Template8</span><span class="special">&lt;</span><span class="identifier">Top3</span><span class="special">&gt;::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// false</span>
    165. 

  165. <span class="identifier">has_template_Template8</span><span class="special">&lt;</span><span class="identifier">Top4</span><span class="special">&gt;::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// true</span>
    166. 

  166. 

  167. <span class="identifier">has_template_Template9</span><span class="special">&lt;</span><span class="identifier">Top3</span><span class="special">&gt;::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// false</span>
    168. 

  168. <span class="identifier">has_template_Template9</span><span class="special">&lt;</span><span class="identifier">Top4</span><span class="special">&gt;::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// true</span>
    169. 

  169. </pre>
    170. 

  170. <h5>
    171. 

  171. <a name="the_type_traits_introspection_library.tti_detail_has_template.tti_detail_has_template_metafunction.h3"></a>
    172. 

  172.         <span class="phrase"><a name="the_type_traits_introspection_library.tti_detail_has_template.tti_detail_has_template_metafunction.metafunction_re_use"></a></span><a class="link" href="tti_detail_has_template_metafunction.html#the_type_traits_introspection_library.tti_detail_has_template.tti_detail_has_template_metafunction.metafunction_re_use">Metafunction
    173. 

  173.         re-use</a>
    174. 

  174.       </h5>
    175. 

  175. <p>
    176. 

  176.         The macro encodes the name of the inner class template for which we are searching,
    177. 

  177.         the fact that we are introspecting for a class template within an enclosing
    178. 

  178.         type, and optionally the template parameters for that class template.
    179. 

  179.       </p>
    180. 

  180. <p>
    181. 

  181.         Once we create our metafunction for introspecting an inner class template
    182. 

  182.         by name, we can reuse the metafunction for introspecting any enclosing type,
    183. 

  183.         having any inner class template, for that name.
    184. 

  184.       </p>
    185. 

  185. <p>
    186. 

  186.         However we need to understand that we are restricted in our reuse of the
    187. 

  187.         metafunction by whether we originally use the template type parameters form
    188. 

  188.         or the specific form. In either case we are always introspecting an inner
    189. 

  189.         class template which matches that form. In the case of the template type
    190. 

  190.         parameters form, any inner class template for which we are introspecting
    191. 

  191.         must have all template type parameters, as well as the correct name. In the
    192. 

  192.         case of the specific parameters form, any inner class template for which
    193. 

  193.         we are introspecting must have template parameters which match the specific
    194. 

  194.         template parameters passed to the macro, as well as the correct name.
    195. 

  195.       </p>
    196. 

  196. </div>
    197. 

  197. <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
    198. 

  198. <td align="left"></td>
    199. 

  199. <td align="right"><div class="copyright-footer">Copyright &#169; 2011-2013 Tropic Software
    200. 

  200.       East Inc<p>
    201. 

  201.         Distributed under the Boost Software License, Version 1.0. (See accompanying
    202. 

  202.         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>)
    203. 

  203.       </p>
    204. 

  204. </div></td>
    205. 

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    206. 

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