1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
|
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<title>SWIG and Go</title>
<link rel="stylesheet" type="text/css" href="style.css">
<meta http-equiv="content-type" content="text/html; charset=UTF-8">
</head>
<body bgcolor="#FFFFFF">
<H1><a name="Go">24 SWIG and Go</a></H1>
<!-- INDEX -->
<div class="sectiontoc">
<ul>
<li><a href="#Go_overview">Overview</a>
<li><a href="#Go_examples">Examples</a>
<li><a href="#Go_running_swig">Running SWIG with Go</a>
<ul>
<li><a href="#Go_commandline">Go-specific Commandline Options</a>
<li><a href="#Go_outputs">Generated Wrapper Files</a>
</ul>
<li><a href="#Go_basic_tour">A tour of basic C/C++ wrapping</a>
<ul>
<li><a href="#Go_package">Go Package Name</a>
<li><a href="#Go_names">Go Names</a>
<li><a href="#Go_constants">Go Constants</a>
<li><a href="#Go_enumerations">Go Enumerations</a>
<li><a href="#Go_classes">Go Classes</a>
<ul>
<li><a href="#Go_class_memory">Go Class Memory Management</a>
<li><a href="#Go_class_inheritance">Go Class Inheritance</a>
</ul>
<li><a href="#Go_templates">Go Templates</a>
<li><a href="#Go_director_classes">Go Director Classes</a>
<ul>
<li><a href="#Go_director_example_cpp_code">Example C++ code</a>
<li><a href="#Go_director_enable">Enable director feature</a>
<li><a href="#Go_director_ctor_dtor">Constructor and destructor</a>
<li><a href="#Go_director_overriding">Override virtual methods</a>
<li><a href="#Go_director_base_methods">Call base methods</a>
<li><a href="#Go_director_subclass">Subclass via embedding</a>
<li><a href="#Go_director_finalizer">Memory management with runtime.SetFinalizer</a>
<li><a href="#Go_director_foobargo_class">Complete FooBarGo example class</a>
</ul>
<li><a href="#Go_primitive_type_mappings">Default Go primitive type mappings</a>
<li><a href="#Go_output_arguments">Output arguments</a>
<li><a href="#Go_adding_additional_code">Adding additional go code</a>
<li><a href="#Go_typemaps">Go typemaps</a>
</ul>
</ul>
</div>
<!-- INDEX -->
<p>
This chapter describes SWIG's support of Go. For more information on
the Go programming language
see <a href="http://golang.org/">golang.org</a>.
</p>
<H2><a name="Go_overview">24.1 Overview</a></H2>
<p>
Go does not support direct calling of functions written in C/C++. The
<a href="https://golang.org/cmd/cgo/">cgo program</a> may be used to generate
wrappers to call C code from Go, but there is no convenient way to call C++
code. SWIG fills this gap.
</p>
<p>
There are (at least) two different Go compilers. The first is the gc compiler
of the <a href="https://golang.org/doc/install">Go distribution</a>, normally
invoked via the <a href="https://golang.org/cmd/go/">go tool</a>.
The second Go compiler is the <a href="https://golang.org/doc/install/gccgo">
gccgo compiler</a>, which is a frontend to the GCC compiler suite.
The interface to C/C++ code is completely different for the two Go compilers.
SWIG supports both Go compilers, selected by the <tt>-gccgo</tt> command line
option.
</p>
<p>
Go is a type-safe compiled language and the wrapper code generated by SWIG is
type-safe as well. In case of type issues the build will fail and hence SWIG's
<a href="Modules.html#Modules_nn2">runtime library</a> and
<a href="Typemaps.html#Typemaps_runtime_type_checker">runtime type checking</a>
are not used.
</p>
<H2><a name="Go_examples">24.2 Examples</a></H2>
<p>
Working examples can be found in the
<a href="https://github.com/swig/swig/tree/master/Examples/go">SWIG source tree
</a>.
</p>
<p>
Please note that the examples in the SWIG source tree use makefiles with the .i
SWIG interface file extension for backwards compatibility with Go 1.
</p>
<H2><a name="Go_running_swig">24.3 Running SWIG with Go</a></H2>
<p>
Most Go programs are built using the <a href="https://golang.org/cmd/go/">go
tool</a>. Since Go 1.1 the go tool has support for SWIG. To use it, give your
SWIG interface file the extension .swig (for C code) or .swigcxx (for C++ code).
Put that file in a GOPATH/src directory as usual for Go sources. Put other
C/C++ code in the same directory with extensions of .c and .cxx. The
<tt>go build</tt> and <tt>go install</tt> commands will automatically run SWIG
for you and compile the generated wrapper code. To check the SWIG command line
options the go tool uses run <tt>go build -x</tt>. To access the automatically
generated files run <tt>go build -work</tt>. You'll find the files under the
temporary WORK directory.
</p>
<p>
To manually generate and compile C/C++ wrapper code for Go, use the <tt>-go</tt>
option with SWIG. By default SWIG will generate code for the Go compiler of the
Go distribution. To generate code for gccgo, you should also use the
<tt>-gccgo</tt> option.
</p>
<p>
By default SWIG will generate files that can be used directly
by <tt>go build</tt>. This requires Go 1.2 or later. Put your SWIG
interface file in a directory under GOPATH/src, and give it a name
that does <b>not</b> end in the .swig or .swigcxx extension.
Typically the SWIG interface file extension is .i in this case.
</p>
<div class="code"><pre>
% swig -go example.i
% go install
</pre></div>
<p>
You will now have a Go package that you can import from other Go packages as
usual.
</p>
<p>
SWIG can be used without cgo, via the <tt>-no-cgo</tt> option, but
more steps are required. This only works with Go versions before 1.5.
When using Go version 1.2 or later, or when using gccgo, the code
generated by SWIG can be linked directly into the Go program. A
typical command sequence when using the Go compiler of the Go
distribution would look like this:
</p>
<div class="code"><pre>
% swig -go -no-cgo example.i
% gcc -c code.c # The C library being wrapped.
% gcc -c example_wrap.c
% go tool 6g example.go
% go tool 6c example_gc.c
% go tool pack grc example.a example.6 example_gc.6 code.o example_wrap.o
% go tool 6g main.go
% go tool 6l main.6
</pre></div>
<p>
You can also put the wrapped code into a shared library, and when using the Go
versions before 1.2 this is the only supported option. A typical command
sequence for this approach would look like this:
</p>
<div class="code"><pre>
% swig -go -no-cgo -use-shlib example.i
% gcc -c -fpic example.c
% gcc -c -fpic example_wrap.c
% gcc -shared example.o example_wrap.o -o example.so
% go tool 6g example.go
% go tool 6c example_gc.c
% go tool pack grc example.a example.6 example_gc.6
% go tool 6g main.go # your code, not generated by SWIG
% go tool 6l main.6
</pre></div>
<H3><a name="Go_commandline">24.3.1 Go-specific Commandline Options</a></H3>
<p>
These are the command line options for SWIG's Go module. They can
also be seen by using:
</p>
<div class="code"><pre>
swig -go -help
</pre></div>
<table summary="Go-specific options">
<tr>
<th>Go-specific options</th>
</tr>
<tr>
<td>-cgo</td>
<td>Generate files to be used as input for the Go cgo tool. This is
the default.</td>
</tr>
<tr>
<td>-no-cgo</td>
<td>Generate files that can be used directly, rather than via the Go
cgo tool. This option does not work with Go 1.5 or later. It is
required for versions of Go before 1.2.</td>
</tr>
<tr>
<td>-intgosize <s></td>
<td>Set the size for the Go type <tt>int</tt>. This controls the size
that the C/C++ code expects to see. The <s> argument should
be 32 or 64. This option is currently required during the
transition from Go 1.0 to Go 1.1, as the size of <tt>int</tt> on
64-bit x86 systems changes between those releases (from 32 bits to
64 bits). In the future the option may become optional, and SWIG
will assume that the size of <tt>int</tt> is the size of a C
pointer.</td>
</tr>
<tr>
<td>-gccgo</td>
<td>Generate code for gccgo. The default is to generate code for
the Go compiler of the Go distribution.</td>
</tr>
<tr>
<td>-package <name></td>
<td>Set the name of the Go package to <name>. The default
package name is the SWIG module name.</td>
</tr>
<tr>
<td>-use-shlib</td>
<td>Tell SWIG to emit code that uses a shared library. This is only
meaningful for the Go compiler of the Go distribution, which needs to know at
compile time whether a shared library will be used.</td>
</tr>
<tr>
<td>-soname <name></td>
<td>Set the runtime name of the shared library that the dynamic linker
should include at runtime. The default is the package name with
".so" appended. This is only used when generating code for
the Go compiler of the Go distribution; when using gccgo, the equivalent name
will be taken from the <code>-soname</code> option passed to the linker.
Using this option implies the -use-shlib option.</td>
</tr>
<tr>
<td>-go-pkgpath <pkgpath></td>
<td>When generating code for gccgo, set the pkgpath to use. This
corresponds to the <tt>-fgo-pkgpath</tt> option to gccgo.</td>
</tr>
<tr>
<td>-go-prefix <prefix></td>
<td>When generating code for gccgo, set the prefix to use. This
corresponds to the <tt>-fgo-prefix</tt> option to gccgo.
If <tt>-go-pkgpath</tt> is used, <tt>-go-prefix</tt> will be
ignored.</td>
</tr>
<tr>
<td>-import-prefix <prefix></td>
<td>A prefix to add when turning a %import prefix in the SWIG
interface file into an import statement in the Go file. For
example, with <code>-import-prefix mymodule</code>, a SWIG
interface file <code>%import mypackage</code> will become a Go
import statement <code>import "mymodule/mypackage"</code>.</td>
</table>
<H3><a name="Go_outputs">24.3.2 Generated Wrapper Files</a></H3>
<p>There are two different approaches to generating wrapper files,
controlled by SWIG's <tt>-no-cgo</tt> option. The <tt>-no-cgo</tt>
option only works with version of Go before 1.5. It is required
when using versions of Go before 1.2.</p>
<p>With or without the <tt>-no-cgo</tt> option, SWIG will generate the
following files when generating wrapper code:</p>
<ul>
<li>
MODULE.go will contain the Go functions that your Go code will call.
These functions will be wrappers for the C++ functions defined by your
module. This file should, of course, be compiled with the Go
compiler.
</li>
<li>
MODULE_wrap.c or MODULE_wrap.cxx will contain C/C++ functions will be
invoked by the Go wrapper code. This file should be compiled with the
usual C or C++ compiler.
</li>
<li>
MODULE_wrap.h will be generated if you use the directors feature. It
provides a definition of the generated C++ director classes. It is
generally not necessary to use this file, but in some special cases it
may be helpful to include it in your code, compiled with the usual C
or C++ compiler.
</li>
</ul>
<p>When the <tt>-no-cgo</tt> option is used, and the <tt>-gccgo</tt>
option is not used, SWIG will also generate an additional file:</p>
<ul>
<li>
MODULE_gc.c will contain C code which should be compiled with the C
compiler distributed as part of the gc compiler. It should then be
combined with the compiled MODULE.go using go tool pack.
</li>
</ul>
<H2><a name="Go_basic_tour">24.4 A tour of basic C/C++ wrapping</a></H2>
<p>
By default, SWIG attempts to build a natural Go interface to your
C/C++ code. However, the languages are somewhat different, so some
modifications have to occur. This section briefly covers the
essential aspects of this wrapping.
</p>
<H3><a name="Go_package">24.4.1 Go Package Name</a></H3>
<p>
All Go source code lives in a package. The name of this package will
default to the name of the module from SWIG's <tt>%module</tt>
directive. You may override this by using SWIG's <tt>-package</tt>
command line option.
</p>
<H3><a name="Go_names">24.4.2 Go Names</a></H3>
<p>
In Go, a function is only visible outside the current package if the
first letter of the name is uppercase. This is quite different from
C/C++. Because of this, C/C++ names are modified when generating the
Go interface: the first letter is forced to be uppercase if it is not
already. This affects the names of functions, methods, variables,
constants, enums, and classes.
</p>
<p>
C/C++ variables are wrapped with setter and getter functions in Go.
First the first letter of the variable name will be forced to
uppercase, and then <tt>Get</tt> or <tt>Set</tt> will be prepended.
For example, if the C/C++ variable is called <tt>var</tt>, then SWIG
will define the functions <tt>GetVar</tt> and <tt>SetVar</tt>. If a
variable is declared as <tt>const</tt>, or if
SWIG's <a href="SWIG.html#SWIG_readonly_variables">
<tt>%immutable</tt> directive</a> is used for the variable, then only
the getter will be defined.
</p>
<p>
C++ classes will be discussed further below. Here we'll note that the
first letter of the class name will be forced to uppercase to give the
name of a type in Go. A constructor will be named <tt>New</tt>
followed by that name, and the destructor will be
named <tt>Delete</tt> followed by that name.
</p>
<H3><a name="Go_constants">24.4.3 Go Constants</a></H3>
<p>
C/C++ constants created via <tt>#define</tt> or the <tt>%constant</tt>
directive become Go constants, declared with a <tt>const</tt>
declaration.
<H3><a name="Go_enumerations">24.4.4 Go Enumerations</a></H3>
<p>
C/C++ enumeration types will cause SWIG to define an integer type with
the name of the enumeration (with first letter forced to uppercase as
usual). The values of the enumeration will become variables in Go;
code should avoid modifying those variables.
</p>
<H3><a name="Go_classes">24.4.5 Go Classes</a></H3>
<p>
Go has interfaces, methods and inheritance, but it does not have
classes in the same sense as C++. This sections describes how SWIG
represents C++ classes represented in Go.
</p>
<p>
For a C++ class <tt>ClassName</tt>, SWIG will define two types in Go:
an underlying type, which will just hold a pointer to the C++ type,
and an interface type. The interface type will be
named <tt>ClassName</tt>. SWIG will define a
function <tt>NewClassName</tt> which will take any constructor
arguments and return a value of the interface
type <tt>ClassName</tt>. SWIG will also define a
destructor <tt>DeleteClassName</tt>.
</p>
<p>
SWIG will represent any methods of the C++ class as methods on the
underlying type, and also as methods of the interface type. Thus C++
methods may be invoked directly using the
usual <tt>val.MethodName</tt> syntax. Public members of the C++ class
will be given getter and setter functions defined as methods of the
class.
</p>
<p>
SWIG will represent static methods of C++ classes as ordinary Go
functions. SWIG will use names like <tt>ClassNameMethodName</tt>.
SWIG will give static members getter and setter functions with names
like <tt>GetClassName_VarName</tt>.
</p>
<p>
Given a value of the interface type, Go code can retrieve the pointer
to the C++ type by calling the <tt>Swigcptr</tt> method. This will
return a value of type <tt>SwigcptrClassName</tt>, which is just a
name for <tt>uintptr</tt>. A Go type conversion can be used to
convert this value to a different C++ type, but note that this
conversion will not be type checked and is essentially equivalent
to <tt>reinterpret_cast</tt>. This should only be used for very
special cases, such as where C++ would use a <tt>dynamic_cast</tt>.
</p>
<p>Note that C++ pointers to compound objects are represented in go as objects
themselves, not as go pointers. So, for example, if you wrap the following
function:</p>
<div class="code">
<pre>
class MyClass {
int MyMethod();
static MyClass *MyFactoryFunction();
};
</pre>
</div>
<p>You will get go code that looks like this:</p>
<div class="code">
<pre>
type MyClass interface {
Swigcptr() uintptr
SwigIsMyClass()
MyMethod() int
}
func MyClassMyFactoryFunction() MyClass {
// swig magic here
}
</pre>
</div>
<p>Note that the factory function does not return a go pointer; it actually
returns a go interface. If the returned pointer can be null, you can check
for this by calling the Swigcptr() method.
</p>
<H4><a name="Go_class_memory">24.4.5.1 Go Class Memory Management</a></H4>
<p>
Calling <tt>NewClassName</tt> for a C++ class <tt>ClassName</tt> will allocate
memory using the C++ memory allocator. This memory will not be automatically
freed by Go's garbage collector as the object ownership is not tracked. When
you are done with the C++ object you must free it using
<tt>DeleteClassName</tt>.<br>
<br>
The most Go idiomatic way to manage the memory for some C++ class is to call
<tt>NewClassName</tt> followed by a
<tt><a href="https://golang.org/doc/effective_go.html#defer">defer</a></tt> of
the <tt>DeleteClassName</tt> call. Using <tt>defer</tt> ensures that the memory
of the C++ object is freed as soon as the function containing the <tt>defer</tt>
statement returns. Furthermore <tt>defer</tt> works great for short-lived
objects and fits nicely C++'s RAII idiom. Example:
</p>
<div class="code">
<pre>
func UseClassName(...) ... {
o := NewClassName(...)
defer DeleteClassName(o)
// Use the ClassName object
return ...
}
</pre>
</div>
<p>
With increasing complexity, especially complex C++ object hierarchies, the
correct placement of <tt>defer</tt> statements becomes harder and harder as C++
objects need to be freed in the correct order. This problem can be eased by
keeping a C++ object function local so that it is only available to the function
that creates a C++ object and functions called by this function. Example:
</p>
<div class="code">
<pre>
func WithClassName(constructor args, f func(ClassName, ...interface{}) error, data ...interface{}) error {
o := NewClassName(constructor args)
defer DeleteClassName(o)
return f(o, data...)
}
func UseClassName(o ClassName, data ...interface{}) (err error) {
// Use the ClassName object and additional data and return error.
}
func main() {
WithClassName(constructor args, UseClassName, additional data)
}
</pre>
</div>
<p>
Using <tt>defer</tt> has limitations though, especially when it comes to
long-lived C++ objects whose lifetimes are hard to predict. For such C++
objects a common technique is to store the C++ object into a Go object, and to
use the Go function <tt>runtime.SetFinalizer</tt> to add a finalizer which frees
the C++ object when the Go object is freed. It is strongly recommended to read
the <a href="https://golang.org/pkg/runtime/#SetFinalizer">runtime.SetFinalizer
</a> documentation before using this technique to understand the
<tt>runtime.SetFinalizer</tt> limitations.<br>
</p>
<p>
Common pitfalls with <tt>runtime.SetFinalizer</tt> are:
</p>
<ul>
<li>
If a hierarchy of C++ objects will be automatically freed by Go finalizers then
the Go objects that store the C++ objects need to replicate the hierarchy of the
C++ objects to prevent that C++ objects are freed prematurely while other C++
objects still rely on them.
</li>
<li>
The usage of Go finalizers is problematic with C++'s RAII idiom as it isn't
predictable when the finalizer will run and this might require a Close or Delete
method to be added the Go object that stores a C++ object to mitigate.
</li>
<li>
The Go finalizer function typically runs in a different OS thread which can be
problematic with C++ code that uses thread-local storage.
</li>
</ul>
<p>
<tt>runtime.SetFinalizer</tt> Example:
</p>
<div class="code">
<pre>
import (
"runtime"
"wrap" // SWIG generated wrapper code
)
type GoClassName struct {
wcn wrap.ClassName
}
func NewGoClassName() *GoClassName {
o := &GoClassName{wcn: wrap.NewClassName()}
runtime.SetFinalizer(o, deleteGoClassName)
return o
}
func deleteGoClassName(o *GoClassName) {
// Runs typically in a different OS thread!
wrap.DeleteClassName(o.wcn)
o.wcn = nil
}
func (o *GoClassName) Close() {
// If the C++ object has a Close method.
o.wcn.Close()
// If the GoClassName object is no longer in an usable state.
runtime.SetFinalizer(o, nil) // Remove finalizer.
deleteGoClassName() // Free the C++ object.
}
</pre>
</div>
<H4><a name="Go_class_inheritance">24.4.5.2 Go Class Inheritance</a></H4>
<p>
C++ class inheritance is automatically represented in Go due to its
use of interfaces. The interface for a child class will be a superset
of the interface of its parent class. Thus a value of the child class
type in Go may be passed to a function which expects the parent class.
Doing the reverse will require an explicit type assertion, which will
be checked dynamically.
</p>
<H3><a name="Go_templates">24.4.6 Go Templates</a></H3>
<p>
In order to use C++ templates in Go, you must tell SWIG to create
wrappers for a particular template instantiation. To do this, use
the <tt>%template</tt> directive.
<H3><a name="Go_director_classes">24.4.7 Go Director Classes</a></H3>
<p>
SWIG's director feature permits a Go type to act as the subclass of a C++ class.
This is complicated by the fact that C++ and Go define inheritance differently.
SWIG normally represents the C++ class inheritance automatically in Go via
interfaces but with a Go type representing a subclass of a C++ class some manual
work is necessary.
</p>
<p>
This subchapter gives a step by step guide how to properly subclass a C++ class
with a Go type. In general it is strongly recommended to follow this guide
completely to avoid common pitfalls with directors in Go.
</p>
<H4><a name="Go_director_example_cpp_code">24.4.7.1 Example C++ code</a></H4>
<p>
The step by step guide is based on two example C++ classes. FooBarAbstract is
an abstract C++ class and the FooBarCpp class inherits from it. This guide
explains how to implement a FooBarGo class similar to the FooBarCpp class.
</p>
<p>
<tt>FooBarAbstract</tt> abstract C++ class:
</p>
<div class="code">
<pre>
class FooBarAbstract
{
public:
FooBarAbstract() {};
virtual ~FooBarAbstract() {};
std::string FooBar() {
return this->Foo() + ", " + this->Bar();
};
protected:
virtual std::string Foo() {
return "Foo";
};
virtual std::string Bar() = 0;
};
</pre>
</div>
<p>
<tt>FooBarCpp</tt> C++ class:
</p>
<div class="code">
<pre>
class FooBarCpp : public FooBarAbstract
{
protected:
virtual std::string Foo() {
return "C++ " + FooBarAbstract::Foo();
}
virtual std::string Bar() {
return "C++ Bar";
}
};
</pre>
</div>
<p>
Returned string by the <tt>FooBarCpp::FooBar</tt> method is:
</p>
<div class="code">
<pre>
C++ Foo, C++ Bar
</pre>
</div>
<p>
The complete example, including the <tt>FooBarGoo</tt> class implementation, can
be found in <a href="#Go_director_foobargo_class">the end of the guide</a>.
</p>
<H4><a name="Go_director_enable">24.4.7.2 Enable director feature</a></H4>
<p>
The director feature is disabled by default. To use directors you must make two
changes to the interface file. First, add the "directors" option to the %module
directive, like this:
</p>
<div class="code">
<pre>
%module(directors="1") modulename
</pre>
</div>
<p>
Second, you must use the %feature("director") directive to tell SWIG which
classes should get directors. In the example the FooBarAbstract class needs the
director feature enabled so that the FooBarGo class can inherit from it, like
this:
</p>
<div class="code">
<pre>
%feature("director") FooBarAbstract;
</pre>
</div>
<p>
For a more detailed documentation of the director feature and how to enable or
disable it for specific classes and virtual methods see SWIG's Java
documentation on directors.
</p>
<H4><a name="Go_director_ctor_dtor">24.4.7.3 Constructor and destructor</a></H4>
<p>
SWIG creates an additional set of constructor and destructor functions once the
director feature has been enabled for a C++ class.
<tt>NewDirectorClassName</tt> allows overriding virtual methods on the new
object instance and <tt>DeleteDirectorClassName</tt> needs to be used to free a
director object instance created with <tt>NewDirectorClassName</tt>.
More on overriding virtual methods follows later in this guide under
<a href="#Go_director_overriding">overriding virtual methods</a>.
</p>
<p>
The default constructor and destructor functions <tt>NewClassName</tt> and
<tt>DeleteClassName</tt> can still be used as before so that existing code
doesn't break just because the director feature has been enabled for a C++
class. The behavior is undefined if the default and director constructor and
destructor functions get mixed and so great care needs to be taken that only one
of the constructor and destructor function pairs is used for any object
instance. Both constructor functions, the default and the director one, return
the same interface type. This makes it potentially hard to know which
destructor function, the default or the director one, needs to be called to
delete an object instance.
</p>
<p>
In <b>theory</b> the <tt>DirectorInterface</tt> method could be used to
determine if an object instance was created via <tt>NewDirectorClassName</tt>:
</p>
<div class="code">
<pre>
if o.DirectorInterface() != nil {
DeleteDirectorClassName(o)
} else {
DeleteClassName(o)
}
</pre>
</div>
<p>
In <b>practice</b> it is strongly recommended to embed a director object
instance in a Go struct so that a director object instance will be represented
as a distinct Go type that subclasses a C++ class. For this Go type custom
constructor and destructor functions take care of the director constructor and
destructor function calls and the resulting Go class will appear to the user as
any other SWIG wrapped C++ class. More on properly subclassing a C++ class
follows later in this guide under <a href="#Go_director_subclass">subclass via
embedding</a>.
</p>
<H4><a name="Go_director_overriding">24.4.7.4 Override virtual methods</a></H4>
<p>
In order to override virtual methods on a C++ class with Go methods the
<tt>NewDirectorClassName</tt> constructor functions receives a
<tt>DirectorInterface</tt> argument. The methods in the <tt>
DirectorInterface</tt> are a subset of the public and protected virtual methods
of the C++ class.
Virtual methods that have a final specifier are unsurprisingly excluded.
If the <tt>DirectorInterface</tt> contains a method with a
matching signature to a virtual method of the C++ class then the virtual C++
method will be overwritten with the Go method. As Go doesn't support protected
methods all overridden protected virtual C++ methods will be public in Go.
</p>
<p>
As an example see part of the <tt>FooBarGo</tt> class:
</p>
<div class="code">
<pre>
type overwrittenMethodsOnFooBarAbstract struct {
fb FooBarAbstract
}
func (om *overwrittenMethodsOnFooBarAbstract) Foo() string {
...
}
func (om *overwrittenMethodsOnFooBarAbstract) Bar() string {
...
}
func NewFooBarGo() FooBarGo {
om := &overwrittenMethodsOnFooBarAbstract{}
fb := NewDirectorFooBarAbstract(om)
om.fb = fb
...
}
</pre>
</div>
<p>
The complete example, including the <tt>FooBarGoo</tt> class implementation, can
be found in <a href="#Go_director_foobargo_class">the end of the guide</a>. In
this part of the example the virtual methods <tt>FooBarAbstract::Foo</tt> and
<tt>FooBarAbstract::Bar</tt> have been overwritten with Go methods similarly to
how the <tt>FooBarAbstract</tt> virtual methods are overwritten by the
<tt>FooBarCpp</tt> class.
</p>
<p>
The <tt>DirectorInterface</tt> in the example is implemented by the
<tt>overwrittenMethodsOnFooBarAbstract</tt> Go struct type. A pointer to a
<tt>overwrittenMethodsOnFooBarAbstract</tt> struct instance will be given to the
<tt>NewDirectorFooBarAbstract</tt> constructor function. The constructor return
value implements the <tt>FooBarAbstract</tt> interface.
<tt>overwrittenMethodsOnFooBarAbstract</tt> could in theory be any Go type but
in practice a struct is used as it typically contains at least a value of the
C++ class interface so that the overwritten methods can use the rest of the
C++ class. If the <tt>FooBarGo</tt> class would receive additional constructor
arguments then these would also typically be stored in the
<tt>overwrittenMethodsOnFooBarAbstract</tt> struct so that they can be used by
the Go methods.
</p>
<H4><a name="Go_director_base_methods">24.4.7.5 Call base methods</a></H4>
<p>
Often a virtual method will be overwritten to extend the original behavior of
the method in the base class. This is also the case for the
<tt>FooBarCpp::Foo</tt> method of the example code:
</p>
<div class="code">
<pre>
virtual std::string Foo() {
return "C++ " + FooBarAbstract::Foo();
}
</pre>
</div>
<p>
To use base methods the <tt>DirectorClassNameMethodName</tt> wrapper functions
are automatically generated by SWIG for public and protected virtual methods.
The <tt>FooBarGo.Foo</tt> implementation in the example looks like this:
</p>
<div class="code">
<pre>
func (om *overwrittenMethodsOnFooBarAbstract) Foo() string {
return "Go " + DirectorFooBarAbstractFoo(om.fb)
}
</pre>
</div>
<p>
The complete example, including the <tt>FooBarGoo</tt> class implementation, can
be found in <a href="#Go_director_foobargo_class">the end of the guide</a>.
</p>
<H4><a name="Go_director_subclass">24.4.7.6 Subclass via embedding</a></H4>
<p>
<a href="#Go_director_ctor_dtor">As previously mentioned in this guide</a> the
default and director constructor functions return the same interface type. To
properly subclass a C++ class with a Go type the director object instance
returned by the <tt>NewDirectorClassName</tt> constructor function should be
embedded into a Go struct so that it represents a distinct but compatible type
in Go's type system. This Go struct should be private and the constructor and
destructor functions should instead work with a public interface type so that
the Go class that subclasses a C++ class can be used as a compatible drop in.
</p>
<p>
The subclassing part of the <tt>FooBarGo</tt> class for an example looks like
this:
</p>
<div class="code">
<pre>
type FooBarGo interface {
FooBarAbstract
deleteFooBarAbstract()
IsFooBarGo()
}
type fooBarGo struct {
FooBarAbstract
}
func (fbgs *fooBarGo) deleteFooBarAbstract() {
DeleteDirectorFooBarAbstract(fbgs.FooBarAbstract)
}
func (fbgs *fooBarGo) IsFooBarGo() {}
func NewFooBarGo() FooBarGo {
om := &overwrittenMethodsOnFooBarAbstract{}
fb := NewDirectorFooBarAbstract(om)
om.fb = fb
return &fooBarGo{FooBarAbstract: fb}
}
func DeleteFooBarGo(fbg FooBarGo) {
fbg.deleteFooBarAbstract()
}
</pre>
</div>
<p>
The complete example, including the <tt>FooBarGoo</tt> class implementation, can
be found in <a href="#Go_director_foobargo_class">the end of the guide</a>. In
this part of the example the private <tt>fooBarGo</tt> struct embeds <tt>
FooBarAbstract</tt> which lets the <tt>fooBarGo</tt> Go type "inherit" all the
methods of the <tt>FooBarAbstract</tt> C++ class by means of embedding. The
public <tt>FooBarGo</tt> interface type includes the <tt>FooBarAbstract</tt>
interface and hence <tt>FooBarGo</tt> can be used as a drop in replacement for
<tt>FooBarAbstract</tt> while the reverse isn't possible and would raise a
compile time error. Furthermore the constructor and destructor functions <tt>
NewFooBarGo</tt> and <tt>DeleteFooBarGo</tt> take care of all the director
specifics and to the user the class appears as any other SWIG wrapped C++
class.
</p>
<H4><a name="Go_director_finalizer">24.4.7.7 Memory management with runtime.SetFinalizer</a></H4>
<p>
In general all guidelines for <a href="#Go_class_memory">C++ class memory
management</a> apply as well to director classes. One often overlooked
limitation with <tt>runtime.SetFinalizer</tt> is that a finalizer doesn't run
in case of a cycle and director classes typically have a cycle. The cycle
in the <tt>FooBarGo</tt> class is here:
</p>
<div class="code">
<pre>
type overwrittenMethodsOnFooBarAbstract struct {
fb FooBarAbstract
}
func NewFooBarGo() FooBarGo {
om := &overwrittenMethodsOnFooBarAbstract{}
fb := NewDirectorFooBarAbstract(om) // fb.v = om
om.fb = fb // Backlink causes cycle as fb.v = om!
...
}
</pre>
</div>
<p>
In order to be able to use <tt>runtime.SetFinalizer</tt> nevertheless the
finalizer needs to be set on something that isn't in a cycle and that references
the director object instance. In the <tt>FooBarGo</tt> class example the <tt>
FooBarAbstract</tt> director instance can be automatically deleted by setting
the finalizer on <tt>fooBarGo</tt>:
</p>
<div class="code">
<pre>
type fooBarGo struct {
FooBarAbstract
}
type overwrittenMethodsOnFooBarAbstract struct {
fb FooBarAbstract
}
func NewFooBarGo() FooBarGo {
om := &overwrittenMethodsOnFooBarAbstract{}
fb := NewDirectorFooBarAbstract(om)
om.fb = fb // Backlink causes cycle as fb.v = om!
fbgs := &fooBarGo{FooBarAbstract: fb}
runtime.SetFinalizer(fbgs, FooBarGo.deleteFooBarAbstract)
return fbgs
}
</pre>
</div>
<p>
Furthermore if <tt>runtime.SetFinalizer</tt> is in use either the <tt>
DeleteClassName</tt> destructor function needs to be removed or the <tt>
fooBarGo</tt> struct needs additional data to prevent double deletion. Please
read the <a href="#Go_class_memory">C++ class memory management</a> subchapter
before using <tt>runtime.SetFinalizer</tt> to know all of its gotchas.
</p>
<H4><a name="Go_director_foobargo_class">24.4.7.8 Complete FooBarGo example class</a></H4>
<p>
The complete and annotated <tt>FooBarGo</tt> class looks like this:
</p>
<div class="code">
<pre>
// FooBarGo is a superset of FooBarAbstract and hence FooBarGo can be used as a
// drop in replacement for FooBarAbstract but the reverse causes a compile time
// error.
type FooBarGo interface {
FooBarAbstract
deleteFooBarAbstract()
IsFooBarGo()
}
// Via embedding fooBarGo "inherits" all methods of FooBarAbstract.
type fooBarGo struct {
FooBarAbstract
}
func (fbgs *fooBarGo) deleteFooBarAbstract() {
DeleteDirectorFooBarAbstract(fbgs.FooBarAbstract)
}
// The IsFooBarGo method ensures that FooBarGo is a superset of FooBarAbstract.
// This is also how the class hierarchy gets represented by the SWIG generated
// wrapper code. For an instance FooBarCpp has the IsFooBarAbstract and
// IsFooBarCpp methods.
func (fbgs *fooBarGo) IsFooBarGo() {}
// Go type that defines the DirectorInterface. It contains the Foo and Bar
// methods that overwrite the respective virtual C++ methods on FooBarAbstract.
type overwrittenMethodsOnFooBarAbstract struct {
// Backlink to FooBarAbstract so that the rest of the class can be used by
// the overridden methods.
fb FooBarAbstract
// If additional constructor arguments have been given they are typically
// stored here so that the overridden methods can use them.
}
func (om *overwrittenMethodsOnFooBarAbstract) Foo() string {
// DirectorFooBarAbstractFoo calls the base method FooBarAbstract::Foo.
return "Go " + DirectorFooBarAbstractFoo(om.fb)
}
func (om *overwrittenMethodsOnFooBarAbstract) Bar() string {
return "Go Bar"
}
func NewFooBarGo() FooBarGo {
// Instantiate FooBarAbstract with selected methods overridden. The methods
// that will be overwritten are defined on
// overwrittenMethodsOnFooBarAbstract and have a compatible signature to the
// respective virtual C++ methods. Furthermore additional constructor
// arguments will be typically stored in the
// overwrittenMethodsOnFooBarAbstract struct.
om := &overwrittenMethodsOnFooBarAbstract{}
fb := NewDirectorFooBarAbstract(om)
om.fb = fb // Backlink causes cycle as fb.v = om!
fbgs := &fooBarGo{FooBarAbstract: fb}
// The memory of the FooBarAbstract director object instance can be
// automatically freed once the FooBarGo instance is garbage collected by
// uncommenting the following line. Please make sure to understand the
// runtime.SetFinalizer specific gotchas before doing this. Furthermore
// DeleteFooBarGo should be deleted if a finalizer is in use or the fooBarGo
// struct needs additional data to prevent double deletion.
// runtime.SetFinalizer(fbgs, FooBarGo.deleteFooBarAbstract)
return fbgs
}
// Recommended to be removed if runtime.SetFinalizer is in use.
func DeleteFooBarGo(fbg FooBarGo) {
fbg.deleteFooBarAbstract()
}
</pre>
</div>
<p>
Returned string by the <tt>FooBarGo.FooBar</tt> method is:
</p>
<div class="code">
<pre>
Go Foo, Go Bar
</pre>
</div>
<p>
For comparison the <tt>FooBarCpp</tt> class looks like this:
</p>
<div class="code">
<pre>
class FooBarCpp : public FooBarAbstract
{
protected:
virtual std::string Foo() {
return "C++ " + FooBarAbstract::Foo();
}
virtual std::string Bar() {
return "C++ Bar";
}
};
</pre>
</div>
<p>
For comparison the returned string by the <tt>FooBarCpp::FooBar</tt> method is:
</p>
<div class="code">
<pre>
C++ Foo, C++ Bar
</pre>
</div>
<p>
The complete source of this example can be found under
<a href="https://github.com/swig/swig/tree/master/Examples/go/director">
SWIG/Examples/go/director/</a>.
</p>
<H3><a name="Go_primitive_type_mappings">24.4.8 Default Go primitive type mappings</a></H3>
<p>
The following table lists the default type mapping from C/C++ to Go.
This table will tell you which Go type to expect for a function which
uses a given C/C++ type.
</p>
<table BORDER summary="Go primitive type mappings">
<tr>
<td><b>C/C++ type</b></td>
<td><b>Go type</b></td>
</tr>
<tr>
<td>bool</td>
<td>bool</td>
</tr>
<tr>
<td>char</td>
<td>byte</td>
</tr>
<tr>
<td>signed char</td>
<td>int8</td>
</tr>
<tr>
<td>unsigned char</td>
<td>byte</td>
</tr>
<tr>
<td>short</td>
<td>int16</td>
</tr>
<tr>
<td>unsigned short</td>
<td>uint16</td>
</tr>
<tr>
<td>int</td>
<td>int</td>
</tr>
<tr>
<td>unsigned int</td>
<td>uint</td>
</tr>
<tr>
<td>long</td>
<td>int64</td>
</tr>
<tr>
<td>unsigned long</td>
<td>uint64</td>
</tr>
<tr>
<td>long long</td>
<td>int64</td>
</tr>
<tr>
<td>unsigned long long</td>
<td>uint64</td>
</tr>
<tr>
<td>float</td>
<td>float32</td>
</tr>
<tr>
<td>double</td>
<td>float64</td>
</tr>
<tr>
<td>char *<br>char []</td>
<td>string</td>
</tr>
</table>
<p>
Note that SWIG wraps the C <tt>char</tt> type as a character. Pointers
and arrays of this type are wrapped as strings. The <tt>signed
char</tt> type can be used if you want to treat <tt>char</tt> as a
signed number rather than a character. Also note that all const
references to primitive types are treated as if they are passed by
value.
</p>
<p>
These type mappings are defined by the "gotype" typemap. You may change
that typemap, or add new values, to control how C/C++ types are mapped
into Go types.
</p>
<H3><a name="Go_output_arguments">24.4.9 Output arguments</a></H3>
<p>Because of limitations in the way output arguments are processed in swig,
a function with output arguments will not have multiple return values.
Instead, you must pass a pointer into the C++ function to tell it where to
store the output value. In go, you supply a slice in the place of the output
argument.</p>
<p>For example, suppose you were trying to wrap the modf() function in the
C math library which splits x into integral and fractional parts (and
returns the integer part in one of its parameters):</p>
<div class="code">
<pre>
double modf(double x, double *ip);
</pre>
</div>
<p>You could wrap it with SWIG as follows:</p>
<div class="code">
<pre>
%include <typemaps.i>
double modf(double x, double *OUTPUT);
</pre>
</div>
<p>or you can use the <code>%apply</code> directive:</p>
<div class="code">
<pre>
%include <typemaps.i>
%apply double *OUTPUT { double *ip };
double modf(double x, double *ip);
</pre>
</div>
<p>In Go you would use it like this:</p>
<div class="code">
<pre>
ptr := []float64{0.0}
fraction := modulename.Modf(5.0, ptr)
</pre>
</div>
<p>Since this is ugly, you may want to wrap the swig-generated API with
some <a href="#Go_adding_additional_code">additional functions written in go</a> that
hide the ugly details.</p>
<p>There are no <code>char *OUTPUT</code> typemaps. However you can
apply the <code>signed char *</code> typemaps instead:</p>
<div class="code">
<pre>
%include <typemaps.i>
%apply signed char *OUTPUT {char *output};
void f(char *output);
</pre>
</div>
<H3><a name="Go_adding_additional_code">24.4.10 Adding additional go code</a></H3>
<p>Often the APIs generated by swig are not very natural in go, especially if
there are output arguments. You can
insert additional go wrapping code to add new APIs
with <code>%insert(go_wrapper)</code>, like this:</p>
<div class="code">
<pre>
%include <typemaps.i>
// Change name of what swig generates to Wrapped_modf. This function will
// have the following signature in go:
// func Wrapped_modf(float64, []float64) float64
%rename(wrapped_modf) modf(double x, double *ip);
%apply double *OUTPUT { double *ip };
double modf(double x, double *ip);
%insert(go_wrapper) %{
// The improved go interface to this function, which has two return values,
// in the more natural go idiom:
func Modf(x float64) (fracPart float64, intPart float64) {
ip := []float64{0.0}
fracPart = Wrapped_modf(x, ip)
intPart = ip[0]
return
}
%}
</pre>
</div>
<p>For classes, since swig generates an interface, you can add additional
methods by defining another interface that includes the swig-generated
interface. For example,</p>
<div class="code">
<pre>
%rename(Wrapped_MyClass) MyClass;
%rename(Wrapped_GetAValue) MyClass::GetAValue(int *x);
%apply int *OUTPUT { int *x };
class MyClass {
public:
MyClass();
int AFineMethod(const char *arg); // Swig's wrapping is fine for this one.
bool GetAValue(int *x);
};
%insert(go_wrapper) %{
type MyClass interface {
Wrapped_MyClass
GetAValue() (int, bool)
}
func (arg SwigcptrWrapped_MyClass) GetAValue() (int, bool) {
ip := []int{0}
ok := arg.Wrapped_GetAValue(ip)
return ip[0], ok
}
%}
</pre>
</div>
<p>Of course, if you have to rewrite most of the methods, instead of just a
few, then you might as well define your own struct that includes the
swig-wrapped object, instead of adding methods to the swig-generated object.</p>
<p>If you need to import other go packages, you can do this with
<code>%go_import</code>. For example,</p>
<div class="code">
<pre>
%go_import("fmt", _ "unusedPackage", rp "renamed/package")
%insert(go_wrapper) %{
func foo() {
fmt.Println("Some string:", rp.GetString())
}
// Importing the same package twice is permitted,
// Go code will be generated with only the first instance of the import.
%go_import("fmt")
%insert(go_wrapper) %{
func bar() {
fmt.Println("Hello world!")
}
%}
</pre>
</div>
<H3><a name="Go_typemaps">24.4.11 Go typemaps</a></H3>
<p>
You can use the <tt>%typemap</tt> directive to modify SWIG's default
wrapping behavior for specific C/C++ types. You need to be familiar
with the material in the general
"<a href="Typemaps.html#Typemaps">Typemaps</a>" chapter. That chapter
explains how to define a typemap. This section describes some
specific typemaps used for Go.
</p>
<p>
In general type conversion code may be written either in C/C++ or in
Go. The choice to make normally depends on where memory should be
allocated. To allocate memory controlled by the Go garbage collector,
write Go code. To allocate memory in the C/C++ heap, write C code.
</p>
<table BORDER summary="Go Typemaps">
<tr>
<td><b>Typemap</b></td>
<td><b>Description</b></td>
</tr>
<tr>
<td>gotype</td>
<td>
The Go type to use for a C++ type. This type will appear in the
generated Go wrapper function. If this is not defined SWIG will use a
default as <a href="#Go_primitive_type_mappings">described above</a>.
</td>
</tr>
<tr>
<td>imtype</td>
<td>
An intermediate Go type used by the "goin", "goout", "godirectorin",
and "godirectorout" typemaps. If this typemap is not defined for a
C/C++ type, the gotype typemape will be used. This is useful when
gotype is best converted to C/C++ using Go code.
</td>
</tr>
<tr>
<td>goin</td>
<td>
Go code to convert from gotype to imtype when calling a C/C++
function. SWIG will then internally convert imtype to a C/C++ type
and pass it down. If this is not defined, or is the empty string, no
conversion is done.
</td>
</tr>
<tr>
<td>in</td>
<td>
C/C++ code to convert the internally generated C/C++ type, based on
imtype, into the C/C++ type that a function call expects. If this is
not defined the value will simply be cast to the desired type.
</td>
</tr>
<tr>
<td>out</td>
<td>
C/C++ code to convert the C/C++ type that a function call returns into
the internally generated C/C++ type, based on imtype, that will be
returned to Go. If this is not defined the value will simply be cast
to the desired type.
</td>
</tr>
<tr>
<td>goout</td>
<td>
Go code to convert a value returned from a C/C++ function from imtype
to gotype. If this is not defined, or is the empty string, no
conversion is done.
</td>
</tr>
<tr>
<td>argout</td>
<td>
C/C++ code to adjust an argument value when returning from a function.
This is called after the real C/C++ function has run. This uses the
internally generated C/C++ type, based on imtype. This is only useful
for a pointer type of some sort. If this is not defined nothing will
be done.
</td>
</tr>
<tr>
<td>goargout</td>
<td>
Go code to adjust an argument value when returning from a function.
This is called after the real C/C++ function has run. The value will
be in imtype. This is only useful for a pointer type of some sort.
If this is not defined, or is the empty string, nothing will be done.
</td>
</tr>
<tr>
<td>directorin</td>
<td>
C/C++ code to convert the C/C++ type used to call a director method
into the internally generated C/C++ type, based on imtype, that will
be passed to Go. If this is not defined the value will simply be cast
to the desired type.
</td>
</tr>
<tr>
<td>godirectorin</td>
<td>
Go code to convert a value used to call a director method from imtype
to gotype. If this is not defined, or is the empty string, no
conversion is done.
</td>
</tr>
<tr>
<td>godirectorout</td>
<td>
Go code to convert a value returned from a director method from gotype
to imtype. If this is not defined, or is the empty string, no
conversion is done.
</td>
</tr>
<tr>
<td>directorout</td>
<td>
C/C++ code to convert a value returned from a director method from the
internally generated C/C++ type, based on imtype, into the type that
the method should return If this is not defined the value will simply
be cast to the desired type.
</td>
</tr>
</table>
</body>
</html>
|
