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
|
<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<meta name="generator" content="Docutils 0.3.6: http://docutils.sourceforge.net/" />
<title>Iterator Facade</title>
<meta name="author" content="David Abrahams, Jeremy Siek, Thomas Witt" />
<meta name="organization" content="Boost Consulting, Indiana University Open Systems Lab, University of Hanover Institute for Transport Railway Operation and Construction" />
<meta name="date" content="2004-11-01" />
<meta name="copyright" content="Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003." />
<link rel="stylesheet" href="default.css" type="text/css" />
</head>
<body>
<h1 class="title">Iterator Facade</h1>
<table class="docinfo" frame="void" rules="none">
<col class="docinfo-name" />
<col class="docinfo-content" />
<tbody valign="top">
<tr><th class="docinfo-name">Author:</th>
<td>David Abrahams, Jeremy Siek, Thomas Witt</td></tr>
<tr><th class="docinfo-name">Contact:</th>
<td><a class="first reference" href="mailto:dave@boost-consulting.com">dave@boost-consulting.com</a>, <a class="reference" href="mailto:jsiek@osl.iu.edu">jsiek@osl.iu.edu</a>, <a class="last reference" href="mailto:witt@ive.uni-hannover.de">witt@ive.uni-hannover.de</a></td></tr>
<tr><th class="docinfo-name">Organization:</th>
<td><a class="first reference" href="http://www.boost-consulting.com">Boost Consulting</a>, Indiana University <a class="reference" href="http://www.osl.iu.edu">Open Systems
Lab</a>, University of Hanover <a class="last reference" href="http://www.ive.uni-hannover.de">Institute for Transport
Railway Operation and Construction</a></td></tr>
<tr><th class="docinfo-name">Date:</th>
<td>2004-11-01</td></tr>
<tr><th class="docinfo-name">Copyright:</th>
<td>Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003.</td></tr>
</tbody>
</table>
<div class="document" id="iterator-facade">
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">abstract:</th><td class="field-body"><tt class="literal"><span class="pre">iterator_facade</span></tt> is a base class template that implements the
interface of standard iterators in terms of a few core functions
and associated types, to be supplied by a derived iterator class.</td>
</tr>
</tbody>
</table>
<div class="contents topic" id="table-of-contents">
<p class="topic-title first"><a name="table-of-contents">Table of Contents</a></p>
<ul class="simple">
<li><a class="reference" href="#overview" id="id23" name="id23">Overview</a><ul>
<li><a class="reference" href="#usage" id="id24" name="id24">Usage</a></li>
<li><a class="reference" href="#iterator-core-access" id="id25" name="id25">Iterator Core Access</a></li>
<li><a class="reference" href="#operator" id="id26" name="id26"><tt class="literal"><span class="pre">operator[]</span></tt></a></li>
<li><a class="reference" href="#id2" id="id27" name="id27"><tt class="literal"><span class="pre">operator-></span></tt></a></li>
</ul>
</li>
<li><a class="reference" href="#reference" id="id28" name="id28">Reference</a><ul>
<li><a class="reference" href="#iterator-facade-requirements" id="id29" name="id29"><tt class="literal"><span class="pre">iterator_facade</span></tt> Requirements</a></li>
<li><a class="reference" href="#iterator-facade-operations" id="id30" name="id30"><tt class="literal"><span class="pre">iterator_facade</span></tt> operations</a></li>
</ul>
</li>
<li><a class="reference" href="#tutorial-example" id="id31" name="id31">Tutorial Example</a><ul>
<li><a class="reference" href="#the-problem" id="id32" name="id32">The Problem</a></li>
<li><a class="reference" href="#a-basic-iterator-using-iterator-facade" id="id33" name="id33">A Basic Iterator Using <tt class="literal"><span class="pre">iterator_facade</span></tt></a><ul>
<li><a class="reference" href="#template-arguments-for-iterator-facade" id="id34" name="id34">Template Arguments for <tt class="literal"><span class="pre">iterator_facade</span></tt></a><ul>
<li><a class="reference" href="#derived" id="id35" name="id35"><tt class="literal"><span class="pre">Derived</span></tt></a></li>
<li><a class="reference" href="#value" id="id36" name="id36"><tt class="literal"><span class="pre">Value</span></tt></a></li>
<li><a class="reference" href="#categoryortraversal" id="id37" name="id37"><tt class="literal"><span class="pre">CategoryOrTraversal</span></tt></a></li>
<li><a class="reference" href="#id12" id="id38" name="id38"><tt class="literal"><span class="pre">Reference</span></tt></a></li>
<li><a class="reference" href="#difference" id="id39" name="id39"><tt class="literal"><span class="pre">Difference</span></tt></a></li>
</ul>
</li>
<li><a class="reference" href="#constructors-and-data-members" id="id40" name="id40">Constructors and Data Members</a></li>
<li><a class="reference" href="#implementing-the-core-operations" id="id41" name="id41">Implementing the Core Operations</a></li>
</ul>
</li>
<li><a class="reference" href="#a-constant-node-iterator" id="id42" name="id42">A constant <tt class="literal"><span class="pre">node_iterator</span></tt></a></li>
<li><a class="reference" href="#interoperability" id="id43" name="id43">Interoperability</a></li>
<li><a class="reference" href="#telling-the-truth" id="id44" name="id44">Telling the Truth</a></li>
<li><a class="reference" href="#wrap-up" id="id45" name="id45">Wrap Up</a></li>
</ul>
</li>
</ul>
</div>
<div class="section" id="overview">
<h1><a class="toc-backref" href="#id23" name="overview">Overview</a></h1>
<!-- Version 1.1 of this ReStructuredText document corresponds to
n1530_, the paper accepted by the LWG for TR1. -->
<!-- Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003. -->
<p>While the iterator interface is rich, there is a core subset of the
interface that is necessary for all the functionality. We have
identified the following core behaviors for iterators:</p>
<ul class="simple">
<li>dereferencing</li>
<li>incrementing</li>
<li>decrementing</li>
<li>equality comparison</li>
<li>random-access motion</li>
<li>distance measurement</li>
</ul>
<p>In addition to the behaviors listed above, the core interface elements
include the associated types exposed through iterator traits:
<tt class="literal"><span class="pre">value_type</span></tt>, <tt class="literal"><span class="pre">reference</span></tt>, <tt class="literal"><span class="pre">difference_type</span></tt>, and
<tt class="literal"><span class="pre">iterator_category</span></tt>.</p>
<p>Iterator facade uses the Curiously Recurring Template
Pattern (CRTP) <a class="citation-reference" href="#cop95" id="id1" name="id1">[Cop95]</a> so that the user can specify the behavior
of <tt class="literal"><span class="pre">iterator_facade</span></tt> in a derived class. Former designs used
policy objects to specify the behavior, but that approach was
discarded for several reasons:</p>
<blockquote>
<ol class="arabic simple">
<li>the creation and eventual copying of the policy object may create
overhead that can be avoided with the current approach.</li>
<li>The policy object approach does not allow for custom constructors
on the created iterator types, an essential feature if
<tt class="literal"><span class="pre">iterator_facade</span></tt> should be used in other library
implementations.</li>
<li>Without the use of CRTP, the standard requirement that an
iterator's <tt class="literal"><span class="pre">operator++</span></tt> returns the iterator type itself
would mean that all iterators built with the library would
have to be specializations of <tt class="literal"><span class="pre">iterator_facade<...></span></tt>, rather
than something more descriptive like
<tt class="literal"><span class="pre">indirect_iterator<T*></span></tt>. Cumbersome type generator
metafunctions would be needed to build new parameterized
iterators, and a separate <tt class="literal"><span class="pre">iterator_adaptor</span></tt> layer would be
impossible.</li>
</ol>
</blockquote>
<div class="section" id="usage">
<h2><a class="toc-backref" href="#id24" name="usage">Usage</a></h2>
<p>The user of <tt class="literal"><span class="pre">iterator_facade</span></tt> derives his iterator class from a
specialization of <tt class="literal"><span class="pre">iterator_facade</span></tt> and passes the derived
iterator class as <tt class="literal"><span class="pre">iterator_facade</span></tt>'s first template parameter.
The order of the other template parameters have been carefully
chosen to take advantage of useful defaults. For example, when
defining a constant lvalue iterator, the user can pass a
const-qualified version of the iterator's <tt class="literal"><span class="pre">value_type</span></tt> as
<tt class="literal"><span class="pre">iterator_facade</span></tt>'s <tt class="literal"><span class="pre">Value</span></tt> parameter and omit the
<tt class="literal"><span class="pre">Reference</span></tt> parameter which follows.</p>
<p>The derived iterator class must define member functions implementing
the iterator's core behaviors. The following table describes
expressions which are required to be valid depending on the category
of the derived iterator type. These member functions are described
briefly below and in more detail in the iterator facade
requirements.</p>
<blockquote>
<table border="1" class="table">
<colgroup>
<col width="44%" />
<col width="56%" />
</colgroup>
<thead valign="bottom">
<tr><th>Expression</th>
<th>Effects</th>
</tr>
</thead>
<tbody valign="top">
<tr><td><tt class="literal"><span class="pre">i.dereference()</span></tt></td>
<td>Access the value referred to</td>
</tr>
<tr><td><tt class="literal"><span class="pre">i.equal(j)</span></tt></td>
<td>Compare for equality with <tt class="literal"><span class="pre">j</span></tt></td>
</tr>
<tr><td><tt class="literal"><span class="pre">i.increment()</span></tt></td>
<td>Advance by one position</td>
</tr>
<tr><td><tt class="literal"><span class="pre">i.decrement()</span></tt></td>
<td>Retreat by one position</td>
</tr>
<tr><td><tt class="literal"><span class="pre">i.advance(n)</span></tt></td>
<td>Advance by <tt class="literal"><span class="pre">n</span></tt> positions</td>
</tr>
<tr><td><tt class="literal"><span class="pre">i.distance_to(j)</span></tt></td>
<td>Measure the distance to <tt class="literal"><span class="pre">j</span></tt></td>
</tr>
</tbody>
</table>
</blockquote>
<!-- Should we add a comment that a zero overhead implementation of iterator_facade
is possible with proper inlining? -->
<p>In addition to implementing the core interface functions, an iterator
derived from <tt class="literal"><span class="pre">iterator_facade</span></tt> typically defines several
constructors. To model any of the standard iterator concepts, the
iterator must at least have a copy constructor. Also, if the iterator
type <tt class="literal"><span class="pre">X</span></tt> is meant to be automatically interoperate with another
iterator type <tt class="literal"><span class="pre">Y</span></tt> (as with constant and mutable iterators) then
there must be an implicit conversion from <tt class="literal"><span class="pre">X</span></tt> to <tt class="literal"><span class="pre">Y</span></tt> or from <tt class="literal"><span class="pre">Y</span></tt>
to <tt class="literal"><span class="pre">X</span></tt> (but not both), typically implemented as a conversion
constructor. Finally, if the iterator is to model Forward Traversal
Iterator or a more-refined iterator concept, a default constructor is
required.</p>
</div>
<div class="section" id="iterator-core-access">
<h2><a class="toc-backref" href="#id25" name="iterator-core-access">Iterator Core Access</a></h2>
<p><tt class="literal"><span class="pre">iterator_facade</span></tt> and the operator implementations need to be able
to access the core member functions in the derived class. Making the
core member functions public would expose an implementation detail to
the user. The design used here ensures that implementation details do
not appear in the public interface of the derived iterator type.</p>
<p>Preventing direct access to the core member functions has two
advantages. First, there is no possibility for the user to accidently
use a member function of the iterator when a member of the value_type
was intended. This has been an issue with smart pointer
implementations in the past. The second and main advantage is that
library implementers can freely exchange a hand-rolled iterator
implementation for one based on <tt class="literal"><span class="pre">iterator_facade</span></tt> without fear of
breaking code that was accessing the public core member functions
directly.</p>
<p>In a naive implementation, keeping the derived class' core member
functions private would require it to grant friendship to
<tt class="literal"><span class="pre">iterator_facade</span></tt> and each of the seven operators. In order to
reduce the burden of limiting access, <tt class="literal"><span class="pre">iterator_core_access</span></tt> is
provided, a class that acts as a gateway to the core member functions
in the derived iterator class. The author of the derived class only
needs to grant friendship to <tt class="literal"><span class="pre">iterator_core_access</span></tt> to make his core
member functions available to the library.</p>
<!-- This is no long uptodate -thw -->
<!-- Yes it is; I made sure of it! -DWA -->
<p><tt class="literal"><span class="pre">iterator_core_access</span></tt> will be typically implemented as an empty
class containing only private static member functions which invoke the
iterator core member functions. There is, however, no need to
standardize the gateway protocol. Note that even if
<tt class="literal"><span class="pre">iterator_core_access</span></tt> used public member functions it would not
open a safety loophole, as every core member function preserves the
invariants of the iterator.</p>
</div>
<div class="section" id="operator">
<h2><a class="toc-backref" href="#id26" name="operator"><tt class="literal"><span class="pre">operator[]</span></tt></a></h2>
<p>The indexing operator for a generalized iterator presents special
challenges. A random access iterator's <tt class="literal"><span class="pre">operator[]</span></tt> is only
required to return something convertible to its <tt class="literal"><span class="pre">value_type</span></tt>.
Requiring that it return an lvalue would rule out currently-legal
random-access iterators which hold the referenced value in a data
member (e.g. <a class="reference" href="counting_iterator.html"><tt class="literal"><span class="pre">counting_iterator</span></tt></a>), because <tt class="literal"><span class="pre">*(p+n)</span></tt> is a reference
into the temporary iterator <tt class="literal"><span class="pre">p+n</span></tt>, which is destroyed when
<tt class="literal"><span class="pre">operator[]</span></tt> returns.</p>
<p>Writable iterators built with <tt class="literal"><span class="pre">iterator_facade</span></tt> implement the
semantics required by the preferred resolution to <a class="reference" href="http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/lwg-active.html#299">issue 299</a> and
adopted by proposal <a class="reference" href="http://anubis.dkuug.dk/JTC1/SC22/WG21/docs/papers/2003/n1550.html">n1550</a>: the result of <tt class="literal"><span class="pre">p[n]</span></tt> is an object
convertible to the iterator's <tt class="literal"><span class="pre">value_type</span></tt>, and <tt class="literal"><span class="pre">p[n]</span> <span class="pre">=</span> <span class="pre">x</span></tt> is
equivalent to <tt class="literal"><span class="pre">*(p</span> <span class="pre">+</span> <span class="pre">n)</span> <span class="pre">=</span> <span class="pre">x</span></tt> (Note: This result object may be
implemented as a proxy containing a copy of <tt class="literal"><span class="pre">p+n</span></tt>). This approach
will work properly for any random-access iterator regardless of the
other details of its implementation. A user who knows more about
the implementation of her iterator is free to implement an
<tt class="literal"><span class="pre">operator[]</span></tt> that returns an lvalue in the derived iterator
class; it will hide the one supplied by <tt class="literal"><span class="pre">iterator_facade</span></tt> from
clients of her iterator.</p>
<a class="target" id="operator-arrow" name="operator-arrow"></a></div>
<div class="section" id="id2">
<h2><a class="toc-backref" href="#id27" name="id2"><tt class="literal"><span class="pre">operator-></span></tt></a></h2>
<p>The <tt class="literal"><span class="pre">reference</span></tt> type of a readable iterator (and today's input
iterator) need not in fact be a reference, so long as it is
convertible to the iterator's <tt class="literal"><span class="pre">value_type</span></tt>. When the <tt class="literal"><span class="pre">value_type</span></tt>
is a class, however, it must still be possible to access members
through <tt class="literal"><span class="pre">operator-></span></tt>. Therefore, an iterator whose <tt class="literal"><span class="pre">reference</span></tt>
type is not in fact a reference must return a proxy containing a copy
of the referenced value from its <tt class="literal"><span class="pre">operator-></span></tt>.</p>
<p>The return types for <tt class="literal"><span class="pre">iterator_facade</span></tt>'s <tt class="literal"><span class="pre">operator-></span></tt> and
<tt class="literal"><span class="pre">operator[]</span></tt> are not explicitly specified. Instead, those types
are described in terms of a set of requirements, which must be
satisfied by the <tt class="literal"><span class="pre">iterator_facade</span></tt> implementation.</p>
<table class="citation" frame="void" id="cop95" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a name="cop95">[Cop95]</a></td><td><em>(<a class="fn-backref" href="#id1">1</a>, <a class="fn-backref" href="#id10">2</a>)</em> [Coplien, 1995] Coplien, J., Curiously Recurring Template
Patterns, C++ Report, February 1995, pp. 24-27.</td></tr>
</tbody>
</table>
</div>
</div>
<div class="section" id="reference">
<h1><a class="toc-backref" href="#id28" name="reference">Reference</a></h1>
<!-- Version 1.3 of this ReStructuredText document corresponds to
n1530_, the paper accepted by the LWG for TR1. -->
<!-- Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003. -->
<pre class="literal-block">
template <
class Derived
, class Value
, class CategoryOrTraversal
, class Reference = Value&
, class Difference = ptrdiff_t
>
class iterator_facade {
public:
typedef remove_const<Value>::type value_type;
typedef Reference reference;
typedef Value* pointer;
typedef Difference difference_type;
typedef /* see <a class="reference" href="#iterator-category">below</a> */ iterator_category;
reference operator*() const;
/* see <a class="reference" href="#operator-arrow">below</a> */ operator->() const;
/* see <a class="reference" href="#brackets">below</a> */ operator[](difference_type n) const;
Derived& operator++();
Derived operator++(int);
Derived& operator--();
Derived operator--(int);
Derived& operator+=(difference_type n);
Derived& operator-=(difference_type n);
Derived operator-(difference_type n) const;
protected:
typedef iterator_facade iterator_facade_;
};
// Comparison operators
template <class Dr1, class V1, class TC1, class R1, class D1,
class Dr2, class V2, class TC2, class R2, class D2>
typename enable_if_interoperable<Dr1,Dr2,bool>::type // exposition
operator ==(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);
template <class Dr1, class V1, class TC1, class R1, class D1,
class Dr2, class V2, class TC2, class R2, class D2>
typename enable_if_interoperable<Dr1,Dr2,bool>::type
operator !=(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);
template <class Dr1, class V1, class TC1, class R1, class D1,
class Dr2, class V2, class TC2, class R2, class D2>
typename enable_if_interoperable<Dr1,Dr2,bool>::type
operator <(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);
template <class Dr1, class V1, class TC1, class R1, class D1,
class Dr2, class V2, class TC2, class R2, class D2>
typename enable_if_interoperable<Dr1,Dr2,bool>::type
operator <=(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);
template <class Dr1, class V1, class TC1, class R1, class D1,
class Dr2, class V2, class TC2, class R2, class D2>
typename enable_if_interoperable<Dr1,Dr2,bool>::type
operator >(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);
template <class Dr1, class V1, class TC1, class R1, class D1,
class Dr2, class V2, class TC2, class R2, class D2>
typename enable_if_interoperable<Dr1,Dr2,bool>::type
operator >=(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);
// Iterator difference
template <class Dr1, class V1, class TC1, class R1, class D1,
class Dr2, class V2, class TC2, class R2, class D2>
/* see <a class="reference" href="#minus">below</a> */
operator-(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);
// Iterator addition
template <class Dr, class V, class TC, class R, class D>
Derived operator+ (iterator_facade<Dr,V,TC,R,D> const&,
typename Derived::difference_type n);
template <class Dr, class V, class TC, class R, class D>
Derived operator+ (typename Derived::difference_type n,
iterator_facade<Dr,V,TC,R,D> const&);
</pre>
<a class="target" id="iterator-category" name="iterator-category"></a><p>The <tt class="literal"><span class="pre">iterator_category</span></tt> member of <tt class="literal"><span class="pre">iterator_facade</span></tt> is</p>
<pre class="literal-block">
<em>iterator-category</em>(CategoryOrTraversal, value_type, reference)
</pre>
<p>where <em>iterator-category</em> is defined as follows:</p>
<a class="target" id="id7" name="id7"></a><pre class="literal-block">
<em>iterator-category</em>(C,R,V) :=
if (C is convertible to std::input_iterator_tag
|| C is convertible to std::output_iterator_tag
)
return C
else if (C is not convertible to incrementable_traversal_tag)
<em>the program is ill-formed</em>
else return a type X satisfying the following two constraints:
1. X is convertible to X1, and not to any more-derived
type, where X1 is defined by:
if (R is a reference type
&& C is convertible to forward_traversal_tag)
{
if (C is convertible to random_access_traversal_tag)
X1 = random_access_iterator_tag
else if (C is convertible to bidirectional_traversal_tag)
X1 = bidirectional_iterator_tag
else
X1 = forward_iterator_tag
}
else
{
if (C is convertible to single_pass_traversal_tag
&& R is convertible to V)
X1 = input_iterator_tag
else
X1 = C
}
2. <a class="reference" href="new-iter-concepts.html#category-to-traversal"><em>category-to-traversal</em></a>(X) is convertible to the most
derived traversal tag type to which X is also
convertible, and not to any more-derived traversal tag
type.
</pre>
<p>[Note: the intention is to allow <tt class="literal"><span class="pre">iterator_category</span></tt> to be one of
the five original category tags when convertibility to one of the
traversal tags would add no information]</p>
<!-- Copyright David Abrahams 2004. Use, modification and distribution is -->
<!-- subject to the Boost Software License, Version 1.0. (See accompanying -->
<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
<p>The <tt class="literal"><span class="pre">enable_if_interoperable</span></tt> template used above is for exposition
purposes. The member operators should only be in an overload set
provided the derived types <tt class="literal"><span class="pre">Dr1</span></tt> and <tt class="literal"><span class="pre">Dr2</span></tt> are interoperable,
meaning that at least one of the types is convertible to the other. The
<tt class="literal"><span class="pre">enable_if_interoperable</span></tt> approach uses SFINAE to take the operators
out of the overload set when the types are not interoperable.
The operators should behave <em>as-if</em> <tt class="literal"><span class="pre">enable_if_interoperable</span></tt>
were defined to be:</p>
<pre class="literal-block">
template <bool, typename> enable_if_interoperable_impl
{};
template <typename T> enable_if_interoperable_impl<true,T>
{ typedef T type; };
template<typename Dr1, typename Dr2, typename T>
struct enable_if_interoperable
: enable_if_interoperable_impl<
is_convertible<Dr1,Dr2>::value || is_convertible<Dr2,Dr1>::value
, T
>
{};
</pre>
<div class="section" id="iterator-facade-requirements">
<h2><a class="toc-backref" href="#id29" name="iterator-facade-requirements"><tt class="literal"><span class="pre">iterator_facade</span></tt> Requirements</a></h2>
<p>The following table describes the typical valid expressions on
<tt class="literal"><span class="pre">iterator_facade</span></tt>'s <tt class="literal"><span class="pre">Derived</span></tt> parameter, depending on the
iterator concept(s) it will model. The operations in the first
column must be made accessible to member functions of class
<tt class="literal"><span class="pre">iterator_core_access</span></tt>. In addition,
<tt class="literal"><span class="pre">static_cast<Derived*>(iterator_facade*)</span></tt> shall be well-formed.</p>
<p>In the table below, <tt class="literal"><span class="pre">F</span></tt> is <tt class="literal"><span class="pre">iterator_facade<X,V,C,R,D></span></tt>, <tt class="literal"><span class="pre">a</span></tt> is an
object of type <tt class="literal"><span class="pre">X</span></tt>, <tt class="literal"><span class="pre">b</span></tt> and <tt class="literal"><span class="pre">c</span></tt> are objects of type <tt class="literal"><span class="pre">const</span> <span class="pre">X</span></tt>,
<tt class="literal"><span class="pre">n</span></tt> is an object of <tt class="literal"><span class="pre">F::difference_type</span></tt>, <tt class="literal"><span class="pre">y</span></tt> is a constant
object of a single pass iterator type interoperable with <tt class="literal"><span class="pre">X</span></tt>, and <tt class="literal"><span class="pre">z</span></tt>
is a constant object of a random access traversal iterator type
interoperable with <tt class="literal"><span class="pre">X</span></tt>.</p>
<a class="target" id="core-operations" name="core-operations"></a><div class="topic">
<p class="topic-title first"><tt class="literal"><span class="pre">iterator_facade</span></tt> Core Operations</p>
<table border="1" class="table">
<colgroup>
<col width="21%" />
<col width="23%" />
<col width="27%" />
<col width="29%" />
</colgroup>
<thead valign="bottom">
<tr><th>Expression</th>
<th>Return Type</th>
<th>Assertion/Note</th>
<th>Used to implement Iterator
Concept(s)</th>
</tr>
</thead>
<tbody valign="top">
<tr><td><tt class="literal"><span class="pre">c.dereference()</span></tt></td>
<td><tt class="literal"><span class="pre">F::reference</span></tt></td>
<td> </td>
<td>Readable Iterator, Writable
Iterator</td>
</tr>
<tr><td><tt class="literal"><span class="pre">c.equal(y)</span></tt></td>
<td>convertible to bool</td>
<td>true iff <tt class="literal"><span class="pre">c</span></tt> and <tt class="literal"><span class="pre">y</span></tt>
refer to the same
position.</td>
<td>Single Pass Iterator</td>
</tr>
<tr><td><tt class="literal"><span class="pre">a.increment()</span></tt></td>
<td>unused</td>
<td> </td>
<td>Incrementable Iterator</td>
</tr>
<tr><td><tt class="literal"><span class="pre">a.decrement()</span></tt></td>
<td>unused</td>
<td> </td>
<td>Bidirectional Traversal
Iterator</td>
</tr>
<tr><td><tt class="literal"><span class="pre">a.advance(n)</span></tt></td>
<td>unused</td>
<td> </td>
<td>Random Access Traversal
Iterator</td>
</tr>
<tr><td><tt class="literal"><span class="pre">c.distance_to(z)</span></tt></td>
<td>convertible to
<tt class="literal"><span class="pre">F::difference_type</span></tt></td>
<td>equivalent to
<tt class="literal"><span class="pre">distance(c,</span> <span class="pre">X(z))</span></tt>.</td>
<td>Random Access Traversal
Iterator</td>
</tr>
</tbody>
</table>
</div>
</div>
<div class="section" id="iterator-facade-operations">
<h2><a class="toc-backref" href="#id30" name="iterator-facade-operations"><tt class="literal"><span class="pre">iterator_facade</span></tt> operations</a></h2>
<p>The operations in this section are described in terms of operations on
the core interface of <tt class="literal"><span class="pre">Derived</span></tt> which may be inaccessible
(i.e. private). The implementation should access these operations
through member functions of class <tt class="literal"><span class="pre">iterator_core_access</span></tt>.</p>
<p><tt class="literal"><span class="pre">reference</span> <span class="pre">operator*()</span> <span class="pre">const;</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><tt class="literal"><span class="pre">static_cast<Derived</span> <span class="pre">const*>(this)->dereference()</span></tt></td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">operator->()</span> <span class="pre">const;</span></tt> (see <a class="reference" href="#operator-arrow">below</a>)</p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><p class="first">If <tt class="literal"><span class="pre">reference</span></tt> is a reference type, an object
of type <tt class="literal"><span class="pre">pointer</span></tt> equal to:</p>
<pre class="literal-block">
&static_cast<Derived const*>(this)->dereference()
</pre>
<p class="last">Otherwise returns an object of unspecified type such that,
<tt class="literal"><span class="pre">(*static_cast<Derived</span> <span class="pre">const*>(this))->m</span></tt> is equivalent to <tt class="literal"><span class="pre">(w</span> <span class="pre">=</span> <span class="pre">**static_cast<Derived</span> <span class="pre">const*>(this),</span>
<span class="pre">w.m)</span></tt> for some temporary object <tt class="literal"><span class="pre">w</span></tt> of type <tt class="literal"><span class="pre">value_type</span></tt>.</p>
</td>
</tr>
</tbody>
</table>
<a class="target" id="brackets" name="brackets"></a><p><em>unspecified</em> <tt class="literal"><span class="pre">operator[](difference_type</span> <span class="pre">n)</span> <span class="pre">const;</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body">an object convertible to <tt class="literal"><span class="pre">value_type</span></tt>. For constant
objects <tt class="literal"><span class="pre">v</span></tt> of type <tt class="literal"><span class="pre">value_type</span></tt>, and <tt class="literal"><span class="pre">n</span></tt> of type
<tt class="literal"><span class="pre">difference_type</span></tt>, <tt class="literal"><span class="pre">(*this)[n]</span> <span class="pre">=</span> <span class="pre">v</span></tt> is equivalent to
<tt class="literal"><span class="pre">*(*this</span> <span class="pre">+</span> <span class="pre">n)</span> <span class="pre">=</span> <span class="pre">v</span></tt>, and <tt class="literal"><span class="pre">static_cast<value_type</span>
<span class="pre">const&>((*this)[n])</span></tt> is equivalent to
<tt class="literal"><span class="pre">static_cast<value_type</span> <span class="pre">const&>(*(*this</span> <span class="pre">+</span> <span class="pre">n))</span></tt></td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">Derived&</span> <span class="pre">operator++();</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Effects:</th><td class="field-body"><pre class="first last literal-block">
static_cast<Derived*>(this)->increment();
return *static_cast<Derived*>(this);
</pre>
</td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">Derived</span> <span class="pre">operator++(int);</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Effects:</th><td class="field-body"><pre class="first last literal-block">
Derived tmp(static_cast<Derived const*>(this));
++*this;
return tmp;
</pre>
</td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">Derived&</span> <span class="pre">operator--();</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Effects:</th><td class="field-body"><pre class="first last literal-block">
static_cast<Derived*>(this)->decrement();
return *static_cast<Derived*>(this);
</pre>
</td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">Derived</span> <span class="pre">operator--(int);</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Effects:</th><td class="field-body"><pre class="first last literal-block">
Derived tmp(static_cast<Derived const*>(this));
--*this;
return tmp;
</pre>
</td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">Derived&</span> <span class="pre">operator+=(difference_type</span> <span class="pre">n);</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Effects:</th><td class="field-body"><pre class="first last literal-block">
static_cast<Derived*>(this)->advance(n);
return *static_cast<Derived*>(this);
</pre>
</td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">Derived&</span> <span class="pre">operator-=(difference_type</span> <span class="pre">n);</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Effects:</th><td class="field-body"><pre class="first last literal-block">
static_cast<Derived*>(this)->advance(-n);
return *static_cast<Derived*>(this);
</pre>
</td>
</tr>
</tbody>
</table>
<p><tt class="literal"><span class="pre">Derived</span> <span class="pre">operator-(difference_type</span> <span class="pre">n)</span> <span class="pre">const;</span></tt></p>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Effects:</th><td class="field-body"><pre class="first last literal-block">
Derived tmp(static_cast<Derived const*>(this));
return tmp -= n;
</pre>
</td>
</tr>
</tbody>
</table>
<pre class="literal-block">
template <class Dr, class V, class TC, class R, class D>
Derived operator+ (iterator_facade<Dr,V,TC,R,D> const&,
typename Derived::difference_type n);
template <class Dr, class V, class TC, class R, class D>
Derived operator+ (typename Derived::difference_type n,
iterator_facade<Dr,V,TC,R,D> const&);
</pre>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Effects:</th><td class="field-body"><pre class="first last literal-block">
Derived tmp(static_cast<Derived const*>(this));
return tmp += n;
</pre>
</td>
</tr>
</tbody>
</table>
<pre class="literal-block">
template <class Dr1, class V1, class TC1, class R1, class D1,
class Dr2, class V2, class TC2, class R2, class D2>
typename enable_if_interoperable<Dr1,Dr2,bool>::type
operator ==(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);
</pre>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><p class="first">if <tt class="literal"><span class="pre">is_convertible<Dr2,Dr1>::value</span></tt></p>
<dl class="last">
<dt>then </dt>
<dd><tt class="literal"><span class="pre">((Dr1</span> <span class="pre">const&)lhs).equal((Dr2</span> <span class="pre">const&)rhs)</span></tt>.</dd>
<dt>Otherwise, </dt>
<dd><tt class="literal"><span class="pre">((Dr2</span> <span class="pre">const&)rhs).equal((Dr1</span> <span class="pre">const&)lhs)</span></tt>.</dd>
</dl>
</td>
</tr>
</tbody>
</table>
<pre class="literal-block">
template <class Dr1, class V1, class TC1, class R1, class D1,
class Dr2, class V2, class TC2, class R2, class D2>
typename enable_if_interoperable<Dr1,Dr2,bool>::type
operator !=(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);
</pre>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><p class="first">if <tt class="literal"><span class="pre">is_convertible<Dr2,Dr1>::value</span></tt></p>
<dl class="last">
<dt>then </dt>
<dd><tt class="literal"><span class="pre">!((Dr1</span> <span class="pre">const&)lhs).equal((Dr2</span> <span class="pre">const&)rhs)</span></tt>.</dd>
<dt>Otherwise, </dt>
<dd><tt class="literal"><span class="pre">!((Dr2</span> <span class="pre">const&)rhs).equal((Dr1</span> <span class="pre">const&)lhs)</span></tt>.</dd>
</dl>
</td>
</tr>
</tbody>
</table>
<pre class="literal-block">
template <class Dr1, class V1, class TC1, class R1, class D1,
class Dr2, class V2, class TC2, class R2, class D2>
typename enable_if_interoperable<Dr1,Dr2,bool>::type
operator <(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);
</pre>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><p class="first">if <tt class="literal"><span class="pre">is_convertible<Dr2,Dr1>::value</span></tt></p>
<dl class="last">
<dt>then </dt>
<dd><tt class="literal"><span class="pre">((Dr1</span> <span class="pre">const&)lhs).distance_to((Dr2</span> <span class="pre">const&)rhs)</span> <span class="pre"><</span> <span class="pre">0</span></tt>.</dd>
<dt>Otherwise, </dt>
<dd><tt class="literal"><span class="pre">((Dr2</span> <span class="pre">const&)rhs).distance_to((Dr1</span> <span class="pre">const&)lhs)</span> <span class="pre">></span> <span class="pre">0</span></tt>.</dd>
</dl>
</td>
</tr>
</tbody>
</table>
<pre class="literal-block">
template <class Dr1, class V1, class TC1, class R1, class D1,
class Dr2, class V2, class TC2, class R2, class D2>
typename enable_if_interoperable<Dr1,Dr2,bool>::type
operator <=(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);
</pre>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><p class="first">if <tt class="literal"><span class="pre">is_convertible<Dr2,Dr1>::value</span></tt></p>
<dl class="last">
<dt>then </dt>
<dd><tt class="literal"><span class="pre">((Dr1</span> <span class="pre">const&)lhs).distance_to((Dr2</span> <span class="pre">const&)rhs)</span> <span class="pre"><=</span> <span class="pre">0</span></tt>.</dd>
<dt>Otherwise, </dt>
<dd><tt class="literal"><span class="pre">((Dr2</span> <span class="pre">const&)rhs).distance_to((Dr1</span> <span class="pre">const&)lhs)</span> <span class="pre">>=</span> <span class="pre">0</span></tt>.</dd>
</dl>
</td>
</tr>
</tbody>
</table>
<pre class="literal-block">
template <class Dr1, class V1, class TC1, class R1, class D1,
class Dr2, class V2, class TC2, class R2, class D2>
typename enable_if_interoperable<Dr1,Dr2,bool>::type
operator >(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);
</pre>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><p class="first">if <tt class="literal"><span class="pre">is_convertible<Dr2,Dr1>::value</span></tt></p>
<dl class="last">
<dt>then </dt>
<dd><tt class="literal"><span class="pre">((Dr1</span> <span class="pre">const&)lhs).distance_to((Dr2</span> <span class="pre">const&)rhs)</span> <span class="pre">></span> <span class="pre">0</span></tt>.</dd>
<dt>Otherwise, </dt>
<dd><tt class="literal"><span class="pre">((Dr2</span> <span class="pre">const&)rhs).distance_to((Dr1</span> <span class="pre">const&)lhs)</span> <span class="pre"><</span> <span class="pre">0</span></tt>.</dd>
</dl>
</td>
</tr>
</tbody>
</table>
<pre class="literal-block">
template <class Dr1, class V1, class TC1, class R1, class D1,
class Dr2, class V2, class TC2, class R2, class D2>
typename enable_if_interoperable<Dr1,Dr2,bool>::type
operator >=(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);
</pre>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><p class="first">if <tt class="literal"><span class="pre">is_convertible<Dr2,Dr1>::value</span></tt></p>
<dl class="last">
<dt>then </dt>
<dd><tt class="literal"><span class="pre">((Dr1</span> <span class="pre">const&)lhs).distance_to((Dr2</span> <span class="pre">const&)rhs)</span> <span class="pre">>=</span> <span class="pre">0</span></tt>.</dd>
<dt>Otherwise, </dt>
<dd><tt class="literal"><span class="pre">((Dr2</span> <span class="pre">const&)rhs).distance_to((Dr1</span> <span class="pre">const&)lhs)</span> <span class="pre"><=</span> <span class="pre">0</span></tt>.</dd>
</dl>
</td>
</tr>
</tbody>
</table>
<a class="target" id="minus" name="minus"></a><pre class="literal-block">
template <class Dr1, class V1, class TC1, class R1, class D1,
class Dr2, class V2, class TC2, class R2, class D2>
typename enable_if_interoperable<Dr1,Dr2,difference>::type
operator -(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);
</pre>
<table class="field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Return Type:</th><td class="field-body"><p class="first">if <tt class="literal"><span class="pre">is_convertible<Dr2,Dr1>::value</span></tt></p>
<blockquote class="last">
<dl>
<dt>then </dt>
<dd><tt class="literal"><span class="pre">difference</span></tt> shall be
<tt class="literal"><span class="pre">iterator_traits<Dr1>::difference_type</span></tt>.</dd>
<dt>Otherwise </dt>
<dd><tt class="literal"><span class="pre">difference</span></tt> shall be <tt class="literal"><span class="pre">iterator_traits<Dr2>::difference_type</span></tt></dd>
</dl>
</blockquote>
</td>
</tr>
<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><p class="first">if <tt class="literal"><span class="pre">is_convertible<Dr2,Dr1>::value</span></tt></p>
<dl class="last">
<dt>then </dt>
<dd><tt class="literal"><span class="pre">-((Dr1</span> <span class="pre">const&)lhs).distance_to((Dr2</span> <span class="pre">const&)rhs)</span></tt>.</dd>
<dt>Otherwise, </dt>
<dd><tt class="literal"><span class="pre">((Dr2</span> <span class="pre">const&)rhs).distance_to((Dr1</span> <span class="pre">const&)lhs)</span></tt>.</dd>
</dl>
</td>
</tr>
</tbody>
</table>
</div>
</div>
<div class="section" id="tutorial-example">
<h1><a class="toc-backref" href="#id31" name="tutorial-example">Tutorial Example</a></h1>
<!-- Copyright David Abrahams 2004. Use, modification and distribution is -->
<!-- subject to the Boost Software License, Version 1.0. (See accompanying -->
<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
<p>In this section we'll walk through the implementation of a few
iterators using <tt class="literal"><span class="pre">iterator_facade</span></tt>, based around the simple
example of a linked list of polymorphic objects. This example was
inspired by a <a class="reference" href="http://thread.gmane.org/gmane.comp.lib.boost.user/5100">posting</a> by Keith Macdonald on the <a class="reference" href="../../../more/mailing_lists.htm#users">Boost-Users</a>
mailing list.</p>
<div class="section" id="the-problem">
<h2><a class="toc-backref" href="#id32" name="the-problem">The Problem</a></h2>
<p>Say we've written a polymorphic linked list node base class:</p>
<pre class="literal-block">
# include <iostream>
struct node_base
{
node_base() : m_next(0) {}
// Each node manages all of its tail nodes
virtual ~node_base() { delete m_next; }
// Access the rest of the list
node_base* next() const { return m_next; }
// print to the stream
virtual void print(std::ostream& s) const = 0;
// double the value
virtual void double_me() = 0;
void append(node_base* p)
{
if (m_next)
m_next->append(p);
else
m_next = p;
}
private:
node_base* m_next;
};
</pre>
<p>Lists can hold objects of different types by linking together
specializations of the following template:</p>
<pre class="literal-block">
template <class T>
struct node : node_base
{
node(T x)
: m_value(x)
{}
void print(std::ostream& s) const { s << this->m_value; }
void double_me() { m_value += m_value; }
private:
T m_value;
};
</pre>
<p>And we can print any node using the following streaming operator:</p>
<pre class="literal-block">
inline std::ostream& operator<<(std::ostream& s, node_base const& n)
{
n.print(s);
return s;
}
</pre>
<p>Our first challenge is to build an appropriate iterator over these
lists.</p>
</div>
<div class="section" id="a-basic-iterator-using-iterator-facade">
<h2><a class="toc-backref" href="#id33" name="a-basic-iterator-using-iterator-facade">A Basic Iterator Using <tt class="literal"><span class="pre">iterator_facade</span></tt></a></h2>
<p>We will construct a <tt class="literal"><span class="pre">node_iterator</span></tt> class using inheritance from
<tt class="literal"><span class="pre">iterator_facade</span></tt> to implement most of the iterator's operations.</p>
<pre class="literal-block">
# include "node.hpp"
# include <boost/iterator/iterator_facade.hpp>
class node_iterator
: public boost::iterator_facade<...>
{
...
};
</pre>
<div class="section" id="template-arguments-for-iterator-facade">
<h3><a class="toc-backref" href="#id34" name="template-arguments-for-iterator-facade">Template Arguments for <tt class="literal"><span class="pre">iterator_facade</span></tt></a></h3>
<p><tt class="literal"><span class="pre">iterator_facade</span></tt> has several template parameters, so we must decide
what types to use for the arguments. The parameters are <tt class="literal"><span class="pre">Derived</span></tt>,
<tt class="literal"><span class="pre">Value</span></tt>, <tt class="literal"><span class="pre">CategoryOrTraversal</span></tt>, <tt class="literal"><span class="pre">Reference</span></tt>, and <tt class="literal"><span class="pre">Difference</span></tt>.</p>
<div class="section" id="derived">
<h4><a class="toc-backref" href="#id35" name="derived"><tt class="literal"><span class="pre">Derived</span></tt></a></h4>
<p>Because <tt class="literal"><span class="pre">iterator_facade</span></tt> is meant to be used with the CRTP
<a class="citation-reference" href="#cop95" id="id10" name="id10">[Cop95]</a> the first parameter is the iterator class name itself,
<tt class="literal"><span class="pre">node_iterator</span></tt>.</p>
</div>
<div class="section" id="value">
<h4><a class="toc-backref" href="#id36" name="value"><tt class="literal"><span class="pre">Value</span></tt></a></h4>
<p>The <tt class="literal"><span class="pre">Value</span></tt> parameter determines the <tt class="literal"><span class="pre">node_iterator</span></tt>'s
<tt class="literal"><span class="pre">value_type</span></tt>. In this case, we are iterating over <tt class="literal"><span class="pre">node_base</span></tt>
objects, so <tt class="literal"><span class="pre">Value</span></tt> will be <tt class="literal"><span class="pre">node_base</span></tt>.</p>
</div>
<div class="section" id="categoryortraversal">
<h4><a class="toc-backref" href="#id37" name="categoryortraversal"><tt class="literal"><span class="pre">CategoryOrTraversal</span></tt></a></h4>
<p>Now we have to determine which <a class="reference" href="new-iter-concepts.html#iterator-traversal-concepts-lib-iterator-traversal">iterator traversal concept</a> our
<tt class="literal"><span class="pre">node_iterator</span></tt> is going to model. Singly-linked lists only have
forward links, so our iterator can't can't be a <a class="reference" href="new-iter-concepts.html#bidirectional-traversal-iterators-lib-bidirectional-traversal-iterators">bidirectional
traversal iterator</a>. Our iterator should be able to make multiple
passes over the same linked list (unlike, say, an
<tt class="literal"><span class="pre">istream_iterator</span></tt> which consumes the stream it traverses), so it
must be a <a class="reference" href="new-iter-concepts.html#forward-traversal-iterators-lib-forward-traversal-iterators">forward traversal iterator</a>. Therefore, we'll pass
<tt class="literal"><span class="pre">boost::forward_traversal_tag</span></tt> in this position <a class="footnote-reference" href="#category" id="id11" name="id11"><sup>1</sup></a>.</p>
<table class="footnote" frame="void" id="category" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id11" name="category">[1]</a></td><td><tt class="literal"><span class="pre">iterator_facade</span></tt> also supports old-style category
tags, so we could have passed <tt class="literal"><span class="pre">std::forward_iterator_tag</span></tt> here;
either way, the resulting iterator's <tt class="literal"><span class="pre">iterator_category</span></tt> will
end up being <tt class="literal"><span class="pre">std::forward_iterator_tag</span></tt>.</td></tr>
</tbody>
</table>
</div>
<div class="section" id="id12">
<h4><a class="toc-backref" href="#id38" name="id12"><tt class="literal"><span class="pre">Reference</span></tt></a></h4>
<p>The <tt class="literal"><span class="pre">Reference</span></tt> argument becomes the type returned by
<tt class="literal"><span class="pre">node_iterator</span></tt>'s dereference operation, and will also be the
same as <tt class="literal"><span class="pre">std::iterator_traits<node_iterator>::reference</span></tt>. The
library's default for this parameter is <tt class="literal"><span class="pre">Value&</span></tt>; since
<tt class="literal"><span class="pre">node_base&</span></tt> is a good choice for the iterator's <tt class="literal"><span class="pre">reference</span></tt>
type, we can omit this argument, or pass <tt class="literal"><span class="pre">use_default</span></tt>.</p>
</div>
<div class="section" id="difference">
<h4><a class="toc-backref" href="#id39" name="difference"><tt class="literal"><span class="pre">Difference</span></tt></a></h4>
<p>The <tt class="literal"><span class="pre">Difference</span></tt> argument determines how the distance between
two <tt class="literal"><span class="pre">node_iterator</span></tt>s will be measured and will also be the
same as <tt class="literal"><span class="pre">std::iterator_traits<node_iterator>::difference_type</span></tt>.
The library's default for <tt class="literal"><span class="pre">Difference</span></tt> is <tt class="literal"><span class="pre">std::ptrdiff_t</span></tt>, an
appropriate type for measuring the distance between any two
addresses in memory, and one that works for almost any iterator,
so we can omit this argument, too.</p>
<p>The declaration of <tt class="literal"><span class="pre">node_iterator</span></tt> will therefore look something
like:</p>
<pre class="literal-block">
# include "node.hpp"
# include <boost/iterator/iterator_facade.hpp>
class node_iterator
: public boost::iterator_facade<
node_iterator
, node_base
, boost::forward_traversal_tag
>
{
...
};
</pre>
</div>
</div>
<div class="section" id="constructors-and-data-members">
<h3><a class="toc-backref" href="#id40" name="constructors-and-data-members">Constructors and Data Members</a></h3>
<p>Next we need to decide how to represent the iterator's position.
This representation will take the form of data members, so we'll
also need to write constructors to initialize them. The
<tt class="literal"><span class="pre">node_iterator</span></tt>'s position is quite naturally represented using
a pointer to a <tt class="literal"><span class="pre">node_base</span></tt>. We'll need a constructor to build an
iterator from a <tt class="literal"><span class="pre">node_base*</span></tt>, and a default constructor to
satisfy the <a class="reference" href="new-iter-concepts.html#forward-traversal-iterators-lib-forward-traversal-iterators">forward traversal iterator</a> requirements <a class="footnote-reference" href="#default" id="id13" name="id13"><sup>2</sup></a>.
Our <tt class="literal"><span class="pre">node_iterator</span></tt> then becomes:</p>
<pre class="literal-block">
# include "node.hpp"
# include <boost/iterator/iterator_facade.hpp>
class node_iterator
: public boost::iterator_facade<
node_iterator
, node_base
, boost::forward_traversal_tag
>
{
public:
node_iterator()
: m_node(0)
{}
explicit node_iterator(node_base* p)
: m_node(p)
{}
private:
...
node_base* m_node;
};
</pre>
<table class="footnote" frame="void" id="default" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id13" name="default">[2]</a></td><td>Technically, the C++ standard places almost no
requirements on a default-constructed iterator, so if we were
really concerned with efficiency, we could've written the
default constructor to leave <tt class="literal"><span class="pre">m_node</span></tt> uninitialized.</td></tr>
</tbody>
</table>
</div>
<div class="section" id="implementing-the-core-operations">
<h3><a class="toc-backref" href="#id41" name="implementing-the-core-operations">Implementing the Core Operations</a></h3>
<p>The last step is to implement the <a class="reference" href="#core-operations">core operations</a> required by
the concepts we want our iterator to model. Referring to the
<a class="reference" href="#core-operations">table</a>, we can see that the first three rows are applicable
because <tt class="literal"><span class="pre">node_iterator</span></tt> needs to satisfy the requirements for
<a class="reference" href="new-iter-concepts.html#readable-iterators-lib-readable-iterators">readable iterator</a>, <a class="reference" href="new-iter-concepts.html#single-pass-iterators-lib-single-pass-iterators">single pass iterator</a>, and <a class="reference" href="new-iter-concepts.html#incrementable-iterators-lib-incrementable-iterators">incrementable
iterator</a>.</p>
<p>We therefore need to supply <tt class="literal"><span class="pre">dereference</span></tt>,
<tt class="literal"><span class="pre">equal</span></tt>, and <tt class="literal"><span class="pre">increment</span></tt> members. We don't want these members
to become part of <tt class="literal"><span class="pre">node_iterator</span></tt>'s public interface, so we can
make them private and grant friendship to
<tt class="literal"><span class="pre">boost::iterator_core_access</span></tt>, a "back-door" that
<tt class="literal"><span class="pre">iterator_facade</span></tt> uses to get access to the core operations:</p>
<pre class="literal-block">
# include "node.hpp"
# include <boost/iterator/iterator_facade.hpp>
class node_iterator
: public boost::iterator_facade<
node_iterator
, node_base
, boost::forward_traversal_tag
>
{
public:
node_iterator()
: m_node(0) {}
explicit node_iterator(node_base* p)
: m_node(p) {}
private:
friend class boost::iterator_core_access;
void increment() { m_node = m_node->next(); }
bool equal(node_iterator const& other) const
{
return this->m_node == other.m_node;
}
node_base& dereference() const { return *m_node; }
node_base* m_node;
};
</pre>
<p>Voilà; a complete and conforming readable, forward-traversal
iterator! For a working example of its use, see <a class="reference" href="../example/node_iterator1.cpp">this program</a>.</p>
</div>
</div>
<div class="section" id="a-constant-node-iterator">
<h2><a class="toc-backref" href="#id42" name="a-constant-node-iterator">A constant <tt class="literal"><span class="pre">node_iterator</span></tt></a></h2>
<div class="sidebar">
<p class="sidebar-title first">Constant and Mutable iterators</p>
<p>The term <strong>mutable iterator</strong> means an iterator through which
the object it references (its "referent") can be modified. A
<strong>constant iterator</strong> is one which doesn't allow modification of
its referent.</p>
<p>The words <em>constant</em> and <em>mutable</em> don't refer to the ability to
modify the iterator itself. For example, an <tt class="literal"><span class="pre">int</span> <span class="pre">const*</span></tt> is a
non-<tt class="literal"><span class="pre">const</span></tt> <em>constant iterator</em>, which can be incremented
but doesn't allow modification of its referent, and <tt class="literal"><span class="pre">int*</span>
<span class="pre">const</span></tt> is a <tt class="literal"><span class="pre">const</span></tt> <em>mutable iterator</em>, which cannot be
modified but which allows modification of its referent.</p>
<p>Confusing? We agree, but those are the standard terms. It
probably doesn't help much that a container's constant iterator
is called <tt class="literal"><span class="pre">const_iterator</span></tt>.</p>
</div>
<p>Now, our <tt class="literal"><span class="pre">node_iterator</span></tt> gives clients access to both <tt class="literal"><span class="pre">node</span></tt>'s <tt class="literal"><span class="pre">print(std::ostream&)</span> <span class="pre">const</span></tt> member function, but also its
mutating <tt class="literal"><span class="pre">double_me()</span></tt> member. If we wanted to build a
<em>constant</em> <tt class="literal"><span class="pre">node_iterator</span></tt>, we'd only have to make three
changes:</p>
<pre class="literal-block">
class const_node_iterator
: public boost::iterator_facade<
node_iterator
, node_base <strong>const</strong>
, boost::forward_traversal_tag
>
{
public:
const_node_iterator()
: m_node(0) {}
explicit const_node_iterator(node_base* p)
: m_node(p) {}
private:
friend class boost::iterator_core_access;
void increment() { m_node = m_node->next(); }
bool equal(const_node_iterator const& other) const
{
return this->m_node == other.m_node;
}
node_base <strong>const</strong>& dereference() const { return *m_node; }
node_base <strong>const</strong>* m_node;
};
</pre>
<div class="sidebar">
<p class="sidebar-title first"><tt class="literal"><span class="pre">const</span></tt> and an iterator's <tt class="literal"><span class="pre">value_type</span></tt></p>
<p>The C++ standard requires an iterator's <tt class="literal"><span class="pre">value_type</span></tt> <em>not</em> be
<tt class="literal"><span class="pre">const</span></tt>-qualified, so <tt class="literal"><span class="pre">iterator_facade</span></tt> strips the
<tt class="literal"><span class="pre">const</span></tt> from its <tt class="literal"><span class="pre">Value</span></tt> parameter in order to produce the
iterator's <tt class="literal"><span class="pre">value_type</span></tt>. Making the <tt class="literal"><span class="pre">Value</span></tt> argument
<tt class="literal"><span class="pre">const</span></tt> provides a useful hint to <tt class="literal"><span class="pre">iterator_facade</span></tt> that the
iterator is a <em>constant iterator</em>, and the default <tt class="literal"><span class="pre">Reference</span></tt>
argument will be correct for all lvalue iterators.</p>
</div>
<p>As a matter of fact, <tt class="literal"><span class="pre">node_iterator</span></tt> and <tt class="literal"><span class="pre">const_node_iterator</span></tt>
are so similar that it makes sense to factor the common code out
into a template as follows:</p>
<pre class="literal-block">
template <class Value>
class node_iter
: public boost::iterator_facade<
node_iter<Value>
, Value
, boost::forward_traversal_tag
>
{
public:
node_iter()
: m_node(0) {}
explicit node_iter(Value* p)
: m_node(p) {}
private:
friend class boost::iterator_core_access;
bool equal(node_iter<Value> const& other) const
{
return this->m_node == other.m_node;
}
void increment()
{ m_node = m_node->next(); }
Value& dereference() const
{ return *m_node; }
Value* m_node;
};
typedef node_iter<node_base> node_iterator;
typedef node_iter<node_base const> node_const_iterator;
</pre>
</div>
<div class="section" id="interoperability">
<h2><a class="toc-backref" href="#id43" name="interoperability">Interoperability</a></h2>
<p>Our <tt class="literal"><span class="pre">const_node_iterator</span></tt> works perfectly well on its own, but
taken together with <tt class="literal"><span class="pre">node_iterator</span></tt> it doesn't quite meet
expectations. For example, we'd like to be able to pass a
<tt class="literal"><span class="pre">node_iterator</span></tt> where a <tt class="literal"><span class="pre">node_const_iterator</span></tt> was expected,
just as you can with <tt class="literal"><span class="pre">std::list<int></span></tt>'s <tt class="literal"><span class="pre">iterator</span></tt> and
<tt class="literal"><span class="pre">const_iterator</span></tt>. Furthermore, given a <tt class="literal"><span class="pre">node_iterator</span></tt> and a
<tt class="literal"><span class="pre">node_const_iterator</span></tt> into the same list, we should be able to
compare them for equality.</p>
<p>This expected ability to use two different iterator types together
is known as <a class="reference" href="new-iter-concepts.html#interoperable-iterators-lib-interoperable-iterators"><strong>interoperability</strong></a>. Achieving interoperability in
our case is as simple as templatizing the <tt class="literal"><span class="pre">equal</span></tt> function and
adding a templatized converting constructor <a class="footnote-reference" href="#broken" id="id16" name="id16"><sup>3</sup></a> <a class="footnote-reference" href="#random" id="id17" name="id17"><sup>4</sup></a>:</p>
<pre class="literal-block">
template <class Value>
class node_iter
: public boost::iterator_facade<
node_iter<Value>
, Value
, boost::forward_traversal_tag
>
{
public:
node_iter()
: m_node(0) {}
explicit node_iter(Value* p)
: m_node(p) {}
template <class OtherValue>
node_iter(node_iter<OtherValue> const& other)
: m_node(other.m_node) {}
private:
friend class boost::iterator_core_access;
template <class> friend class node_iter;
template <class OtherValue>
bool equal(node_iter<OtherValue> const& other) const
{
return this->m_node == other.m_node;
}
void increment()
{ m_node = m_node->next(); }
Value& dereference() const
{ return *m_node; }
Value* m_node;
};
typedef impl::node_iterator<node_base> node_iterator;
typedef impl::node_iterator<node_base const> node_const_iterator;
</pre>
<table class="footnote" frame="void" id="broken" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id16" name="broken">[3]</a></td><td>If you're using an older compiler and it can't handle
this example, see the <a class="reference" href="../example/node_iterator2.hpp">example code</a> for workarounds.</td></tr>
</tbody>
</table>
<table class="footnote" frame="void" id="random" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id17" name="random">[4]</a></td><td>If <tt class="literal"><span class="pre">node_iterator</span></tt> had been a <a class="reference" href="new-iter-concepts.html#random-access-traversal-iterators-lib-random-access-traversal-iterators">random access
traversal iterator</a>, we'd have had to templatize its
<tt class="literal"><span class="pre">distance_to</span></tt> function as well.</td></tr>
</tbody>
</table>
<p>You can see an example program which exercises our interoperable
iterators <a class="reference" href="../example/node_iterator2.cpp">here</a>.</p>
</div>
<div class="section" id="telling-the-truth">
<h2><a class="toc-backref" href="#id44" name="telling-the-truth">Telling the Truth</a></h2>
<p>Now <tt class="literal"><span class="pre">node_iterator</span></tt> and <tt class="literal"><span class="pre">node_const_iterator</span></tt> behave exactly as
you'd expect... almost. We can compare them and we can convert in
one direction: from <tt class="literal"><span class="pre">node_iterator</span></tt> to <tt class="literal"><span class="pre">node_const_iterator</span></tt>.
If we try to convert from <tt class="literal"><span class="pre">node_const_iterator</span></tt> to
<tt class="literal"><span class="pre">node_iterator</span></tt>, we'll get an error when the converting
constructor tries to initialize <tt class="literal"><span class="pre">node_iterator</span></tt>'s <tt class="literal"><span class="pre">m_node</span></tt>, a
<tt class="literal"><span class="pre">node*</span></tt> with a <tt class="literal"><span class="pre">node</span> <span class="pre">const*</span></tt>. So what's the problem?</p>
<p>The problem is that
<tt class="literal"><span class="pre">boost::</span></tt><a class="reference" href="../../type_traits/index.html#relationships"><tt class="literal"><span class="pre">is_convertible</span></tt></a><tt class="literal"><span class="pre"><node_const_iterator,node_iterator>::value</span></tt>
will be <tt class="literal"><span class="pre">true</span></tt>, but it should be <tt class="literal"><span class="pre">false</span></tt>. <a class="reference" href="../../type_traits/index.html#relationships"><tt class="literal"><span class="pre">is_convertible</span></tt></a>
lies because it can only see as far as the <em>declaration</em> of
<tt class="literal"><span class="pre">node_iter</span></tt>'s converting constructor, but can't look inside at
the <em>definition</em> to make sure it will compile. A perfect solution
would make <tt class="literal"><span class="pre">node_iter</span></tt>'s converting constructor disappear when
the <tt class="literal"><span class="pre">m_node</span></tt> conversion would fail.</p>
<p>In fact, that sort of magic is possible using
<a class="reference" href="../../utility/enable_if.html"><tt class="literal"><span class="pre">boost::enable_if</span></tt></a>. By rewriting the converting constructor as
follows, we can remove it from the overload set when it's not
appropriate:</p>
<pre class="literal-block">
#include <boost/type_traits/is_convertible.hpp>
#include <boost/utility/enable_if.hpp>
...
private:
struct enabler {};
public:
template <class OtherValue>
node_iter(
node_iter<OtherValue> const& other
, typename boost::enable_if<
boost::is_convertible<OtherValue*,Value*>
, enabler
>::type = enabler()
)
: m_node(other.m_node) {}
</pre>
</div>
<div class="section" id="wrap-up">
<h2><a class="toc-backref" href="#id45" name="wrap-up">Wrap Up</a></h2>
<p>This concludes our <tt class="literal"><span class="pre">iterator_facade</span></tt> tutorial, but before you
stop reading we urge you to take a look at <a class="reference" href="iterator_adaptor.html"><tt class="literal"><span class="pre">iterator_adaptor</span></tt></a>.
There's another way to approach writing these iterators which might
even be superior.</p>
</div>
</div>
</div>
<hr class="footer" />
<div class="footer">
<a class="reference" href="iterator_facade.rst">View document source</a>.
Generated by <a class="reference" href="http://docutils.sourceforge.net/">Docutils</a> from <a class="reference" href="http://docutils.sourceforge.net/rst.html">reStructuredText</a> source.
</div>
</body>
</html>
|
