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  1. <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
    2. 

  2. <html>
    3. 

  3. <head>
    4. 

  4. <title>SWIG and Go</title>
    5. 

  5. <link rel="stylesheet" type="text/css" href="style.css">
    6. 

  6. </head>
    7. 

  7. <body bgcolor="#FFFFFF">
    8. 

  8. <H1><a name="Go"></a>22 SWIG and Go</H1>
    9. 

  9. <!-- INDEX -->
    10. 

  10. <div class="sectiontoc">
    11. 

  11. <ul>
    12. 

  12. <li><a href="#Go_overview">Overview</a>
    13. 

  13. <li><a href="#Go_running_swig">Running SWIG with Go</a>
    14. 

  14. <ul>
    15. 

  15. <li><a href="#Go_commandline">Additional Commandline Options</a>
    16. 

  16. <li><a href="#Go_outputs">Go Output Files</a>
    17. 

  17. </ul>
    18. 

  18. <li><a href="#Go_basic_tour">A tour of basic C/C++ wrapping</a>
    19. 

  19. <ul>
    20. 

  20. <li><a href="#Go_package">Go Package Name</a>
    21. 

  21. <li><a href="#Go_names">Go Names</a>
    22. 

  22. <li><a href="#Go_constants">Go Constants</a>
    23. 

  23. <li><a href="#Go_enumerations">Go Enumerations</a>
    24. 

  24. <li><a href="#Go_classes">Go Classes</a>
    25. 

  25. <ul>
    26. 

  26. <li><a href="#Go_class_inheritance">Go Class Inheritance</a>
    27. 

  27. </ul>
    28. 

  28. <li><a href="#Go_templates">Go Templates</a>
    29. 

  29. <li><a href="#Go_director_classes">Go Director Classes</a>
    30. 

  30. <li><a href="#Go_primitive_type_mappings">Default Go primitive type mappings</a>
    31. 

  31. <li><a href="#Go_output_arguments">Output arguments</a>
    32. 

  32. <li><a href="#Go_adding_additional_code">Adding additional go code</a>
    33. 

  33. </ul>
    34. 

  34. </ul>
    35. 

  35. </div>
    36. 

  36. <!-- INDEX -->
    37. 

  37. 

  38. 

  39. 

  40. <p>
    41. 

  41. This chapter describes SWIG's support of Go.  For more information on
    42. 

  42. the Go programming language
    43. 

  43. see <a href="http://golang.org/">golang.org</a>.
    44. 

  44. </p>
    45. 

  45. 

  46. <H2><a name="Go_overview"></a>22.1 Overview</H2>
    47. 

  47. 

  48. 

  49. <p>
    50. 

  50. Go is a compiled language, not a scripting language.  However, it does
    51. 

  51. not support direct calling of functions written in C/C++.  The cgo
    52. 

  52. program may be used to generate wrappers to call C code from Go, but
    53. 

  53. there is no convenient way to call C++ code.  SWIG fills this gap.
    54. 

  54. </p>
    55. 

  55. 

  56. <p>
    57. 

  57. There are (at least) two different Go compilers.  One is the gc
    58. 

  58. compiler, normally invoked under the names 6g, 8g, or 5g.  The other
    59. 

  59. is the gccgo compiler, which is a frontend to the gcc compiler suite.
    60. 

  60. The interface to C/C++ code is completely different for the two Go
    61. 

  61. compilers.  SWIG supports both, selected by a command line option.
    62. 

  62. </p>
    63. 

  63. 

  64. <p>
    65. 

  65. Because Go is a type-safe compiled language, SWIG's runtime type
    66. 

  66. checking and runtime library are not used with Go.  This should be
    67. 

  67. borne in mind when reading the rest of the SWIG documentation.
    68. 

  68. </p>
    69. 

  69. 

  70. <H2><a name="Go_running_swig"></a>22.2 Running SWIG with Go</H2>
    71. 

  71. 

  72. 

  73. <p>
    74. 

  74. To generate Go code, use the <tt>-go</tt> option with SWIG.  By
    75. 

  75. default SWIG will generate code for the gc compilers.  To generate
    76. 

  76. code for gccgo, you should also use the <tt>-gccgo</tt> option.
    77. 

  77. </p>
    78. 

  78. 

  79. <H3><a name="Go_commandline"></a>22.2.1 Additional Commandline Options</H3>
    80. 

  80. 

  81. 

  82. <p>
    83. 

  83. These are the command line options for SWIG's GO module.  They can
    84. 

  84. also be seen by using:
    85. 

  85. </p>
    86. 

  86. 

  87. <div class="code"><pre>
    88. 

  88. swig -go -help
    89. 

  89. </pre></div>
    90. 

  90. 

  91. <table summary="Go specific options">
    92. 

  92. <tr>
    93. 

  93. <th>Go specific options</th>
    94. 

  94. </tr>
    95. 

  95. 

  96. <tr>
    97. 

  97. <td>-gccgo</td>
    98. 

  98. <td>Generate code for gccgo.  The default is to generate code for
    99. 

  99.   6g/8g/5g.</td>
    100. 

  100. </tr>
    101. 

  101. 

  102. <tr>
    103. 

  103. <td>-gccgo-46</td>
    104. 

  104. <td>Generate code for gccgo 4.6.  The default is set by the configure
    105. 

  105.   script.</td>  This generates code that does not use some facilities
    106. 

  106.   that are only available in gccgo 4.7 and later.
    107. 

  107. </tr>
    108. 

  108. 

  109. <tr>
    110. 

  110. <td>-no-gccgo-46</td>
    111. 

  111. <td>Turn off <code>-gccgo-46</code>, whether set by default or earlier
    112. 

  112.   on the command line.
    113. 

  113. </tr>
    114. 

  114. 

  115. <tr>
    116. 

  116. <td>-package &lt;name&gt;</td>
    117. 

  117. <td>Set the name of the Go package to &lt;name&gt;.  The default
    118. 

  118.   package name is the SWIG module name.</td>
    119. 

  119. </tr>
    120. 

  120. 

  121. <tr>
    122. 

  122. <td>-soname %lt;name%gt;</td>
    123. 

  123. <td>Set the runtime name of the shared library that the dynamic linker
    124. 

  124.   should include at runtime.  The default is the package name with
    125. 

  125.   ".so" appended.  This is only used when generating code for
    126. 

  126.   6g/8g/5g; when using gccgo, the equivalent name will be taken from
    127. 

  127.   the <code>-soname</code> option passed to the linker.</td>
    128. 

  128. </tr>
    129. 

  129. 

  130. <tr>
    131. 

  131. <td>-go-prefix &lt;prefix&gt;</td>
    132. 

  132. <td>When generating code for gccgo, set the prefix to use.  This
    133. 

  133.   corresponds to the <tt>-fgo-prefix</tt> option to gccgo.</td>
    134. 

  134. </tr>
    135. 

  135. 

  136. <tr>
    137. 

  137. <td>-long-type-size &lt;s&gt;</td>
    138. 

  138. <td>Set the size for the C/C++ type <tt>long</tt>.  This controls
    139. 

  139.   whether <tt>long</tt> is converted to the Go type <tt>int32</tt>
    140. 

  140.   or <tt>int64</tt>.  The &lt;s&gt; argument should be 32 or 64.</td>
    141. 

  141. </tr>
    142. 

  142. 

  143. </table>
    144. 

  144. 

  145. <H3><a name="Go_outputs"></a>22.2.2 Go Output Files</H3>
    146. 

  146. 

  147. 

  148. <p> When generating Go code, SWIG will generate the following
    149. 

  149.   files:</p>
    150. 

  150. 

  151. <ul>
    152. 

  152. <li>
    153. 

  153. MODULE.go will contain the Go functions that your Go code will call.
    154. 

  154. These functions will be wrappers for the C++ functions defined by your
    155. 

  155. module.  This file should, of course, be compiled with the Go
    156. 

  156. compiler.
    157. 

  157. <li>
    158. 

  158. MODULE_wrap.c or MODULE_wrap.cxx will contain C/C++ functions will be
    159. 

  159. invoked by the Go wrapper code.  This file should be compiled with the
    160. 

  160. usual C or C++ compiler and linked into a shared library.
    161. 

  161. <li>
    162. 

  162. MODULE_wrap.h will be generated if you use the directors feature.  It
    163. 

  163. provides a definition of the generated C++ director classes.  It is
    164. 

  164. generally not necessary to use this file, but in some special cases it
    165. 

  165. may be helpful to include it in your code, compiled with the usual C
    166. 

  166. or C++ compiler.
    167. 

  167. <li>
    168. 

  168. If using the gc compiler, MODULE_gc.c will contain C code which should
    169. 

  169. be compiled with the C compiler distributed as part of the gc compiler: 6c, 8c,
    170. 

  170. or 5c.  It should then be combined with the compiled MODULE.go using
    171. 

  171. gopack.  This file will not be generated when using gccgo.
    172. 

  172. </ul>
    173. 

  173. 

  174. <p>
    175. 

  175. A typical command sequence would look like this:
    176. 

  176. </p>
    177. 

  177. 

  178. <div class="code"><pre>
    179. 

  179. % swig -go example.i
    180. 

  180. % gcc -c -fpic example.c
    181. 

  181. % gcc -c -fpic example_wrap.c
    182. 

  182. % gcc -shared example.o example_wrap.o -o example.so
    183. 

  183. % 6g example.go
    184. 

  184. % 6c example_gc.c
    185. 

  185. % gopack grc example.a example.6 example_gc.6
    186. 

  186. % 6g main.go  # your code, not generated by SWIG
    187. 

  187. % 6l main.6
    188. 

  188. </pre></div>
    189. 

  189. 

  190. <H2><a name="Go_basic_tour"></a>22.3 A tour of basic C/C++ wrapping</H2>
    191. 

  191. 

  192. 

  193. <p>
    194. 

  194. By default, SWIG attempts to build a natural Go interface to your
    195. 

  195. C/C++ code.  However, the languages are somewhat different, so some
    196. 

  196. modifications have to occur.  This section briefly covers the
    197. 

  197. essential aspects of this wrapping.
    198. 

  198. </p>
    199. 

  199. 

  200. <H3><a name="Go_package"></a>22.3.1 Go Package Name</H3>
    201. 

  201. 

  202. 

  203. <p>
    204. 

  204. All Go source code lives in a package.  The name of this package will
    205. 

  205. default to the name of the module from SWIG's <tt>%module</tt>
    206. 

  206. directive.  You may override this by using SWIG's <tt>-package</tt>
    207. 

  207. command line option.
    208. 

  208. </p>
    209. 

  209. 

  210. <H3><a name="Go_names"></a>22.3.2 Go Names</H3>
    211. 

  211. 

  212. 

  213. <p>
    214. 

  214. In Go, a function is only visible outside the current package if the
    215. 

  215. first letter of the name is uppercase.  This is quite different from
    216. 

  216. C/C++.  Because of this, C/C++ names are modified when generating the
    217. 

  217. Go interface: the first letter is forced to be uppercase if it is not
    218. 

  218. already.  This affects the names of functions, methods, variables,
    219. 

  219. constants, enums, and classes.
    220. 

  220. </p>
    221. 

  221. 

  222. <p>
    223. 

  223. C/C++ variables are wrapped with setter and getter functions in Go.
    224. 

  224. First the first letter of the variable name will be forced to
    225. 

  225. uppercase, and then <tt>Get</tt> or <tt>Set</tt> will be prepended.
    226. 

  226. For example, if the C/C++ variable is called <tt>var</tt>, then SWIG
    227. 

  227. will define the functions <tt>GetVar</tt> and <tt>SetVar</tt>.  If a
    228. 

  228. variable is declared as <tt>const</tt>, or if
    229. 

  229. SWIG's <a href="SWIG.html#SWIG_readonly_variables">
    230. 

  230. <tt>%immutable</tt> directive</a> is used for the variable, then only
    231. 

  231. the getter will be defined.
    232. 

  232. </p>
    233. 

  233. 

  234. <p>
    235. 

  235. C++ classes will be discussed further below.  Here we'll note that the
    236. 

  236. first letter of the class name will be forced to uppercase to give the
    237. 

  237. name of a type in Go.  A constructor will be named <tt>New</tt>
    238. 

  238. followed by that name, and the destructor will be
    239. 

  239. named <tt>Delete</tt> followed by that name.
    240. 

  240. </p>
    241. 

  241. 

  242. <H3><a name="Go_constants"></a>22.3.3 Go Constants</H3>
    243. 

  243. 

  244. 

  245. <p>
    246. 

  246. C/C++ constants created via <tt>#define</tt> or the <tt>%constant</tt>
    247. 

  247. directive become Go constants, declared with a <tt>const</tt>
    248. 

  248. declaration.
    249. 

  249. 

  250. <H3><a name="Go_enumerations"></a>22.3.4 Go Enumerations</H3>
    251. 

  251. 

  252. 

  253. <p>
    254. 

  254. C/C++ enumeration types will cause SWIG to define an integer type with
    255. 

  255. the name of the enumeration (with first letter forced to uppercase as
    256. 

  256. usual).  The values of the enumeration will become variables in Go;
    257. 

  257. code should avoid modifying those variables.
    258. 

  258. </p>
    259. 

  259. 

  260. <H3><a name="Go_classes"></a>22.3.5 Go Classes</H3>
    261. 

  261. 

  262. 

  263. <p>
    264. 

  264. Go has interfaces, methods and inheritance, but it does not have
    265. 

  265. classes in the same sense as C++.  This sections describes how SWIG
    266. 

  266. represents C++ classes represented in Go.
    267. 

  267. </p>
    268. 

  268. 

  269. <p>
    270. 

  270. For a C++ class <tt>ClassName</tt>, SWIG will define two types in Go:
    271. 

  271. an underlying type, which will just hold a pointer to the C++ type,
    272. 

  272. and an interface type.  The interface type will be
    273. 

  273. named <tt>ClassName</tt>.  SWIG will define a
    274. 

  274. function <tt>NewClassName</tt> which will take any constructor
    275. 

  275. arguments and return a value of the interface
    276. 

  276. type <tt>ClassName</tt>.  SWIG will also define a
    277. 

  277. destructor <tt>DeleteClassName</tt>.
    278. 

  278. </p>
    279. 

  279. 

  280. <p>
    281. 

  281. SWIG will represent any methods of the C++ class as methods on the
    282. 

  282. underlying type, and also as methods of the interface type.  Thus C++
    283. 

  283. methods may be invoked directly using the
    284. 

  284. usual <tt>val.MethodName</tt> syntax.  Public members of the C++ class
    285. 

  285. will be given getter and setter functions defined as methods of the
    286. 

  286. class.
    287. 

  287. </p>
    288. 

  288. 

  289. <p>
    290. 

  290. SWIG will represent static methods of C++ classes as ordinary Go
    291. 

  291. functions.  SWIG will use names like <tt>ClassNameMethodName</tt>.
    292. 

  292. SWIG will give static members getter and setter functions with names
    293. 

  293. like <tt>GetClassName_VarName</tt>.
    294. 

  294. </p>
    295. 

  295. 

  296. <p>
    297. 

  297. Given a value of the interface type, Go code can retrieve the pointer
    298. 

  298. to the C++ type by calling the <tt>Swigcptr</tt> method.  This will
    299. 

  299. return a value of type <tt>SwigcptrClassName</tt>, which is just a
    300. 

  300. name for <tt>uintptr</tt>.  A Go type conversion can be used to
    301. 

  301. convert this value to a different C++ type, but note that this
    302. 

  302. conversion will not be type checked and is essentially equivalent
    303. 

  303. to <tt>reinterpret_cast</tt>.  This should only be used for very
    304. 

  304. special cases, such as where C++ would use a <tt>dynamic_cast</tt>.
    305. 

  305. </p>
    306. 

  306. 

  307. <p>Note that C++ pointers to compound objects are represented in go as objects
    308. 

  308. themselves, not as go pointers.  So, for example, if you wrap the following
    309. 

  309. function:</p>
    310. 

  310. <div class="code">
    311. 

  311. <pre>
    312. 

  312. class MyClass {
    313. 

  313.   int MyMethod();
    314. 

  314.   static MyClass *MyFactoryFunction();
    315. 

  315. };
    316. 

  316. 

  317. </pre>
    318. 

  318. </div>
    319. 

  319. <p>You will get go code that looks like this:</p>
    320. 

  320. <div class="code">
    321. 

  321. <pre>
    322. 

  322. type MyClass interface {
    323. 

  323.   Swigcptr() uintptr
    324. 

  324.   SwigIsMyClass()
    325. 

  325.   MyMethod() int
    326. 

  326. }
    327. 

  327. 

  328. MyClassMyFactoryFunction() MyClass {
    329. 

  329.   // swig magic here
    330. 

  330. }
    331. 

  331. </pre>
    332. 

  332. </div>
    333. 

  333. <p>Note that the factory function does not return a go pointer; it actually
    334. 

  334. returns a go interface.  If the returned pointer can be null, you can check
    335. 

  335. for this by calling the Swigcptr() method.
    336. 

  336. </p>
    337. 

  337. 

  338. <H4><a name="Go_class_inheritance"></a>22.3.5.1 Go Class Inheritance</H4>
    339. 

  339. 

  340. 

  341. <p>
    342. 

  342. C++ class inheritance is automatically represented in Go due to its
    343. 

  343. use of interfaces.  The interface for a child class will be a superset
    344. 

  344. of the interface of its parent class.  Thus a value of the child class
    345. 

  345. type in Go may be passed to a function which expects the parent class.
    346. 

  346. Doing the reverse will require an explicit type assertion, which will
    347. 

  347. be checked dynamically.
    348. 

  348. </p>
    349. 

  349. 

  350. <H3><a name="Go_templates"></a>22.3.6 Go Templates</H3>
    351. 

  351. 

  352. 

  353. <p>
    354. 

  354. In order to use C++ templates in Go, you must tell SWIG to create
    355. 

  355. wrappers for a particular template instantation.  To do this, use
    356. 

  356. the <tt>%template</tt> directive.
    357. 

  357. 

  358. <H3><a name="Go_director_classes"></a>22.3.7 Go Director Classes</H3>
    359. 

  359. 

  360. 

  361. <p>
    362. 

  362. SWIG's director feature permits a Go type to act as the subclass of a
    363. 

  363. C++ class with virtual methods.  This is complicated by the fact that
    364. 

  364. C++ and Go define inheritance differently.  In Go, structs can inherit
    365. 

  365. methods via anonymous field embedding.  However, when a method is
    366. 

  366. called for an embedded struct, if that method calls any other methods,
    367. 

  367. they are called for the embedded struct, not for the original type.
    368. 

  368. Therefore, SWIG must use Go interfaces to represent C++ inheritance.
    369. 

  369. </p>
    370. 

  370. 

  371. <p>
    372. 

  372. In order to use the director feature in Go, you must define a type in
    373. 

  373. your Go code.  You must then add methods for the type.  Define a
    374. 

  374. method in Go for each C++ virtual function that you want to override.
    375. 

  375. You must then create a value of your new type, and pass a pointer to
    376. 

  376. it to the function <tt>NewDirectorClassName</tt>,
    377. 

  377. where <tt>ClassName</tt> is the name of the C++ class.  That will
    378. 

  378. return a value of type <tt>ClassName</tt>.
    379. 

  379. </p>
    380. 

  380. 

  381. <p>
    382. 

  382. For example:
    383. 

  383. </p>
    384. 

  384. 

  385. <div class="code">
    386. 

  386. <pre>
    387. 

  387. type GoClass struct { }
    388. 

  388. func (p *GoClass) VirtualFunction() { }
    389. 

  389. func MakeClass() ClassName {
    390. 

  390. 	return NewDirectorClassName(&amp;GoClass{})
    391. 

  391. }
    392. 

  392. </pre>
    393. 

  393. </div>
    394. 

  394. 

  395. <p>
    396. 

  396. Any call in C++ code to the virtual function will wind up calling the
    397. 

  397. method defined in Go.  The Go code may of course call other methods on
    398. 

  398. itself, and those methods may be defined either in Go or in C++.
    399. 

  399. </p>
    400. 

  400. 

  401. <H3><a name="Go_primitive_type_mappings"></a>22.3.8 Default Go primitive type mappings</H3>
    402. 

  402. 

  403. 

  404. <p>
    405. 

  405. The following table lists the default type mapping from C/C++ to Go.
    406. 

  406. This table will tell you which Go type to expect for a function which
    407. 

  407. uses a given C/C++ type.
    408. 

  408. </p>
    409. 

  409. 

  410. <table BORDER summary="Go primitive type mappings">
    411. 

  411. <tr>
    412. 

  412. <td><b>C/C++ type</b></td>
    413. 

  413. <td><b>Go type</b></td>
    414. 

  414. </tr>
    415. 

  415. 

  416. <tr>
    417. 

  417. <td>bool</td>
    418. 

  418. <td>bool</td>
    419. 

  419. </tr>
    420. 

  420. 

  421. <tr>
    422. 

  422. <td>char</td>
    423. 

  423. <td>byte</td>
    424. 

  424. </tr>
    425. 

  425. 

  426. <tr>
    427. 

  427. <td>signed char</td>
    428. 

  428. <td>int8</td>
    429. 

  429. </tr>
    430. 

  430. 

  431. <tr>
    432. 

  432. <td>unsigned char</td>
    433. 

  433. <td>byte</td>
    434. 

  434. </tr>
    435. 

  435. 

  436. <tr>
    437. 

  437. <td>short</td>
    438. 

  438. <td>int16</td>
    439. 

  439. </tr>
    440. 

  440. 

  441. <tr>
    442. 

  442. <td>unsigned short</td>
    443. 

  443. <td>uint16</td>
    444. 

  444. </tr>
    445. 

  445. 

  446. <tr>
    447. 

  447. <td>int</td>
    448. 

  448. <td>int</td>
    449. 

  449. </tr>
    450. 

  450. 

  451. <tr>
    452. 

  452. <td>unsigned int</td>
    453. 

  453. <td>uint</td>
    454. 

  454. </tr>
    455. 

  455. 

  456. <tr>
    457. 

  457. <td>long</td>
    458. 

  458. <td>int32 or int64, depending on <tt>-long-type-size</tt></td>
    459. 

  459. </tr>
    460. 

  460. 

  461. <tr>
    462. 

  462. <td>unsigned long</td>
    463. 

  463. <td>uint32 or uint64, depending on <tt>-long-type-size</tt></td>
    464. 

  464. </tr>
    465. 

  465. 

  466. <tr>
    467. 

  467. <td>long long</td>
    468. 

  468. <td>int64</td>
    469. 

  469. </tr>
    470. 

  470. 

  471. <tr>
    472. 

  472. <td>unsigned long long</td>
    473. 

  473. <td>uint64</td>
    474. 

  474. </tr>
    475. 

  475. 

  476. <tr>
    477. 

  477. <td>float</td>
    478. 

  478. <td>float32</td>
    479. 

  479. </tr>
    480. 

  480. 

  481. <tr>
    482. 

  482. <td>double</td>
    483. 

  483. <td>float64</td>
    484. 

  484. </tr>
    485. 

  485. 

  486. <tr>
    487. 

  487. <td>char *<br>char []</td>
    488. 

  488. <td>string</td>
    489. 

  489. </tr>
    490. 

  490. 

  491. </table>
    492. 

  492. 

  493. <p>
    494. 

  494. Note that SWIG wraps the C <tt>char</tt> type as a character. Pointers
    495. 

  495. and arrays of this type are wrapped as strings.  The <tt>signed
    496. 

  496. char</tt> type can be used if you want to treat <tt>char</tt> as a
    497. 

  497. signed number rather than a character.  Also note that all const
    498. 

  498. references to primitive types are treated as if they are passed by
    499. 

  499. value.
    500. 

  500. </p>
    501. 

  501. 

  502. <p>
    503. 

  503. These type mappings are defined by the "gotype" typemap.  You may change
    504. 

  504. that typemap, or add new values, to control how C/C++ types are mapped
    505. 

  505. into Go types.
    506. 

  506. </p>
    507. 

  507. 

  508. <H3><a name="Go_output_arguments"></a>22.3.9 Output arguments</H3>
    509. 

  509. 

  510. 

  511. <p>Because of limitations in the way output arguments are processed in swig,
    512. 

  512. a function with output arguments will not have multiple return values.
    513. 

  513. Instead, you must pass a pointer into the C++ function to tell it where to
    514. 

  514. store the ouput value.  In go, you supply a slice in the place of the output
    515. 

  515. argument.</p>
    516. 

  516. 

  517. <p>For example, suppose you were trying to wrap the modf() function in the
    518. 

  518. C math library which splits x into integral and fractional parts (and
    519. 

  519. returns the integer part in one of its parameters):</p>
    520. 

  520. <div class="code">
    521. 

  521. <pre>
    522. 

  522. double modf(double x, double *ip);
    523. 

  523. </pre>
    524. 

  524. </div>
    525. 

  525. <p>You could wrap it with SWIG as follows:</p>
    526. 

  526. <div class="code">
    527. 

  527. <pre>
    528. 

  528. %include &lt;typemaps.i&gt;
    529. 

  529. double modf(double x, double *OUTPUT);
    530. 

  530. </pre>
    531. 

  531. </div>
    532. 

  532. <p>or you can use the <code>%apply</code> directive:</p>
    533. 

  533. <div class="code">
    534. 

  534. <pre>
    535. 

  535. %include &lt;typemaps.i&gt;
    536. 

  536. %apply double *OUTPUT { double *ip };
    537. 

  537. double modf(double x, double *ip);
    538. 

  538. </pre>
    539. 

  539. </div>
    540. 

  540. <p>In Go you would use it like this:</p>
    541. 

  541. <div class="code">
    542. 

  542. <pre>
    543. 

  543. ptr := []float64{0.0}
    544. 

  544. fraction := modulename.Modf(5.0, ptr)
    545. 

  545. </pre>
    546. 

  546. </div>
    547. 

  547. <p>Since this is ugly, you may want to wrap the swig-generated API with
    548. 

  548. some <a href="#Embedded_go_code">additional functions written in go</a> that
    549. 

  549. hide the ugly details.</p>
    550. 

  550. 

  551. <p>There are no <code>char&nbsp;*OUTPUT</code> typemaps.  However you can
    552. 

  552. apply the <code>signed&nbsp;char&nbsp;*</code> typemaps instead:</p>
    553. 

  553. <div class="code">
    554. 

  554. <pre>
    555. 

  555. %include &lt;typemaps.i&gt;
    556. 

  556. %apply signed char *OUTPUT {char *output};
    557. 

  557. void f(char *output);
    558. 

  558. </pre>
    559. 

  559. </div>
    560. 

  560. 

  561. <H3><a name="Go_adding_additional_code"></a>22.3.10 Adding additional go code</H3>
    562. 

  562. 

  563. 

  564. <p>Often the APIs generated by swig are not very natural in go, especially if
    565. 

  565. there are output arguments.  You can
    566. 

  566. insert additional go wrapping code to add new APIs
    567. 

  567. with <code>%insert(go_wrapper)</code>, like this:</p>
    568. 

  568. <div class="code">
    569. 

  569. <pre>
    570. 

  570. %include &lt;typemaps.i&gt;
    571. 

  571. // Change name of what swig generates to Wrapped_modf.  This function will
    572. 

  572. // have the following signature in go:
    573. 

  573. //   func Wrapped_modf(float64, []float64) float64
    574. 

  574. %rename(wrapped_modf) modf(double x, double *ip);
    575. 

  575. 

  576. %apply double *OUTPUT { double *ip };
    577. 

  577. double modf(double x, double *ip);
    578. 

  578. 

  579. %insert(go_wrapper) %{
    580. 

  580. 

  581. // The improved go interface to this function, which has two return values,
    582. 

  582. // in the more natural go idiom:
    583. 

  583. func Modf(x float64) (fracPart float64, intPart float64) {
    584. 

  584.   ip := []float64{0.0}
    585. 

  585.   fracPart = Wrapped_modf(x, ip)
    586. 

  586.   intPart = ip[0]
    587. 

  587.   return
    588. 

  588. }
    589. 

  589. 

  590. %}
    591. 

  591. </pre>
    592. 

  592. </div>
    593. 

  593. 

  594. <p>For classes, since swig generates an interface, you can add additional
    595. 

  595. methods by defining another interface that includes the swig-generated
    596. 

  596. interface.  For example,</p>
    597. 

  597. <div class="code">
    598. 

  598. <pre>
    599. 

  599. %rename(Wrapped_MyClass) MyClass;
    600. 

  600. %rename(Wrapped_GetAValue) MyClass::GetAValue(int *x);
    601. 

  601. %apply int *OUTPUT { int *x };
    602. 

  602. 

  603. class MyClass {
    604. 

  604.  public:
    605. 

  605.   MyClass();
    606. 

  606.   int AFineMethod(const char *arg); // Swig's wrapping is fine for this one.
    607. 

  607.   bool GetAValue(int *x);
    608. 

  608. };
    609. 

  609. 

  610. %insert(go_wrapper) %{
    611. 

  611. 

  612. type MyClass interface {
    613. 

  613.   Wrapped_MyClass
    614. 

  614.   GetAValue() (int, bool)
    615. 

  615. }
    616. 

  616. 

  617. func (arg SwigcptrWrapped_MyClass) GetAValue() (int, bool) {
    618. 

  618.   ip := []int{0}
    619. 

  619.   ok := arg.Wrapped_GetAValue(ip)
    620. 

  620.   return ip[0], ok
    621. 

  621. }
    622. 

  622. 

  623. %}
    624. 

  624. </pre>
    625. 

  625. </div>
    626. 

  626. <p>Of course, if you have to rewrite most of the methods, instead of just a
    627. 

  627. few, then you might as well define your own struct that includes the
    628. 

  628. swig-wrapped object, instead of adding methods to the swig-generated object.</p>
    629. 

  629. 

  630. <p>This only works if your wrappers do not need to import other go modules.
    631. 

  631. There is at present no way to insert import statements in the correct place
    632. 

  632. in swig-generated go.  If you need to do that, you must put your go code
    633. 

  633. in a separate file.</p>
    634. 

  634. </body>
    635. 

  635. </html>
    636. 

  636.