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  12.       Boost.Python - &lt;boost/python/with_custodian_and_ward.hpp&gt;
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  21.             <a href="../../../../index.htm"><img height="86" width="277"
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  27.           <h1 align="center">
    28. 

  28.             <a href="../index.html">Boost.Python</a>
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  29.           </h1>
    30. 

  30.           <h2 align="center">
    31. 

  31.             Header &lt;boost/python/with_custodian_and_ward.hpp&gt;
    32. 

  32.           </h2>
    33. 

  33.         </td>
    34. 

  34.       </tr>
    35. 

  35.     </table>
    36. 

  36.     <hr>
    37. 

  37.     <h2>
    38. 

  38.       Contents
    39. 

  39.     </h2>
    40. 

  40.     <dl class="page-index">
    41. 

  41.       <dt>
    42. 

  42.         <a href="#introduction">Introduction</a>
    43. 

  43.       </dt>
    44. 

  44.       <dt>
    45. 

  45.         <a href="#classes">Classes</a>
    46. 

  46.       </dt>
    47. 

  47.       <dd>
    48. 

  48.         <dl class="page-index">
    49. 

  49.           <dt>
    50. 

  50.             <a href="#with_custodian_and_ward-spec">Class Template
    51. 

  51.             <code>with_custodian_and_ward</code></a>
    52. 

  52.           </dt>
    53. 

  53.           <dd>
    54. 

  54.             <dl class="page-index">
    55. 

  55.               <dt>
    56. 

  56.                 <a href="#with_custodian_and_ward-spec-synopsis">Class
    57. 

  57.                 Template <code>with_custodian_and_ward</code> synopsis</a>
    58. 

  58.               </dt>
    59. 

  59.               <dt>
    60. 

  60.                 <a href="#with_custodian_and_ward-spec-statics">Class
    61. 

  61.                 <code>with_custodian_and_ward</code> static functions</a>
    62. 

  62.               </dt>
    63. 

  63.             </dl>
    64. 

  64.           </dd>
    65. 

  65.           <dt>
    66. 

  66.             <a href="#with_custodian_and_ward_postcall-spec">Class Template
    67. 

  67.             <code>with_custodian_and_ward_postcall</code></a>
    68. 

  68.           </dt>
    69. 

  69.           <dd>
    70. 

  70.             <dl class="page-index">
    71. 

  71.               <dt>
    72. 

  72.                 <a href=
    73. 

  73.                 "#with_custodian_and_ward_postcall-spec-synopsis">Class
    74. 

  74.                 Template <code>with_custodian_and_ward_postcall</code>
    75. 

  75.                 synopsis</a>
    76. 

  76.               </dt>
    77. 

  77.               <dt>
    78. 

  78.                 <a href=
    79. 

  79.                 "#with_custodian_and_ward_postcall-spec-statics">Class
    80. 

  80.                 <code>with_custodian_and_ward_postcall</code> static
    81. 

  81.                 functions</a>
    82. 

  82.               </dt>
    83. 

  83.             </dl>
    84. 

  84.           </dd>
    85. 

  85.         </dl>
    86. 

  86.       </dd>
    87. 

  87.       <dt>
    88. 

  88.         <a href="#examples">Example</a>
    89. 

  89.       </dt>
    90. 

  90.     </dl>
    91. 

  91.     <hr>
    92. 

  92.     <h2>
    93. 

  93.       <a name="introduction">Introduction</a>
    94. 

  94.     </h2>This header provides facilities for establishing a lifetime
    95. 

  95.     dependency between two of a function's Python argument or result objects.
    96. 

  96.     The <i>ward</i> object will not be destroyed until after the custodian as
    97. 

  97.     long as the <i>custodian</i> object supports <a href=
    98. 

  98.     "http://www.python.org/doc/current/lib/module-weakref.html">weak
    99. 

  99.     references</a> (Boost.Python extension classes all support weak
    100. 

  100.     references). If the <i>custodian</i> object does not support weak
    101. 

  101.     references and is not <code>None</code>, an appropriate exception will be
    102. 

  102.     thrown. The two class templates <code>with_custodian_and_ward</code> and
    103. 

  103.     <code>with_custodian_and_ward_postcall</code> differ in the point at
    104. 

  104.     which they take effect. 
    105. 

  105.     <p>
    106. 

  106.       In order to reduce the chance of inadvertently creating dangling
    107. 

  107.       pointers, the default is to do lifetime binding <i>before</i> the
    108. 

  108.       underlying C++ object is invoked. However, before invocation the result
    109. 

  109.       object is not available, so
    110. 

  110.       <code>with_custodian_and_ward_postcall</code> is provided to bind
    111. 

  111.       lifetimes after invocation. Also, if a C++ exception is thrown after
    112. 

  112.       <code>with_custodian_and_ward&lt;&gt;::precall</code> but before the
    113. 

  113.       underlying C++ object actually stores a pointer, the lifetime of the
    114. 

  114.       custodian and ward objects will be artificially bound together, so one
    115. 

  115.       might choose <code>with_custodian_and_ward_postcall</code> instead,
    116. 

  116.       depending on the semantics of the function being wrapped.
    117. 

  117.     </p>
    118. 

  118.     <p>
    119. 

  119.       Please note that this is <i>not</i> the appropriate tool to use when
    120. 

  120.       wrapping functions which <b>transfer ownership</b> of a raw pointer
    121. 

  121.       across the function-call boundary. Please see the <a href=
    122. 

  122.       "faq.html#ownership">FAQ</a> if you want to do that.
    123. 

  123.     </p>
    124. 

  124.     <h2>
    125. 

  125.       <a name="classes"></a>Classes
    126. 

  126.     </h2>
    127. 

  127.     <h3>
    128. 

  128.       <a name="with_custodian_and_ward-spec"></a>Class template
    129. 

  129.       <code>with_custodian_and_ward</code>
    130. 

  130.     </h3>
    131. 

  131.     <table border="1" summary="with_custodian_and_ward template parameters">
    132. 

  132.       <caption>
    133. 

  133.         <b><code>with_custodian_and_ward</code> template parameters</b>
    134. 

  134.       </caption>
    135. 

  135.       <tr>
    136. 

  136.         <th>
    137. 

  137.           Parameter
    138. 

  138.         </th>
    139. 

  139.         <th>
    140. 

  140.           Requirements
    141. 

  141.         </th>
    142. 

  142.         <th>
    143. 

  143.           Description
    144. 

  144.         </th>
    145. 

  145.         <th>
    146. 

  146.           Default
    147. 

  147.         </th>
    148. 

  148.       </tr>
    149. 

  149.       <tr>
    150. 

  150.         <td>
    151. 

  151.           <code>custodian</code>
    152. 

  152.         </td>
    153. 

  153.         <td>
    154. 

  154.           A positive compile-time constant of type <code>std::size_t</code>.
    155. 

  155.         </td>
    156. 

  156.         <td>
    157. 

  157.           The 1-based index of the parameter which is the dependency in the
    158. 

  158.           lifetime relationship to be established. If used to wrap a member
    159. 

  159.           function, parameter 1 is the target object (<code>*this</code>).
    160. 

  160.           Note that if the target Python object type doesn't support weak
    161. 

  161.           references, a Python <code>TypeError</code> exception will be
    162. 

  162.           raised when the C++ object being wrapped is called.
    163. 

  163.         </td>
    164. 

  164.       </tr>
    165. 

  165.       <tr>
    166. 

  166.         <td>
    167. 

  167.           <code>ward</code>
    168. 

  168.         </td>
    169. 

  169.         <td>
    170. 

  170.           A positive compile-time constant of type <code>std::size_t</code>.
    171. 

  171.         </td>
    172. 

  172.         <td>
    173. 

  173.           The 1-based index of the parameter which is the dependent in the
    174. 

  174.           lifetime relationship to be established. If used to wrap a member
    175. 

  175.           function, parameter 1 is the target object (<code>*this</code>).
    176. 

  176.         </td>
    177. 

  177.       </tr>
    178. 

  178.       <tr>
    179. 

  179.         <td>
    180. 

  180.           <code>Base</code>
    181. 

  181.         </td>
    182. 

  182.         <td>
    183. 

  183.           A model of <a href="CallPolicies.html">CallPolicies</a>
    184. 

  184.         </td>
    185. 

  185.         <td>
    186. 

  186.           Used for <a href="CallPolicies.html#composition">policy
    187. 

  187.           composition</a>.
    188. 

  188.         </td>
    189. 

  189.         <td>
    190. 

  190.           <code><a href=
    191. 

  191.           "default_call_policies.html#default_call_policies-spec">default_call_policies</a></code>
    192. 

  192.         </td>
    193. 

  193.       </tr>
    194. 

  194.     </table>
    195. 

  195.     <h4>
    196. 

  196.       <a name="with_custodian_and_ward-spec-synopsis"></a>Class template
    197. 

  197.       <code>with_custodian_and_ward</code> synopsis
    198. 

  198.     </h4>
    199. 

  199.     <pre>
    200. 

  200. namespace boost { namespace python
    201. 

  201. {
    202. 

  202.    template &lt;std::size_t custodian, std::size_t ward, class Base = default_call_policies&gt;
    203. 

  203.    struct with_custodian_and_ward : Base
    204. 

  204.    {
    205. 

  205.       static bool precall(PyObject* args);
    206. 

  206.    };
    207. 

  207. }}
    208. 

  208. </pre>
    209. 

  209.     <h4>
    210. 

  210.       <a name="with_custodian_and_ward-spec-statics"></a>Class
    211. 

  211.       <code>with_custodian_and_ward</code> static functions
    212. 

  212.     </h4>
    213. 

  213.     <pre>
    214. 

  214. bool precall(PyObject* args);
    215. 

  215. </pre>
    216. 

  216.     <dl class="function-semantics">
    217. 

  217.       <dt>
    218. 

  218.         <b>Requires:</b> <code><a href=
    219. 

  219.         "http://www.python.org/doc/2.2/api/tupleObjects.html#l2h-476">PyTuple_Check</a>(args)
    220. 

  220.         != 0</code>
    221. 

  221.       </dt>
    222. 

  222.       <dt>
    223. 

  223.         <b>Effects:</b> Makes the lifetime of the argument indicated by
    224. 

  224.         <code>ward</code> dependent on the lifetime of the argument indicated
    225. 

  225.         by <code>custodian</code>.
    226. 

  226.       </dt>
    227. 

  227.       <dt>
    228. 

  228.         <b>Returns:</b> <code>false</code> and <code><a href=
    229. 

  229.         "http://www.python.org/doc/2.2/api/exceptionHandling.html#l2h-71">PyErr_Occurred</a>()&nbsp;!=&nbsp;0</code>
    230. 

  230.         upon failure, <code>true</code> otherwise.
    231. 

  231.       </dt>
    232. 

  232.     </dl><!-- xxxxxx -->
    233. 

  233.     <h3>
    234. 

  234.       <a name="with_custodian_and_ward_postcall-spec"></a>Class template
    235. 

  235.       <code>with_custodian_and_ward_postcall</code>
    236. 

  236.     </h3>
    237. 

  237.     <table border="1" summary=
    238. 

  238.     "with_custodian_and_ward_postcall template parameters">
    239. 

  239.       <caption>
    240. 

  240.         <b><code>with_custodian_and_ward_postcall</code> template
    241. 

  241.         parameters</b>
    242. 

  242.       </caption>
    243. 

  243.       <tr>
    244. 

  244.         <th>
    245. 

  245.           Parameter
    246. 

  246.         </th>
    247. 

  247.         <th>
    248. 

  248.           Requirements
    249. 

  249.         </th>
    250. 

  250.         <th>
    251. 

  251.           Description
    252. 

  252.         </th>
    253. 

  253.         <th>
    254. 

  254.           Default
    255. 

  255.         </th>
    256. 

  256.       </tr>
    257. 

  257.       <tr>
    258. 

  258.         <td>
    259. 

  259.           <code>custodian</code>
    260. 

  260.         </td>
    261. 

  261.         <td>
    262. 

  262.           A compile-time constant of type <code>std::size_t</code>.
    263. 

  263.         </td>
    264. 

  264.         <td>
    265. 

  265.           The index of the parameter which is the dependency in the lifetime
    266. 

  266.           relationship to be established. Zero indicates the result object; 1
    267. 

  267.           indicates the first argument. If used to wrap a member function,
    268. 

  268.           parameter 1 is the target object (<code>*this</code>). Note that if
    269. 

  269.           the target Python object type doesn't support weak references, a
    270. 

  270.           Python <code>TypeError</code> exception will be raised when the C++
    271. 

  271.           object being wrapped is called.
    272. 

  272.         </td>
    273. 

  273.       </tr>
    274. 

  274.       <tr>
    275. 

  275.         <td>
    276. 

  276.           <code>ward</code>
    277. 

  277.         </td>
    278. 

  278.         <td>
    279. 

  279.           A compile-time constant of type <code>std::size_t</code>.
    280. 

  280.         </td>
    281. 

  281.         <td>
    282. 

  282.           The index of the parameter which is the dependent in the lifetime
    283. 

  283.           relationship to be established. Zero indicates the result object; 1
    284. 

  284.           indicates the first argument. If used to wrap a member function,
    285. 

  285.           parameter 1 is the target object (<code>*this</code>).
    286. 

  286.         </td>
    287. 

  287.       </tr>
    288. 

  288.       <tr>
    289. 

  289.         <td>
    290. 

  290.           <code>Base</code>
    291. 

  291.         </td>
    292. 

  292.         <td>
    293. 

  293.           A model of <a href="CallPolicies.html">CallPolicies</a>
    294. 

  294.         </td>
    295. 

  295.         <td>
    296. 

  296.           Used for <a href="CallPolicies.html#composition">policy
    297. 

  297.           composition</a>.
    298. 

  298.         </td>
    299. 

  299.         <td>
    300. 

  300.           <code><a href=
    301. 

  301.           "default_call_policies.html#default_call_policies-spec">default_call_policies</a></code>
    302. 

  302.         </td>
    303. 

  303.       </tr>
    304. 

  304.     </table>
    305. 

  305.     <h4>
    306. 

  306.       <a name="with_custodian_and_ward_postcall-spec-synopsis"></a>Class
    307. 

  307.       template <code>with_custodian_and_ward_postcall</code> synopsis
    308. 

  308.     </h4>
    309. 

  309.     <pre>
    310. 

  310. namespace boost { namespace python
    311. 

  311. {
    312. 

  312.    template &lt;std::size_t custodian, std::size_t ward, class Base = default_call_policies&gt;
    313. 

  313.    struct with_custodian_and_ward_postcall : Base
    314. 

  314.    {
    315. 

  315.       static PyObject* postcall(PyObject* args, PyObject* result);
    316. 

  316.    };
    317. 

  317. }}
    318. 

  318. </pre>
    319. 

  319.     <h4>
    320. 

  320.       <a name="with_custodian_and_ward_postcall-spec-statics"></a>Class
    321. 

  321.       <code>with_custodian_and_ward_postcall</code> static functions
    322. 

  322.     </h4>
    323. 

  323.     <pre>
    324. 

  324. PyObject* postcall(PyObject* args, PyObject* result);
    325. 

  325. </pre>
    326. 

  326.     <dl class="function-semantics">
    327. 

  327.       <dt>
    328. 

  328.         <b>Requires:</b> <code><a href=
    329. 

  329.         "http://www.python.org/doc/2.2/api/tupleObjects.html#l2h-476">PyTuple_Check</a>(args)
    330. 

  330.         != 0</code>, <code>result&nbsp;!=&nbsp;0</code>.
    331. 

  331.       </dt>
    332. 

  332.       <dt>
    333. 

  333.         <b>Effects:</b> Makes the lifetime of the object indicated by
    334. 

  334.         <code>ward</code> dependent on the lifetime of the object indicated
    335. 

  335.         by <code>custodian</code>.
    336. 

  336.       </dt>
    337. 

  337.       <dt>
    338. 

  338.         <b>Returns:</b> <code>0</code> and <code><a href=
    339. 

  339.         "http://www.python.org/doc/2.2/api/exceptionHandling.html#l2h-71">PyErr_Occurred</a>()&nbsp;!=&nbsp;0</code>
    340. 

  340.         upon failure, <code>true</code> otherwise.
    341. 

  341.       </dt>
    342. 

  342.     </dl>
    343. 

  343.     <h2>
    344. 

  344.       <a name="examples"></a>Example
    345. 

  345.     </h2>The following example shows how
    346. 

  346.     <code>with_custodian_and_ward_postcall</code> is used by the library to
    347. 

  347.     implement <code><a href=
    348. 

  348.     "return_internal_reference.html#return_internal_reference-spec">return_internal_reference</a></code>
    349. 

  349.     
    350. 

  350.     <pre>
    351. 

  351. template &lt;std::size_t owner_arg = 1, class Base = default_call_policies&gt;
    352. 

  352. struct return_internal_reference
    353. 

  353.     : with_custodian_and_ward_postcall&lt;0, owner_arg, Base&gt;
    354. 

  354. {
    355. 

  355.    typedef <a href=
    356. 

  356. "reference_existing_object.html#reference_existing_object-spec">reference_existing_object</a> result_converter;
    357. 

  357. };
    358. 

  358. </pre>
    359. 

  359.     <p>
    360. 

  360.       Revised 
    361. 

  361.       <!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan -->
    362. 

  362.        13 November, 2002 
    363. 

  363.       <!--webbot bot="Timestamp" endspan i-checksum="39359" -->
    364. 

  364.       </p>
    365. 

  365.     <p>
    366. 

  366.       <i>&copy; Copyright <a href="http://www.boost.org/people/dave_abrahams.htm">Dave
    367. 

  367.       Abrahams</a> 2002. </i>
    368. 

  368.     </p>
    369. 

  369.   </body>
    370. 

  370. </html>
    371. 

  371.