-
Notifications
You must be signed in to change notification settings - Fork 22
/
p2387r3.html
1351 lines (1343 loc) · 134 KB
/
p2387r3.html
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
<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml" lang xml:lang>
<head>
<meta charset="utf-8" />
<meta name="generator" content="mpark/wg21" />
<meta name="viewport" content="width=device-width, initial-scale=1.0, user-scalable=yes" />
<meta name="dcterms.date" content="2021-12-17" />
<title>Pipe support for user-defined range adaptors</title>
<style>
code{white-space: pre-wrap;}
span.smallcaps{font-variant: small-caps;}
span.underline{text-decoration: underline;}
div.column{display: inline-block; vertical-align: top; width: 50%;}
div.hanging-indent{margin-left: 1.5em; text-indent: -1.5em;}
ul.task-list{list-style: none;}
pre > code.sourceCode { white-space: pre; position: relative; }
pre > code.sourceCode > span { display: inline-block; line-height: 1.25; }
pre > code.sourceCode > span:empty { height: 1.2em; }
code.sourceCode > span { color: inherit; text-decoration: inherit; }
div.sourceCode { margin: 1em 0; }
pre.sourceCode { margin: 0; }
@media screen {
div.sourceCode { overflow: auto; }
}
@media print {
pre > code.sourceCode { white-space: pre-wrap; }
pre > code.sourceCode > span { text-indent: -5em; padding-left: 5em; }
}
pre.numberSource code
{ counter-reset: source-line 0; }
pre.numberSource code > span
{ position: relative; left: -4em; counter-increment: source-line; }
pre.numberSource code > span > a:first-child::before
{ content: counter(source-line);
position: relative; left: -1em; text-align: right; vertical-align: baseline;
border: none; display: inline-block;
-webkit-touch-callout: none; -webkit-user-select: none;
-khtml-user-select: none; -moz-user-select: none;
-ms-user-select: none; user-select: none;
padding: 0 4px; width: 4em;
color: #aaaaaa;
}
pre.numberSource { margin-left: 3em; border-left: 1px solid #aaaaaa; padding-left: 4px; }
div.sourceCode
{ background-color: #f6f8fa; }
@media screen {
pre > code.sourceCode > span > a:first-child::before { text-decoration: underline; }
}
code span. { } /* Normal */
code span.al { color: #ff0000; } /* Alert */
code span.an { } /* Annotation */
code span.at { } /* Attribute */
code span.bn { color: #9f6807; } /* BaseN */
code span.bu { color: #9f6807; } /* BuiltIn */
code span.cf { color: #00607c; } /* ControlFlow */
code span.ch { color: #9f6807; } /* Char */
code span.cn { } /* Constant */
code span.co { color: #008000; font-style: italic; } /* Comment */
code span.cv { color: #008000; font-style: italic; } /* CommentVar */
code span.do { color: #008000; } /* Documentation */
code span.dt { color: #00607c; } /* DataType */
code span.dv { color: #9f6807; } /* DecVal */
code span.er { color: #ff0000; font-weight: bold; } /* Error */
code span.ex { } /* Extension */
code span.fl { color: #9f6807; } /* Float */
code span.fu { } /* Function */
code span.im { } /* Import */
code span.in { color: #008000; } /* Information */
code span.kw { color: #00607c; } /* Keyword */
code span.op { color: #af1915; } /* Operator */
code span.ot { } /* Other */
code span.pp { color: #6f4e37; } /* Preprocessor */
code span.re { } /* RegionMarker */
code span.sc { color: #9f6807; } /* SpecialChar */
code span.ss { color: #9f6807; } /* SpecialString */
code span.st { color: #9f6807; } /* String */
code span.va { } /* Variable */
code span.vs { color: #9f6807; } /* VerbatimString */
code span.wa { color: #008000; font-weight: bold; } /* Warning */
code.diff {color: #898887}
code.diff span.va {color: #006e28}
code.diff span.st {color: #bf0303}
</style>
<style type="text/css">
body {
margin: 5em;
font-family: serif;
hyphens: auto;
line-height: 1.35;
}
div.wrapper {
max-width: 60em;
margin: auto;
}
ul {
list-style-type: none;
padding-left: 2em;
margin-top: -0.2em;
margin-bottom: -0.2em;
}
a {
text-decoration: none;
color: #4183C4;
}
a.hidden_link {
text-decoration: none;
color: inherit;
}
li {
margin-top: 0.6em;
margin-bottom: 0.6em;
}
h1, h2, h3, h4 {
position: relative;
line-height: 1;
}
a.self-link {
position: absolute;
top: 0;
left: calc(-1 * (3.5rem - 26px));
width: calc(3.5rem - 26px);
height: 2em;
text-align: center;
border: none;
transition: opacity .2s;
opacity: .5;
font-family: sans-serif;
font-weight: normal;
font-size: 83%;
}
a.self-link:hover { opacity: 1; }
a.self-link::before { content: "§"; }
ul > li:before {
content: "\2014";
position: absolute;
margin-left: -1.5em;
}
:target { background-color: #C9FBC9; }
:target .codeblock { background-color: #C9FBC9; }
:target ul { background-color: #C9FBC9; }
.abbr_ref { float: right; }
.folded_abbr_ref { float: right; }
:target .folded_abbr_ref { display: none; }
:target .unfolded_abbr_ref { float: right; display: inherit; }
.unfolded_abbr_ref { display: none; }
.secnum { display: inline-block; min-width: 35pt; }
.header-section-number { display: inline-block; min-width: 35pt; }
.annexnum { display: block; }
div.sourceLinkParent {
float: right;
}
a.sourceLink {
position: absolute;
opacity: 0;
margin-left: 10pt;
}
a.sourceLink:hover {
opacity: 1;
}
a.itemDeclLink {
position: absolute;
font-size: 75%;
text-align: right;
width: 5em;
opacity: 0;
}
a.itemDeclLink:hover { opacity: 1; }
span.marginalizedparent {
position: relative;
left: -5em;
}
li span.marginalizedparent { left: -7em; }
li ul > li span.marginalizedparent { left: -9em; }
li ul > li ul > li span.marginalizedparent { left: -11em; }
li ul > li ul > li ul > li span.marginalizedparent { left: -13em; }
div.footnoteNumberParent {
position: relative;
left: -4.7em;
}
a.marginalized {
position: absolute;
font-size: 75%;
text-align: right;
width: 5em;
}
a.enumerated_item_num {
position: relative;
left: -3.5em;
display: inline-block;
margin-right: -3em;
text-align: right;
width: 3em;
}
div.para { margin-bottom: 0.6em; margin-top: 0.6em; text-align: justify; }
div.section { text-align: justify; }
div.sentence { display: inline; }
span.indexparent {
display: inline;
position: relative;
float: right;
right: -1em;
}
a.index {
position: absolute;
display: none;
}
a.index:before { content: "⟵"; }
a.index:target {
display: inline;
}
.indexitems {
margin-left: 2em;
text-indent: -2em;
}
div.itemdescr {
margin-left: 3em;
}
.bnf {
font-family: serif;
margin-left: 40pt;
margin-top: 0.5em;
margin-bottom: 0.5em;
}
.ncbnf {
font-family: serif;
margin-top: 0.5em;
margin-bottom: 0.5em;
margin-left: 40pt;
}
.ncsimplebnf {
font-family: serif;
font-style: italic;
margin-top: 0.5em;
margin-bottom: 0.5em;
margin-left: 40pt;
background: inherit;
}
span.textnormal {
font-style: normal;
font-family: serif;
white-space: normal;
display: inline-block;
}
span.rlap {
display: inline-block;
width: 0px;
}
span.descr { font-style: normal; font-family: serif; }
span.grammarterm { font-style: italic; }
span.term { font-style: italic; }
span.terminal { font-family: monospace; font-style: normal; }
span.nonterminal { font-style: italic; }
span.tcode { font-family: monospace; font-style: normal; }
span.textbf { font-weight: bold; }
span.textsc { font-variant: small-caps; }
a.nontermdef { font-style: italic; font-family: serif; }
span.emph { font-style: italic; }
span.techterm { font-style: italic; }
span.mathit { font-style: italic; }
span.mathsf { font-family: sans-serif; }
span.mathrm { font-family: serif; font-style: normal; }
span.textrm { font-family: serif; }
span.textsl { font-style: italic; }
span.mathtt { font-family: monospace; font-style: normal; }
span.mbox { font-family: serif; font-style: normal; }
span.ungap { display: inline-block; width: 2pt; }
span.textit { font-style: italic; }
span.texttt { font-family: monospace; }
span.tcode_in_codeblock { font-family: monospace; font-style: normal; }
span.phantom { color: white; }
span.math { font-style: normal; }
span.mathblock {
display: block;
margin-left: auto;
margin-right: auto;
margin-top: 1.2em;
margin-bottom: 1.2em;
text-align: center;
}
span.mathalpha {
font-style: italic;
}
span.synopsis {
font-weight: bold;
margin-top: 0.5em;
display: block;
}
span.definition {
font-weight: bold;
display: block;
}
.codeblock {
margin-left: 1.2em;
line-height: 127%;
}
.outputblock {
margin-left: 1.2em;
line-height: 127%;
}
div.itemdecl {
margin-top: 2ex;
}
code.itemdeclcode {
white-space: pre;
display: block;
}
span.textsuperscript {
vertical-align: super;
font-size: smaller;
line-height: 0;
}
.footnotenum { vertical-align: super; font-size: smaller; line-height: 0; }
.footnote {
font-size: small;
margin-left: 2em;
margin-right: 2em;
margin-top: 0.6em;
margin-bottom: 0.6em;
}
div.minipage {
display: inline-block;
margin-right: 3em;
}
div.numberedTable {
text-align: center;
margin: 2em;
}
div.figure {
text-align: center;
margin: 2em;
}
table {
border: 1px solid black;
border-collapse: collapse;
margin-left: auto;
margin-right: auto;
margin-top: 0.8em;
text-align: left;
hyphens: none;
}
td, th {
padding-left: 1em;
padding-right: 1em;
vertical-align: top;
}
td.empty {
padding: 0px;
padding-left: 1px;
}
td.left {
text-align: left;
}
td.right {
text-align: right;
}
td.center {
text-align: center;
}
td.justify {
text-align: justify;
}
td.border {
border-left: 1px solid black;
}
tr.rowsep, td.cline {
border-top: 1px solid black;
}
tr.even, tr.odd {
border-bottom: 1px solid black;
}
tr.capsep {
border-top: 3px solid black;
border-top-style: double;
}
tr.header {
border-bottom: 3px solid black;
border-bottom-style: double;
}
th {
border-bottom: 1px solid black;
}
span.centry {
font-weight: bold;
}
div.table {
display: block;
margin-left: auto;
margin-right: auto;
text-align: center;
width: 90%;
}
span.indented {
display: block;
margin-left: 2em;
margin-bottom: 1em;
margin-top: 1em;
}
ol.enumeratea { list-style-type: none; background: inherit; }
ol.enumerate { list-style-type: none; background: inherit; }
code.sourceCode > span { display: inline; }
</style>
<style type="text/css">a {
color : #4183C4;
text-decoration: underline;
}
a.marginalized {
text-decoration: none;
}
a.self-link {
text-decoration: none;
}
h1#toctitle {
border-bottom: 1px solid #cccccc;
}
#TOC li {
margin-top: 1px;
margin-bottom: 1px;
}
#TOC ul>li:before { display: none; }
h3.subtitle { margin-top: -15px; }
h1:target { background-color: transparent; }
h2:target { background-color: transparent; }
h3:target { background-color: transparent; }
h4:target { background-color: transparent; }
h5:target { background-color: transparent; }
h6:target { background-color: transparent; }
code span.co { font-family: monospace; }
table tr {
background-color: white;
}
table tr:nth-child(2n) {
background-color: #f6f8fa;
}
#title-block-header > table tr:nth-child(2n) {
background-color: white;
}
td > div.sourceCode {
background-color: inherit;
}
table {
border-collapse: collapse;
}
table td, table th {
border: 1px solid #cccccc;
}
table th {
border-bottom: 1px solid black;
text-align: center;
}
table tr:first-child th {
border-top: 0;
}
table tr:last-child td {
border-bottom: 0;
}
table tr td:first-child,
table tr th:first-child {
border-left: 0;
}
table tr td:last-child,
table tr th:last-child {
border-right: 0;
}
table tbody tr:first-child td {
border-top: 1px solid black;
}
#title-block-header td { border: 0; }
@media all {
body {
margin: 2em;
}
}
@media screen and (min-width: 480px) {
body {
margin: 5em;
}
}
#refs code{padding-left: 0px; text-indent: 0px;}
:root {
--diff-ins: #e6ffed;
--diff-strongins: #acf2bd;
--diff-del: #ffdddd;
--diff-strongdel: #ff8888;
}
span.diffins {
background-color: var(--diff-strongins);
}
span.diffdel {
background-color: var(--diff-strongdel);
}
div.rm { text-decoration: line-through; }
div.rm code.sourceCode { text-decoration: line-through; }
div.addu, span.addu {
color: #006e28;
background-color: var(--diff-ins);
}
div.rm pre, div.add pre { background-color: #f6f8fa; }
div.addu pre { background-color: var(--diff-ins); }
div.add, div.add pre { background-color: var(--diff-ins); }
div.addu blockquote {
border-left: 4px solid #00a000;
padding: 0 15px;
color: #006e28;
text-decoration: none;
}
div.addu blockquote code.sourceCode { text-decoration: none; }
div.addu blockquote pre { text-decoration: none; }
div.addu blockquote pre code { text-decoration: none; }
div.quote {
border-left: 7px solid #ccc;
background: #f9f9f9;
margin: 1.5em 10px;
padding-left: 20px;
}
code.diff span.va { color: #000000; background-color: var(--diff-ins); }
code.diff span.st { color: #000000; background-color: var(--diff-del); }
</style>
<link href="data:image/x-icon;base64,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" rel="icon" />
<!--[if lt IE 9]>
<script src="//cdnjs.cloudflare.com/ajax/libs/html5shiv/3.7.3/html5shiv-printshiv.min.js"></script>
<![endif]-->
</head>
<body>
<div class="wrapper">
<header id="title-block-header">
<h1 class="title" style="text-align:center">Pipe support for user-defined range adaptors</h1>
<table style="border:none;float:right">
<tr>
<td>Document #:</td>
<td>P2387R3</td>
</tr>
<tr>
<td>Date:</td>
<td>2021-12-17</td>
</tr>
<tr>
<td style="vertical-align:top">Project:</td>
<td>Programming Language C++</td>
</tr>
<tr>
<td style="vertical-align:top">Audience:</td>
<td>
LWG<br>
</td>
</tr>
<tr>
<td style="vertical-align:top">Reply-to:</td>
<td>
Barry Revzin<br><<a href="mailto:[email protected]" class="email">[email protected]</a>><br>
</td>
</tr>
</table>
</header>
<div style="clear:both">
<div id="TOC" role="doc-toc">
<h1 id="toctitle">Contents</h1>
<ul>
<li><a href="#revision-history"><span class="toc-section-number">1</span> Revision History<span></span></a></li>
<li><a href="#introduction"><span class="toc-section-number">2</span> Introduction<span></span></a></li>
<li><a href="#implementation-experience"><span class="toc-section-number">3</span> Implementation Experience<span></span></a>
<ul>
<li><a href="#nanorange"><span class="toc-section-number">3.1</span> NanoRange<span></span></a></li>
<li><a href="#range-v3"><span class="toc-section-number">3.2</span> range-v3<span></span></a></li>
<li><a href="#gcc-10"><span class="toc-section-number">3.3</span> gcc 10<span></span></a></li>
<li><a href="#gcc-11"><span class="toc-section-number">3.4</span> gcc 11<span></span></a></li>
<li><a href="#msvc"><span class="toc-section-number">3.5</span> msvc<span></span></a></li>
</ul></li>
<li><a href="#problem-space"><span class="toc-section-number">4</span> Problem Space<span></span></a>
<ul>
<li><a href="#range-adaptor-closure-objects"><span class="toc-section-number">4.1</span> Range Adaptor Closure Objects<span></span></a></li>
<li><a href="#range-adaptor-objects"><span class="toc-section-number">4.2</span> Range Adaptor Objects<span></span></a></li>
</ul></li>
<li><a href="#proposal"><span class="toc-section-number">5</span> Proposal<span></span></a>
<ul>
<li><a href="#wording-for-range_adaptor_closure"><span class="toc-section-number">5.1</span> Wording for <code class="sourceCode cpp">range_adaptor_closure</code><span></span></a></li>
<li><a href="#wording-for-bind_back"><span class="toc-section-number">5.2</span> Wording for <code class="sourceCode cpp">bind_back</code><span></span></a></li>
<li><a href="#feature-test-macros"><span class="toc-section-number">5.3</span> Feature-test macros<span></span></a></li>
</ul></li>
<li><a href="#bibliography"><span class="toc-section-number">6</span> References<span></span></a></li>
</ul>
</div>
<h1 data-number="1" style="border-bottom:1px solid #cccccc" id="revision-history"><span class="header-section-number">1</span> Revision History<a href="#revision-history" class="self-link"></a></h1>
<p>Since <span class="citation" data-cites="P2387R2">[<a href="#ref-P2387R2" role="doc-biblioref">P2387R2</a>]</span>, rebased and updated wording.</p>
<p>Since <span class="citation" data-cites="P2387R1">[<a href="#ref-P2387R1" role="doc-biblioref">P2387R1</a>]</span>, added another feature test macro.</p>
<p>Since <span class="citation" data-cites="P2387R0">[<a href="#ref-P2387R0" role="doc-biblioref">P2387R0</a>]</span>, added a feature test macro.</p>
<h1 data-number="2" style="border-bottom:1px solid #cccccc" id="introduction"><span class="header-section-number">2</span> Introduction<a href="#introduction" class="self-link"></a></h1>
<p>When we presented the C++23 Ranges Plan <span class="citation" data-cites="P2214R0">[<a href="#ref-P2214R0" role="doc-biblioref">P2214R0</a>]</span> to LEWG, one of the arguments we made was that the top priority item on the list was the ability to eagerly collect a range into a type (<code class="sourceCode cpp">ranges<span class="op">::</span>to</code> <span class="citation" data-cites="P1206R3">[<a href="#ref-P1206R3" role="doc-biblioref">P1206R3</a>]</span>). During the telecon discussing that paper, Walter Brown made an excellent observation: if we gave users the tools to write their own range adaptors that would properly inter-operate with standard library adaptors (as well as other users’ adaptors), then it becomes less important to provide more adaptors in the standard library.</p>
<p>The goal of this paper is provide that functionality: provide a standard customization mechanism for range adaptors, so that everybody can write their own adaptors.</p>
<h1 data-number="3" style="border-bottom:1px solid #cccccc" id="implementation-experience"><span class="header-section-number">3</span> Implementation Experience<a href="#implementation-experience" class="self-link"></a></h1>
<p>To start with, there have been several implementations of the range adaptor design, that are all a little bit different. It is worth going through them all to compare how they solved the problem.</p>
<h2 data-number="3.1" id="nanorange"><span class="header-section-number">3.1</span> NanoRange<a href="#nanorange" class="self-link"></a></h2>
<p>In Tristan Brindle’s <span class="citation" data-cites="NanoRange">[<a href="#ref-NanoRange" role="doc-biblioref">NanoRange</a>]</span>, we have the following approach. NanoRange uses the acronyms <code class="sourceCode cpp">raco</code> for <strong>R</strong>ange <strong>A</strong>daptor <strong>C</strong>losure <strong>O</strong>bject and <code class="sourceCode cpp">rao</code> for <strong>R</strong>ange <strong>A</strong>daptor <strong>O</strong>bject (both defined in <span>24.7.2 <a href="https://wg21.link/range.adaptor.object">[range.adaptor.object]</a></span>).</p>
<div class="sourceCode" id="cb1"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb1-1"><a href="#cb1-1"></a><span class="kw">namespace</span> nano<span class="op">::</span>detail <span class="op">{</span></span>
<span id="cb1-2"><a href="#cb1-2"></a> <span class="co">// variable template to identify a Range Adaptor Closure Object</span></span>
<span id="cb1-3"><a href="#cb1-3"></a> <span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span><span class="op">></span></span>
<span id="cb1-4"><a href="#cb1-4"></a> <span class="kw">inline</span> <span class="kw">constexpr</span> <span class="dt">bool</span> is_raco <span class="op">=</span> <span class="kw">false</span>;</span>
<span id="cb1-5"><a href="#cb1-5"></a></span>
<span id="cb1-6"><a href="#cb1-6"></a> <span class="co">// support for R | C to evaluate as C(R)</span></span>
<span id="cb1-7"><a href="#cb1-7"></a> <span class="kw">template</span> <span class="op"><</span>viewable_range R, <span class="kw">typename</span> C<span class="op">></span></span>
<span id="cb1-8"><a href="#cb1-8"></a> <span class="kw">requires</span> is_raco<span class="op"><</span>remove_cvref_t<span class="op"><</span>C<span class="op">>></span></span>
<span id="cb1-9"><a href="#cb1-9"></a> <span class="op">&&</span> invocable<span class="op"><</span>C, R<span class="op">></span></span>
<span id="cb1-10"><a href="#cb1-10"></a> <span class="kw">constexpr</span> <span class="kw">auto</span> <span class="kw">operator</span><span class="op">|(</span>R<span class="op">&&</span> lhs, C<span class="op">&&</span> rhs<span class="op">)</span> <span class="op">-></span> <span class="kw">decltype</span><span class="op">(</span><span class="kw">auto</span><span class="op">)</span> <span class="op">{</span></span>
<span id="cb1-11"><a href="#cb1-11"></a> <span class="cf">return</span> FWD<span class="op">(</span>rhs<span class="op">)(</span>FWD<span class="op">(</span>lhs<span class="op">))</span>;</span>
<span id="cb1-12"><a href="#cb1-12"></a> <span class="op">}</span></span>
<span id="cb1-13"><a href="#cb1-13"></a></span>
<span id="cb1-14"><a href="#cb1-14"></a> <span class="co">// a type to handle merging two Range Adaptor Closure Objects together</span></span>
<span id="cb1-15"><a href="#cb1-15"></a> <span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span> LHS, <span class="kw">typename</span> RHS<span class="op">></span></span>
<span id="cb1-16"><a href="#cb1-16"></a> <span class="kw">struct</span> raco_pipe <span class="op">{</span></span>
<span id="cb1-17"><a href="#cb1-17"></a> LHS lhs;</span>
<span id="cb1-18"><a href="#cb1-18"></a> RHS rhs;</span>
<span id="cb1-19"><a href="#cb1-19"></a></span>
<span id="cb1-20"><a href="#cb1-20"></a> <span class="co">// ...</span></span>
<span id="cb1-21"><a href="#cb1-21"></a></span>
<span id="cb1-22"><a href="#cb1-22"></a> <span class="kw">template</span> <span class="op"><</span>viewable_range R<span class="op">></span></span>
<span id="cb1-23"><a href="#cb1-23"></a> <span class="kw">requires</span> invocable<span class="op"><</span>LHS<span class="op">&</span>, R<span class="op">></span></span>
<span id="cb1-24"><a href="#cb1-24"></a> <span class="op">&&</span> invocable<span class="op"><</span>RHS<span class="op">&</span>, invoke_result_t<span class="op"><</span>LHS<span class="op">&</span>, R<span class="op">>></span></span>
<span id="cb1-25"><a href="#cb1-25"></a> <span class="kw">constexpr</span> <span class="kw">auto</span> <span class="kw">operator</span><span class="op">()(</span>R<span class="op">&&</span> r<span class="op">)</span> <span class="kw">const</span> <span class="op">{</span></span>
<span id="cb1-26"><a href="#cb1-26"></a> <span class="cf">return</span> rhs<span class="op">(</span>lhs<span class="op">(</span>FWD<span class="op">(</span>r<span class="op">)))</span>;</span>
<span id="cb1-27"><a href="#cb1-27"></a> <span class="op">}</span></span>
<span id="cb1-28"><a href="#cb1-28"></a></span>
<span id="cb1-29"><a href="#cb1-29"></a> <span class="co">// ...</span></span>
<span id="cb1-30"><a href="#cb1-30"></a> <span class="op">}</span>;</span>
<span id="cb1-31"><a href="#cb1-31"></a></span>
<span id="cb1-32"><a href="#cb1-32"></a> <span class="co">// ... which is itself a Range Adaptor Closure Object</span></span>
<span id="cb1-33"><a href="#cb1-33"></a> <span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span> LHS, <span class="kw">typename</span> RHS<span class="op">></span></span>
<span id="cb1-34"><a href="#cb1-34"></a> <span class="kw">inline</span> <span class="kw">constexpr</span> <span class="dt">bool</span> is_raco<span class="op"><</span>raco_pipe<span class="op"><</span>LHS, RHS<span class="op">>></span> <span class="op">=</span> <span class="kw">true</span>;</span>
<span id="cb1-35"><a href="#cb1-35"></a></span>
<span id="cb1-36"><a href="#cb1-36"></a> <span class="co">// support for C | D to produce a new Range Adaptor Closure Object</span></span>
<span id="cb1-37"><a href="#cb1-37"></a> <span class="co">// so that (R | C) | D and R | (C | D) can be equivalent</span></span>
<span id="cb1-38"><a href="#cb1-38"></a> <span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span> LHS, <span class="kw">typename</span> RHS<span class="op">></span></span>
<span id="cb1-39"><a href="#cb1-39"></a> <span class="kw">requires</span> is_raco<span class="op"><</span>remove_cvref_t<span class="op"><</span>LHS<span class="op">>></span></span>
<span id="cb1-40"><a href="#cb1-40"></a> <span class="op">&&</span> is_raco<span class="op"><</span>remove_cvref_t<span class="op"><</span>RHS<span class="op">>></span></span>
<span id="cb1-41"><a href="#cb1-41"></a> <span class="kw">constexpr</span> <span class="kw">auto</span> <span class="kw">operator</span><span class="op">|(</span>LHS<span class="op">&&</span>, RHS<span class="op">&&)</span> <span class="op">{</span></span>
<span id="cb1-42"><a href="#cb1-42"></a> <span class="cf">return</span> raco_pipe<span class="op"><</span>decay_t<span class="op"><</span>LHS<span class="op">></span>, decay_t<span class="op"><</span>RHS<span class="op">>>(</span>FWD<span class="op">(</span>lhs<span class="op">)</span>, FWD<span class="op">(</span>rhs<span class="op">))</span>;</span>
<span id="cb1-43"><a href="#cb1-43"></a> <span class="op">}</span></span>
<span id="cb1-44"><a href="#cb1-44"></a></span>
<span id="cb1-45"><a href="#cb1-45"></a> <span class="co">// ... and a convenience type for creating range adaptor objects</span></span>
<span id="cb1-46"><a href="#cb1-46"></a> <span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span> F<span class="op">></span></span>
<span id="cb1-47"><a href="#cb1-47"></a> <span class="kw">struct</span> rao_proxy <span class="op">:</span> F <span class="op">{</span></span>
<span id="cb1-48"><a href="#cb1-48"></a> <span class="kw">constexpr</span> <span class="kw">explicit</span> rao_proxy<span class="op">(</span>F<span class="op">&&</span> f<span class="op">)</span> <span class="op">:</span> F<span class="op">(</span>std<span class="op">::</span>move<span class="op">(</span>f<span class="op">))</span> <span class="op">{</span> <span class="op">}</span></span>
<span id="cb1-49"><a href="#cb1-49"></a> <span class="op">}</span>;</span>
<span id="cb1-50"><a href="#cb1-50"></a></span>
<span id="cb1-51"><a href="#cb1-51"></a> <span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span> F<span class="op">></span></span>
<span id="cb1-52"><a href="#cb1-52"></a> <span class="kw">inline</span> <span class="kw">constexpr</span> <span class="dt">bool</span> is_raco<span class="op"><</span>rao_proxy<span class="op"><</span>F<span class="op">>></span> <span class="op">=</span> <span class="kw">true</span>;</span>
<span id="cb1-53"><a href="#cb1-53"></a><span class="op">}</span></span></code></pre></div>
<p>And with that out of the way, NanoRange can create range adaptors fairly easily whether or not the range adaptor does not take any extra arguments (as in <code class="sourceCode cpp">join</code>) or does (as in <code class="sourceCode cpp">transform</code>). Note that <code class="sourceCode cpp">join_view_fn</code> <em>must</em> be in the <code class="sourceCode cpp">nano<span class="op">::</span>detail</code> namespace in order for <code class="sourceCode cpp">rng <span class="op">|</span> nano<span class="op">::</span>views<span class="op">::</span>join</code> to find the appropriate <code class="sourceCode cpp"><span class="kw">operator</span><span class="op">|</span></code> (but <code class="sourceCode cpp">transform_view_fn</code> does not, since with <code class="sourceCode cpp">rng <span class="op">|</span> nano<span class="op">::</span>views<span class="op">::</span>transform<span class="op">(</span>f<span class="op">)</span></code> the invocation of <code class="sourceCode cpp">transform<span class="op">(</span>f<span class="op">)</span></code> returns a <code class="sourceCode cpp">rao_proxy</code> which is itself a type in the <code class="sourceCode cpp">nano<span class="op">::</span>detail</code> namespace):</p>
<table>
<thead>
<tr class="header">
<th><div style="text-align:center">
<strong><code class="sourceCode cpp">join</code></strong>
</div></th>
<th><div style="text-align:center">
<strong><code class="sourceCode cpp">transform</code></strong>
</div></th>
</tr>
</thead>
<tbody>
<tr class="odd">
<td><div class="sourceCode" id="cb2"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb2-1"><a href="#cb2-1"></a><span class="kw">namespace</span> nano<span class="op">::</span>detail <span class="op">{</span></span>
<span id="cb2-2"><a href="#cb2-2"></a> <span class="kw">struct</span> join_view_fn <span class="op">{</span></span>
<span id="cb2-3"><a href="#cb2-3"></a> <span class="kw">template</span> <span class="op"><</span>viewable_range R<span class="op">></span></span>
<span id="cb2-4"><a href="#cb2-4"></a> <span class="kw">requires</span> <span class="co">/* ... */</span></span>
<span id="cb2-5"><a href="#cb2-5"></a> <span class="kw">constexpr</span> <span class="kw">auto</span> <span class="kw">operator</span><span class="op">(</span>R<span class="op">&&</span> r<span class="op">)</span></span>
<span id="cb2-6"><a href="#cb2-6"></a> <span class="op">-></span> join_view<span class="op"><</span>all_t<span class="op"><</span>R<span class="op">>></span>;</span>
<span id="cb2-7"><a href="#cb2-7"></a> <span class="op">}</span>;</span>
<span id="cb2-8"><a href="#cb2-8"></a></span>
<span id="cb2-9"><a href="#cb2-9"></a> <span class="kw">template</span> <span class="op"><></span></span>
<span id="cb2-10"><a href="#cb2-10"></a> <span class="kw">inline</span> <span class="kw">constexpr</span> is_raco<span class="op"><</span>join_view_fn<span class="op">></span> <span class="op">=</span> <span class="kw">true</span>;</span>
<span id="cb2-11"><a href="#cb2-11"></a><span class="op">}</span></span>
<span id="cb2-12"><a href="#cb2-12"></a></span>
<span id="cb2-13"><a href="#cb2-13"></a><span class="kw">namespace</span> nano<span class="op">::</span>views <span class="op">{</span></span>
<span id="cb2-14"><a href="#cb2-14"></a> <span class="co">// for user consumption</span></span>
<span id="cb2-15"><a href="#cb2-15"></a> <span class="kw">inline</span> <span class="kw">constexpr</span> detail<span class="op">::</span>join_view_fn join<span class="op">{}</span>;</span>
<span id="cb2-16"><a href="#cb2-16"></a><span class="op">}</span></span></code></pre></div></td>
<td><div class="sourceCode" id="cb3"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb3-1"><a href="#cb3-1"></a><span class="kw">namespace</span> nano<span class="op">::</span>detail <span class="op">{</span></span>
<span id="cb3-2"><a href="#cb3-2"></a> <span class="kw">struct</span> transform_view_fn_base <span class="op">{</span></span>
<span id="cb3-3"><a href="#cb3-3"></a> <span class="co">// the overload that has all the information</span></span>
<span id="cb3-4"><a href="#cb3-4"></a> <span class="kw">template</span> <span class="op"><</span>viewable_range E, <span class="kw">typename</span> F<span class="op">></span></span>
<span id="cb3-5"><a href="#cb3-5"></a> <span class="kw">requires</span> <span class="co">/* ... */</span></span>
<span id="cb3-6"><a href="#cb3-6"></a> <span class="kw">constexpr</span> <span class="kw">auto</span> <span class="kw">operator</span><span class="op">()(</span>E<span class="op">&&</span> e, F<span class="op">&&</span> f<span class="op">)</span> <span class="kw">const</span></span>
<span id="cb3-7"><a href="#cb3-7"></a> <span class="op">-></span> transform_view<span class="op"><</span>all_t<span class="op"><</span>E<span class="op">></span>, decay_t<span class="op"><</span>F<span class="op">>></span>;</span>
<span id="cb3-8"><a href="#cb3-8"></a></span>
<span id="cb3-9"><a href="#cb3-9"></a> <span class="co">// the partial overload</span></span>
<span id="cb3-10"><a href="#cb3-10"></a> <span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span> F<span class="op">></span></span>
<span id="cb3-11"><a href="#cb3-11"></a> <span class="kw">constexpr</span> <span class="kw">auto</span> <span class="kw">operator</span><span class="op">()(</span>F f<span class="op">)</span> <span class="kw">const</span> <span class="op">{</span></span>
<span id="cb3-12"><a href="#cb3-12"></a> <span class="cf">return</span> rao_proxy<span class="op">{</span></span>
<span id="cb3-13"><a href="#cb3-13"></a> <span class="op">[</span>f<span class="op">=</span>move<span class="op">(</span>f<span class="op">)](</span>viewable_range <span class="kw">auto</span><span class="op">&&</span> r<span class="op">)</span></span>
<span id="cb3-14"><a href="#cb3-14"></a> <span class="kw">requires</span> <span class="co">/* ... */</span></span>
<span id="cb3-15"><a href="#cb3-15"></a> <span class="op">{</span></span>
<span id="cb3-16"><a href="#cb3-16"></a> <span class="cf">return</span> <span class="co">/* ... */</span>;</span>
<span id="cb3-17"><a href="#cb3-17"></a> <span class="op">}}</span>;</span>
<span id="cb3-18"><a href="#cb3-18"></a> <span class="op">}</span></span>
<span id="cb3-19"><a href="#cb3-19"></a> <span class="op">}</span>;</span>
<span id="cb3-20"><a href="#cb3-20"></a><span class="op">}</span></span>
<span id="cb3-21"><a href="#cb3-21"></a></span>
<span id="cb3-22"><a href="#cb3-22"></a><span class="kw">namespace</span> nano<span class="op">::</span>views <span class="op">{</span></span>
<span id="cb3-23"><a href="#cb3-23"></a> <span class="co">// for user consumption</span></span>
<span id="cb3-24"><a href="#cb3-24"></a> <span class="kw">inline</span> <span class="kw">constexpr</span> detail<span class="op">::</span>transform_view_fn transform<span class="op">{}</span>;</span>
<span id="cb3-25"><a href="#cb3-25"></a><span class="op">}</span></span></code></pre></div></td>
</tr>
</tbody>
</table>
<p>Although practically speaking, users will not just copy again the constraints for <code class="sourceCode cpp"><span class="kw">struct</span> meow_view_fn</code> that they had written for <code class="sourceCode cpp"><span class="kw">struct</span> meow_view</code>. Indeed, NanoRange does not do this. Instead, it uses the trailing-return-type based SFINAE to use the underlying range adaptor’s constraints. So <code class="sourceCode cpp">join_view_fn</code> actually looks like this (and it’s up to <code class="sourceCode cpp">join_view</code> to express its constraints properly:</p>
<div class="sourceCode" id="cb4"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb4-1"><a href="#cb4-1"></a><span class="kw">struct</span> join_view_fn <span class="op">{</span></span>
<span id="cb4-2"><a href="#cb4-2"></a> <span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span> E<span class="op">></span></span>
<span id="cb4-3"><a href="#cb4-3"></a> <span class="kw">constexpr</span> <span class="kw">auto</span> <span class="kw">operator</span><span class="op">()(</span>E<span class="op">&&</span> e<span class="op">)</span> <span class="kw">const</span></span>
<span id="cb4-4"><a href="#cb4-4"></a> <span class="op">-></span> <span class="kw">decltype</span><span class="op">(</span>join_view<span class="op">{</span>FWD<span class="op">(</span>e<span class="op">)})</span></span>
<span id="cb4-5"><a href="#cb4-5"></a> <span class="op">{</span></span>
<span id="cb4-6"><a href="#cb4-6"></a> <span class="cf">return</span> join_view<span class="op">{</span>FWD<span class="op">(</span>e<span class="op">)}</span>;</span>
<span id="cb4-7"><a href="#cb4-7"></a> <span class="op">}</span></span>
<span id="cb4-8"><a href="#cb4-8"></a><span class="op">}</span>;</span></code></pre></div>
<h2 data-number="3.2" id="range-v3"><span class="header-section-number">3.2</span> range-v3<a href="#range-v3" class="self-link"></a></h2>
<p>In <span class="citation" data-cites="range-v3">[<a href="#ref-range-v3" role="doc-biblioref">range-v3</a>]</span>, the approach is a bit more involved but still has the same kind of structure. Here, we have three types: <code class="sourceCode cpp">view_closure<span class="op"><</span>F<span class="op">></span></code> inherits from <code class="sourceCode cpp">view_closure_base</code> inherits from <code class="sourceCode cpp">view_closure_base_</code> (the latter of which is an empty class).</p>
<p><code class="sourceCode cpp">view_closure<span class="op"><</span>F<span class="op">></span></code> is a lot like NanoRange’s <code class="sourceCode cpp">rao_proxy<span class="op"><</span>F<span class="op">></span></code>, just with an extra base class:</p>
<div class="sourceCode" id="cb5"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb5-1"><a href="#cb5-1"></a><span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span> ViewFn<span class="op">></span></span>
<span id="cb5-2"><a href="#cb5-2"></a><span class="kw">struct</span> view_closure <span class="op">:</span> view_closure_base, ViewFn</span>
<span id="cb5-3"><a href="#cb5-3"></a><span class="op">{</span></span>
<span id="cb5-4"><a href="#cb5-4"></a> view_closure<span class="op">()</span> <span class="op">=</span> <span class="cf">default</span>;</span>
<span id="cb5-5"><a href="#cb5-5"></a></span>
<span id="cb5-6"><a href="#cb5-6"></a> <span class="kw">constexpr</span> <span class="kw">explicit</span> view_closure<span class="op">(</span>ViewFn fn<span class="op">)</span> <span class="op">:</span> ViewFn<span class="op">(</span>std<span class="op">::</span>move<span class="op">(</span>fn<span class="op">))</span> <span class="op">{</span> <span class="op">}</span></span>
<span id="cb5-7"><a href="#cb5-7"></a><span class="op">}</span>;</span></code></pre></div>
<p>The interesting class is the intermediate <code class="sourceCode cpp">view_closure_base</code>, which has all the functionality:</p>
<div class="sourceCode" id="cb6"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb6-1"><a href="#cb6-1"></a><span class="kw">namespace</span> ranges<span class="op">::</span>views <span class="op">{</span></span>
<span id="cb6-2"><a href="#cb6-2"></a> <span class="co">// this type is its own namespace for ADL inhibition</span></span>
<span id="cb6-3"><a href="#cb6-3"></a> <span class="kw">namespace</span> view_closure_base_ns <span class="op">{</span> <span class="kw">struct</span> view_closure_base; <span class="op">}</span></span>
<span id="cb6-4"><a href="#cb6-4"></a> <span class="kw">using</span> view_closure_base_ns<span class="op">::</span>view_closure_base;</span>
<span id="cb6-5"><a href="#cb6-5"></a> <span class="kw">namespace</span> detail <span class="op">{</span> <span class="kw">struct</span> view_closure_base_; <span class="op">}</span></span>
<span id="cb6-6"><a href="#cb6-6"></a></span>
<span id="cb6-7"><a href="#cb6-7"></a> <span class="co">// Piping a value into a range adaptor closure object should not yield another closure</span></span>
<span id="cb6-8"><a href="#cb6-8"></a> <span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span> ViewFn, <span class="kw">typename</span> Rng<span class="op">></span></span>
<span id="cb6-9"><a href="#cb6-9"></a> <span class="kw">concept</span> invocable_view_closure <span class="op">=</span></span>
<span id="cb6-10"><a href="#cb6-10"></a> invocable<span class="op"><</span>ViewFn, Rng<span class="op">></span></span>
<span id="cb6-11"><a href="#cb6-11"></a> <span class="op">&&</span> <span class="op">(</span><span class="kw">not</span> derived_from<span class="op"><</span>invoke_result_t<span class="op"><</span>ViewFn, Rng<span class="op">></span>, detail<span class="op">::</span>view_closure_base_<span class="op">>)</span>;</span>
<span id="cb6-12"><a href="#cb6-12"></a></span>
<span id="cb6-13"><a href="#cb6-13"></a> <span class="kw">struct</span> view_closure_base_ns<span class="op">::</span>view_closure_base <span class="op">:</span> detail<span class="op">::</span>view_closure_base_ <span class="op">{</span></span>
<span id="cb6-14"><a href="#cb6-14"></a> <span class="co">// support for R | C to evaluate as C(R)</span></span>
<span id="cb6-15"><a href="#cb6-15"></a> <span class="kw">template</span> <span class="op"><</span>viewable_range R, invocable_view_closure<span class="op"><</span>R<span class="op">></span> ViewFn<span class="op">></span></span>
<span id="cb6-16"><a href="#cb6-16"></a> <span class="kw">friend</span> <span class="kw">constexpr</span> <span class="kw">auto</span> <span class="kw">operator</span><span class="op">|(</span>R<span class="op">&&</span> rng, view_closure<span class="op"><</span>ViewFn<span class="op">></span> vw<span class="op">)</span> <span class="op">{</span></span>
<span id="cb6-17"><a href="#cb6-17"></a> <span class="cf">return</span> std<span class="op">::</span>move<span class="op">(</span>vw<span class="op">)(</span>FWD<span class="op">(</span>rng<span class="op">))</span>;</span>
<span id="cb6-18"><a href="#cb6-18"></a> <span class="op">}</span></span>
<span id="cb6-19"><a href="#cb6-19"></a></span>
<span id="cb6-20"><a href="#cb6-20"></a> <span class="co">// for diagnostic purposes, we delete the overload for R | C</span></span>
<span id="cb6-21"><a href="#cb6-21"></a> <span class="co">// if R is a range but not a viewable_range</span></span>
<span id="cb6-22"><a href="#cb6-22"></a> <span class="kw">template</span> <span class="op"><</span>range R, <span class="kw">typename</span> ViewFn<span class="op">></span></span>
<span id="cb6-23"><a href="#cb6-23"></a> <span class="kw">requires</span> <span class="op">(</span><span class="kw">not</span> viewable_range<span class="op"><</span>R<span class="op">>)</span></span>
<span id="cb6-24"><a href="#cb6-24"></a> <span class="kw">friend</span> <span class="kw">constexpr</span> <span class="kw">auto</span> <span class="kw">operator</span><span class="op">|(</span>R<span class="op">&&</span>, view_closure<span class="op"><</span>ViewFn<span class="op">>)</span> <span class="op">=</span> <span class="kw">delete</span>;</span>
<span id="cb6-25"><a href="#cb6-25"></a></span>
<span id="cb6-26"><a href="#cb6-26"></a> <span class="co">// support for C | D to produce a new Range Adaptor Closure Object</span></span>
<span id="cb6-27"><a href="#cb6-27"></a> <span class="co">// so that (R | C) | D and R | (C | D) can be equivalent</span></span>
<span id="cb6-28"><a href="#cb6-28"></a> <span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span> ViewFn, derived_from<span class="op"><</span>detail<span class="op">::</span>view_closure_base_<span class="op">></span> Pipeable<span class="op">></span></span>
<span id="cb6-29"><a href="#cb6-29"></a> <span class="kw">friend</span> <span class="kw">constexpr</span> <span class="kw">auto</span> <span class="kw">operator</span><span class="op">|(</span>view_closure<span class="op"><</span>ViewFn<span class="op">></span> vw, Pipeable pipe<span class="op">)</span> <span class="op">{</span></span>
<span id="cb6-30"><a href="#cb6-30"></a> <span class="co">// produced a new closure, E, such that E(R) == D(C(R))</span></span>
<span id="cb6-31"><a href="#cb6-31"></a> <span class="cf">return</span> view_closure<span class="op">(</span>compose<span class="op">(</span>std<span class="op">::</span>move<span class="op">(</span>pipe<span class="op">)</span>, std<span class="op">::</span>move<span class="op">(</span>vw<span class="op">)))</span>;</span>
<span id="cb6-32"><a href="#cb6-32"></a> <span class="op">}</span></span>
<span id="cb6-33"><a href="#cb6-33"></a> <span class="op">}</span>;</span>
<span id="cb6-34"><a href="#cb6-34"></a><span class="op">}</span></span></code></pre></div>
<p>And with that, we can implement <code class="sourceCode cpp">join</code> and <code class="sourceCode cpp">transform</code> as follows:</p>
<table>
<thead>
<tr class="header">
<th><div style="text-align:center">
<strong><code class="sourceCode cpp">join</code></strong>
</div></th>
<th><div style="text-align:center">
<strong><code class="sourceCode cpp">transform</code></strong>
</div></th>
</tr>
</thead>
<tbody>
<tr class="odd">
<td><div class="sourceCode" id="cb7"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb7-1"><a href="#cb7-1"></a><span class="kw">namespace</span> ranges<span class="op">::</span>views <span class="op">{</span></span>
<span id="cb7-2"><a href="#cb7-2"></a> <span class="kw">struct</span> join_view_fn <span class="op">{</span></span>
<span id="cb7-3"><a href="#cb7-3"></a> <span class="kw">template</span> <span class="op"><</span>viewable_range R<span class="op">></span></span>
<span id="cb7-4"><a href="#cb7-4"></a> <span class="kw">requires</span> <span class="co">/* ... */</span></span>
<span id="cb7-5"><a href="#cb7-5"></a> <span class="kw">constexpr</span> <span class="kw">auto</span> <span class="kw">operator</span><span class="op">(</span>R<span class="op">&&</span> r<span class="op">)</span></span>
<span id="cb7-6"><a href="#cb7-6"></a> <span class="op">-></span> join_view<span class="op"><</span>all_t<span class="op"><</span>R<span class="op">>></span>;</span>
<span id="cb7-7"><a href="#cb7-7"></a> <span class="op">}</span>;</span>
<span id="cb7-8"><a href="#cb7-8"></a></span>
<span id="cb7-9"><a href="#cb7-9"></a> <span class="co">// for user consumption</span></span>
<span id="cb7-10"><a href="#cb7-10"></a> <span class="kw">inline</span> <span class="kw">constexpr</span> view_closure<span class="op"><</span>join_view_fn<span class="op">></span> join<span class="op">{}</span>;</span>
<span id="cb7-11"><a href="#cb7-11"></a><span class="op">}</span></span></code></pre></div></td>
<td><div class="sourceCode" id="cb8"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb8-1"><a href="#cb8-1"></a><span class="kw">namespace</span> ranges<span class="op">::</span>views <span class="op">{</span></span>
<span id="cb8-2"><a href="#cb8-2"></a> <span class="kw">struct</span> transform_view_fn_base <span class="op">{</span></span>
<span id="cb8-3"><a href="#cb8-3"></a> <span class="co">// the overload that has all the information</span></span>
<span id="cb8-4"><a href="#cb8-4"></a> <span class="kw">template</span> <span class="op"><</span>viewable_range E, <span class="kw">typename</span> F<span class="op">></span></span>
<span id="cb8-5"><a href="#cb8-5"></a> <span class="kw">requires</span> <span class="co">/* ... */</span></span>
<span id="cb8-6"><a href="#cb8-6"></a> <span class="kw">constexpr</span> <span class="kw">auto</span> <span class="kw">operator</span><span class="op">()(</span>E<span class="op">&&</span> e, F<span class="op">&&</span> f<span class="op">)</span> <span class="kw">const</span></span>
<span id="cb8-7"><a href="#cb8-7"></a> <span class="op">-></span> transform_view<span class="op"><</span>all_t<span class="op"><</span>E<span class="op">></span>, decay_t<span class="op"><</span>F<span class="op">>></span>;</span>
<span id="cb8-8"><a href="#cb8-8"></a> <span class="op">}</span>;</span>
<span id="cb8-9"><a href="#cb8-9"></a></span>
<span id="cb8-10"><a href="#cb8-10"></a> <span class="kw">struct</span> transform_view_fn</span>
<span id="cb8-11"><a href="#cb8-11"></a> <span class="op">:</span> transform_view_fn_base</span>
<span id="cb8-12"><a href="#cb8-12"></a> <span class="op">{</span></span>
<span id="cb8-13"><a href="#cb8-13"></a> <span class="kw">using</span> transform_view_fn_base<span class="op">::</span><span class="kw">operator</span><span class="op">()</span>;</span>
<span id="cb8-14"><a href="#cb8-14"></a></span>
<span id="cb8-15"><a href="#cb8-15"></a> <span class="co">// the partial overload</span></span>
<span id="cb8-16"><a href="#cb8-16"></a> <span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span> F<span class="op">></span></span>
<span id="cb8-17"><a href="#cb8-17"></a> <span class="kw">constexpr</span> <span class="kw">auto</span> <span class="kw">operator</span><span class="op">()(</span>F f<span class="op">)</span> <span class="kw">const</span> <span class="op">{</span></span>
<span id="cb8-18"><a href="#cb8-18"></a> <span class="cf">return</span> view_closure<span class="op">(</span>bind_back<span class="op">(</span></span>
<span id="cb8-19"><a href="#cb8-19"></a> transform_view_fn_base<span class="op">{}</span>, std<span class="op">::</span>move<span class="op">(</span>f<span class="op">)))</span>;</span>
<span id="cb8-20"><a href="#cb8-20"></a> <span class="op">}</span></span>
<span id="cb8-21"><a href="#cb8-21"></a> <span class="op">}</span>;</span>
<span id="cb8-22"><a href="#cb8-22"></a></span>
<span id="cb8-23"><a href="#cb8-23"></a> <span class="co">// for user consumption</span></span>
<span id="cb8-24"><a href="#cb8-24"></a> <span class="kw">inline</span> <span class="kw">constexpr</span> transform_view_fn transform<span class="op">{}</span>;</span>
<span id="cb8-25"><a href="#cb8-25"></a><span class="op">}</span></span></code></pre></div></td>
</tr>
</tbody>
</table>
<p>Compared to NanoRange, this looks very similar. We have to manually write both overloads for <code class="sourceCode cpp">transform</code>, where the partial overload returns some kind of special library closure object (<code class="sourceCode cpp">view_closure</code> vs <code class="sourceCode cpp">rao_proxy</code>). The primary difference here is that with NanoRange, <code class="sourceCode cpp">join_view_fn</code> needed to be defined in the <code class="sourceCode cpp">nano<span class="op">::</span>detail</code> namespace and then the variable template <code class="sourceCode cpp">is_raco</code> needed to be specialized to <code class="sourceCode cpp"><span class="kw">true</span></code>, while in range-v3, <code class="sourceCode cpp">join_view_fn</code> can actually be in any namespace as long as the <code class="sourceCode cpp">join</code> object itself has type <code class="sourceCode cpp">view_closure<span class="op"><</span>join_view_fn<span class="op">></span></code>.</p>
<h2 data-number="3.3" id="gcc-10"><span class="header-section-number">3.3</span> gcc 10<a href="#gcc-10" class="self-link"></a></h2>
<p>The implementation of pipe support in <span class="citation" data-cites="gcc-10">[<a href="#ref-gcc-10" role="doc-biblioref">gcc-10</a>]</span> is quite different from either NanoRange or range-v3. There, we had two class templates: <code class="sourceCode cpp">__adaptor<span class="op">::</span>_RangeAdaptorClosure<span class="op"><</span>F<span class="op">></span></code> and <code class="sourceCode cpp">__adaptor<span class="op">::</span>_RangeAdaptor<span class="op"><</span>F<span class="op">></span></code>, which represent range adaptor closure objects and range adaptors, respectively.</p>
<p>The latter either invokes <code class="sourceCode cpp">F</code> if possible (to handle the <code class="sourceCode cpp">adaptor<span class="op">(</span>range, args<span class="op">...)</span></code> case) or, if not, returns a <code class="sourceCode cpp">_RangeAdaptorClosure</code> specialization (to handle the <code class="sourceCode cpp">adaptor<span class="op">(</span>args<span class="op">...)</span></code> case). The following implementation is reduced a bit, to simply convey how it works (and to use non-uglified names):</p>
<div class="sourceCode" id="cb9"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb9-1"><a href="#cb9-1"></a><span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span> Callable<span class="op">></span></span>
<span id="cb9-2"><a href="#cb9-2"></a><span class="kw">struct</span> _RangeAdaptor <span class="op">{</span></span>
<span id="cb9-3"><a href="#cb9-3"></a> <span class="op">[[</span><span class="at">no_unique_address</span><span class="op">]]</span> Callable callable;</span>
<span id="cb9-4"><a href="#cb9-4"></a></span>
<span id="cb9-5"><a href="#cb9-5"></a> <span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span><span class="op">...</span> Args<span class="op">></span></span>
<span id="cb9-6"><a href="#cb9-6"></a> <span class="kw">requires</span> <span class="op">(</span><span class="kw">sizeof</span><span class="op">...(</span>Args<span class="op">)</span> <span class="op">>=</span> <span class="dv">1</span><span class="op">)</span></span>
<span id="cb9-7"><a href="#cb9-7"></a> <span class="kw">constexpr</span> <span class="kw">auto</span> <span class="kw">operator</span><span class="op">()(</span>Args<span class="op">&&...</span> args<span class="op">)</span> <span class="kw">const</span> <span class="op">{</span></span>
<span id="cb9-8"><a href="#cb9-8"></a> <span class="cf">if</span> <span class="kw">constexpr</span> <span class="op">(</span>invocable<span class="op"><</span>Callable, Args<span class="op">...>)</span> <span class="op">{</span></span>
<span id="cb9-9"><a href="#cb9-9"></a> <span class="co">// The adaptor(range, args...) case</span></span>
<span id="cb9-10"><a href="#cb9-10"></a> <span class="cf">return</span> callable<span class="op">(</span>FWD<span class="op">(</span>args<span class="op">)...)</span>;</span>
<span id="cb9-11"><a href="#cb9-11"></a> <span class="op">}</span> <span class="cf">else</span> <span class="op">{</span></span>
<span id="cb9-12"><a href="#cb9-12"></a> <span class="co">// The adaptor(args...)(range) case</span></span>
<span id="cb9-13"><a href="#cb9-13"></a> <span class="cf">return</span> _RangeAdaptorClosure<span class="op">(</span></span>
<span id="cb9-14"><a href="#cb9-14"></a> <span class="op">[...</span>args<span class="op">=</span>FWD<span class="op">(</span>args<span class="op">)</span>, callable<span class="op">]<</span><span class="kw">typename</span> R<span class="op">>(</span>R<span class="op">&&</span> r<span class="op">){</span></span>
<span id="cb9-15"><a href="#cb9-15"></a> <span class="cf">return</span> callable<span class="op">(</span>FWD<span class="op">(</span>r<span class="op">)</span>, args<span class="op">...)</span>;</span>
<span id="cb9-16"><a href="#cb9-16"></a> <span class="op">})</span>;</span>
<span id="cb9-17"><a href="#cb9-17"></a> <span class="op">}</span></span>
<span id="cb9-18"><a href="#cb9-18"></a> <span class="op">}</span></span>
<span id="cb9-19"><a href="#cb9-19"></a><span class="op">}</span>;</span></code></pre></div>
<p>The former provides piping support:</p>
<div class="sourceCode" id="cb10"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb10-1"><a href="#cb10-1"></a><span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span> Callable<span class="op">></span></span>
<span id="cb10-2"><a href="#cb10-2"></a><span class="kw">struct</span> _RangeAdaptorClosure <span class="op">:</span> _RangeAdaptor<span class="op"><</span>Callable<span class="op">></span></span>
<span id="cb10-3"><a href="#cb10-3"></a><span class="op">{</span></span>
<span id="cb10-4"><a href="#cb10-4"></a> <span class="co">// support for C(R)</span></span>
<span id="cb10-5"><a href="#cb10-5"></a> <span class="kw">template</span> <span class="op"><</span>viewable_range R<span class="op">></span> <span class="kw">requires</span> invocable<span class="op"><</span>Callable, R<span class="op">></span></span>
<span id="cb10-6"><a href="#cb10-6"></a> <span class="kw">constexpr</span> <span class="kw">auto</span> <span class="kw">operator</span><span class="op">()(</span>R<span class="op">&&</span> r<span class="op">)</span> <span class="kw">const</span> <span class="op">{</span></span>
<span id="cb10-7"><a href="#cb10-7"></a> <span class="cf">return</span> callable<span class="op">(</span>FWD<span class="op">(</span>r<span class="op">))</span>;</span>
<span id="cb10-8"><a href="#cb10-8"></a> <span class="op">}</span></span>
<span id="cb10-9"><a href="#cb10-9"></a></span>
<span id="cb10-10"><a href="#cb10-10"></a> <span class="co">// support for R | C to evaluate as C(R)</span></span>
<span id="cb10-11"><a href="#cb10-11"></a> <span class="kw">template</span> <span class="op"><</span>viewable_range R<span class="op">></span> <span class="kw">requires</span> invocable<span class="op"><</span>Callable, R<span class="op">></span></span>
<span id="cb10-12"><a href="#cb10-12"></a> <span class="kw">friend</span> <span class="kw">constexpr</span> <span class="kw">auto</span> <span class="kw">operator</span><span class="op">|(</span>R<span class="op">&&</span> r, _RangeAdaptorClosure <span class="kw">const</span><span class="op">&</span> o<span class="op">)</span> <span class="op">{</span></span>
<span id="cb10-13"><a href="#cb10-13"></a> <span class="cf">return</span> o<span class="op">.</span>callable<span class="op">(</span>FWD<span class="op">(</span>r<span class="op">))</span>;</span>
<span id="cb10-14"><a href="#cb10-14"></a> <span class="op">}</span></span>
<span id="cb10-15"><a href="#cb10-15"></a></span>
<span id="cb10-16"><a href="#cb10-16"></a> <span class="co">// support for C | D to produce a new Range Adaptor Closure Object</span></span>
<span id="cb10-17"><a href="#cb10-17"></a> <span class="co">// so that (R | C) | D and R | (C | D) can be equivalent</span></span>
<span id="cb10-18"><a href="#cb10-18"></a> <span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span> T<span class="op">></span></span>
<span id="cb10-19"><a href="#cb10-19"></a> <span class="kw">friend</span> <span class="kw">constexpr</span> <span class="kw">auto</span> <span class="kw">operator</span><span class="op">|(</span>_RangeAdaptorClosure<span class="op"><</span>T<span class="op">></span> <span class="kw">const</span><span class="op">&</span> lhs, _RangeAdaptorClosure <span class="kw">const</span><span class="op">&</span> rhs<span class="op">)</span> <span class="op">{</span></span>
<span id="cb10-20"><a href="#cb10-20"></a> <span class="cf">return</span> _RangeAdaptorClosure<span class="op">([</span>lhs, rhs<span class="op">]<</span><span class="kw">typename</span> R<span class="op">>(</span>R<span class="op">&&</span> r<span class="op">){</span></span>
<span id="cb10-21"><a href="#cb10-21"></a> <span class="cf">return</span> FWD<span class="op">(</span>r<span class="op">)</span> <span class="op">|</span> lhs <span class="op">|</span> rhs;</span>
<span id="cb10-22"><a href="#cb10-22"></a> <span class="op">})</span>;</span>
<span id="cb10-23"><a href="#cb10-23"></a> <span class="op">}</span></span>
<span id="cb10-24"><a href="#cb10-24"></a><span class="op">}</span>;</span></code></pre></div>
<p>And with that, we can implement <code class="sourceCode cpp">join</code> and <code class="sourceCode cpp">transform</code> as follows:</p>
<table>
<thead>
<tr class="header">
<th><div style="text-align:center">
<strong><code class="sourceCode cpp">join</code></strong>
</div></th>
<th><div style="text-align:center">
<strong><code class="sourceCode cpp">transform</code></strong>
</div></th>
</tr>
</thead>
<tbody>
<tr class="odd">
<td><div class="sourceCode" id="cb11"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb11-1"><a href="#cb11-1"></a><span class="kw">namespace</span> std<span class="op">::</span>ranges<span class="op">::</span>views <span class="op">{</span></span>
<span id="cb11-2"><a href="#cb11-2"></a> <span class="co">// for user consumption</span></span>
<span id="cb11-3"><a href="#cb11-3"></a> <span class="kw">inline</span> <span class="kw">constexpr</span> __adaptor<span class="op">::</span>_RangeAdaptorClosure join</span>
<span id="cb11-4"><a href="#cb11-4"></a> <span class="op">=</span> <span class="op">[]<</span>viewable_range R<span class="op">></span> <span class="kw">requires</span> <span class="co">/* ... */</span></span>
<span id="cb11-5"><a href="#cb11-5"></a> <span class="op">(</span>R<span class="op">&&</span> r<span class="op">)</span> <span class="op">{</span></span>
<span id="cb11-6"><a href="#cb11-6"></a> <span class="cf">return</span> join_view<span class="op">(</span>FWD<span class="op">(</span>r<span class="op">))</span>;</span>
<span id="cb11-7"><a href="#cb11-7"></a> <span class="op">}</span>;</span>
<span id="cb11-8"><a href="#cb11-8"></a><span class="op">}</span></span></code></pre></div></td>
<td><div class="sourceCode" id="cb12"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb12-1"><a href="#cb12-1"></a><span class="kw">namespace</span> std<span class="op">::</span>ranges<span class="op">::</span>views <span class="op">{</span></span>
<span id="cb12-2"><a href="#cb12-2"></a> <span class="co">// for user consumption</span></span>
<span id="cb12-3"><a href="#cb12-3"></a> <span class="kw">inline</span> <span class="kw">constexpr</span> __adaptor<span class="op">::</span>_RangeAdaptor transform</span>
<span id="cb12-4"><a href="#cb12-4"></a> <span class="op">=</span> <span class="op">[]<</span>viewable_range R, <span class="kw">typename</span> F<span class="op">></span></span>
<span id="cb12-5"><a href="#cb12-5"></a> <span class="kw">requires</span> <span class="co">/* ... */</span></span>
<span id="cb12-6"><a href="#cb12-6"></a> <span class="op">(</span>R<span class="op">&&</span> r, F<span class="op">&&</span> f<span class="op">){</span></span>
<span id="cb12-7"><a href="#cb12-7"></a> <span class="cf">return</span> transform_view<span class="op">(</span>FWD<span class="op">(</span>r<span class="op">)</span>, FWD<span class="op">(</span>f<span class="op">))</span>;</span>
<span id="cb12-8"><a href="#cb12-8"></a> <span class="op">}</span>;</span>
<span id="cb12-9"><a href="#cb12-9"></a><span class="op">}</span></span></code></pre></div></td>
</tr>
</tbody>
</table>
<p>Compared to either NanoRange or range-v3, this implementation strategy has the significant advantage that we don’t have to write both overloads of <code class="sourceCode cpp">transform</code> manually: we just write a single lambda and use class template argument deduction to wrap its type in the right facility (<code class="sourceCode cpp">_RangeAdaptorClosure</code> for <code class="sourceCode cpp">join</code> and <code class="sourceCode cpp">_RangeAdaptor</code> for <code class="sourceCode cpp">transform</code>) to provide <code class="sourceCode cpp"><span class="op">|</span></code> support.</p>
<p>This becomes clearer if we look at gcc 10’s implementation of <code class="sourceCode cpp">views<span class="op">::</span>transform</code> vs range-v3’s directly:</p>
<table>
<thead>
<tr class="header">
<th><div style="text-align:center">
<strong>range-v3</strong>
</div></th>
<th><div style="text-align:center">
<strong>gcc 10</strong>
</div></th>
</tr>
</thead>
<tbody>
<tr class="odd">
<td><div class="sourceCode" id="cb13"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb13-1"><a href="#cb13-1"></a><span class="kw">namespace</span> ranges<span class="op">::</span>views <span class="op">{</span></span>
<span id="cb13-2"><a href="#cb13-2"></a> <span class="kw">struct</span> transform_view_fn_base <span class="op">{</span></span>
<span id="cb13-3"><a href="#cb13-3"></a> <span class="co">// the overload that has all the information</span></span>
<span id="cb13-4"><a href="#cb13-4"></a> <span class="kw">template</span> <span class="op"><</span>viewable_range E, <span class="kw">typename</span> F<span class="op">></span></span>
<span id="cb13-5"><a href="#cb13-5"></a> <span class="kw">requires</span> <span class="co">/* ... */</span></span>
<span id="cb13-6"><a href="#cb13-6"></a> <span class="kw">constexpr</span> <span class="kw">auto</span> <span class="kw">operator</span><span class="op">()(</span>E<span class="op">&&</span> e, F<span class="op">&&</span> f<span class="op">)</span> <span class="kw">const</span></span>
<span id="cb13-7"><a href="#cb13-7"></a> <span class="op">-></span> transform_view<span class="op"><</span>all_t<span class="op"><</span>E<span class="op">></span>, decay_t<span class="op"><</span>F<span class="op">>></span>;</span>
<span id="cb13-8"><a href="#cb13-8"></a> <span class="op">}</span>;</span>
<span id="cb13-9"><a href="#cb13-9"></a></span>
<span id="cb13-10"><a href="#cb13-10"></a> <span class="kw">struct</span> transform_view_fn</span>
<span id="cb13-11"><a href="#cb13-11"></a> <span class="op">:</span> transform_view_fn_base</span>
<span id="cb13-12"><a href="#cb13-12"></a> <span class="op">{</span></span>
<span id="cb13-13"><a href="#cb13-13"></a> <span class="kw">using</span> transform_view_fn_base<span class="op">::</span><span class="kw">operator</span><span class="op">()</span>;</span>
<span id="cb13-14"><a href="#cb13-14"></a></span>
<span id="cb13-15"><a href="#cb13-15"></a> <span class="co">// the partial overload</span></span>
<span id="cb13-16"><a href="#cb13-16"></a> <span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span> F<span class="op">></span></span>
<span id="cb13-17"><a href="#cb13-17"></a> <span class="kw">constexpr</span> <span class="kw">auto</span> <span class="kw">operator</span><span class="op">()(</span>F f<span class="op">)</span> <span class="kw">const</span> <span class="op">{</span></span>
<span id="cb13-18"><a href="#cb13-18"></a> <span class="cf">return</span> view_closure<span class="op">(</span>bind_back<span class="op">(</span></span>
<span id="cb13-19"><a href="#cb13-19"></a> transform_view_fn_base<span class="op">()</span>, std<span class="op">::</span>move<span class="op">(</span>f<span class="op">)))</span>;</span>
<span id="cb13-20"><a href="#cb13-20"></a> <span class="op">}</span></span>
<span id="cb13-21"><a href="#cb13-21"></a> <span class="op">}</span>;</span>
<span id="cb13-22"><a href="#cb13-22"></a></span>
<span id="cb13-23"><a href="#cb13-23"></a> <span class="co">// for user consumption</span></span>
<span id="cb13-24"><a href="#cb13-24"></a> <span class="kw">inline</span> <span class="kw">constexpr</span> transform_view_fn transform<span class="op">{}</span>;</span>
<span id="cb13-25"><a href="#cb13-25"></a><span class="op">}</span></span></code></pre></div></td>
<td><div class="sourceCode" id="cb14"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb14-1"><a href="#cb14-1"></a><span class="kw">namespace</span> std<span class="op">::</span>ranges<span class="op">::</span>views <span class="op">{</span></span>
<span id="cb14-2"><a href="#cb14-2"></a> <span class="co">// for user consumption</span></span>
<span id="cb14-3"><a href="#cb14-3"></a> <span class="kw">inline</span> <span class="kw">constexpr</span> __adaptor<span class="op">::</span>_RangeAdaptor transform</span>
<span id="cb14-4"><a href="#cb14-4"></a> <span class="op">=</span> <span class="op">[]<</span>viewable_range R, <span class="kw">typename</span> F<span class="op">></span></span>
<span id="cb14-5"><a href="#cb14-5"></a> <span class="kw">requires</span> <span class="co">/* ... */</span></span>
<span id="cb14-6"><a href="#cb14-6"></a> <span class="op">(</span>R<span class="op">&&</span> r, F<span class="op">&&</span> f<span class="op">){</span></span>
<span id="cb14-7"><a href="#cb14-7"></a> <span class="cf">return</span> transform_view<span class="op">(</span>FWD<span class="op">(</span>r<span class="op">)</span>, FWD<span class="op">(</span>f<span class="op">))</span>;</span>
<span id="cb14-8"><a href="#cb14-8"></a> <span class="op">}</span>;</span>
<span id="cb14-9"><a href="#cb14-9"></a><span class="op">}</span></span></code></pre></div></td>
</tr>
</tbody>
</table>
<h2 data-number="3.4" id="gcc-11"><span class="header-section-number">3.4</span> gcc 11<a href="#gcc-11" class="self-link"></a></h2>
<p>The implementation of pipe support in <span class="citation" data-cites="gcc-11">[<a href="#ref-gcc-11" role="doc-biblioref">gcc-11</a>]</span> is closer to the range-v3/NanoRange implementations than the gcc 10 one.</p>
<p>In this implementation, <code class="sourceCode cpp">_RangeAdaptorClosure</code> is an empty type that is the base class of every range adaptor closure, equivalent to range-v3’s <code class="sourceCode cpp">view_closure_base</code>:</p>
<div class="sourceCode" id="cb15"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb15-1"><a href="#cb15-1"></a><span class="kw">struct</span> _RangeAdaptorClosure <span class="op">{</span></span>
<span id="cb15-2"><a href="#cb15-2"></a> <span class="co">// support for R | C to evaluate as C(R)</span></span>
<span id="cb15-3"><a href="#cb15-3"></a> <span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span> Range, <span class="kw">typename</span> Self<span class="op">></span></span>
<span id="cb15-4"><a href="#cb15-4"></a> <span class="kw">requires</span> derived_from<span class="op"><</span>remove_cvref_t<span class="op"><</span>Self<span class="op">></span>, _RangeAdaptorClosure<span class="op">></span></span>
<span id="cb15-5"><a href="#cb15-5"></a> <span class="op">&&</span> invocable<span class="op"><</span>Self, Range<span class="op">></span></span>
<span id="cb15-6"><a href="#cb15-6"></a> <span class="kw">friend</span> <span class="kw">constexpr</span> <span class="kw">auto</span> <span class="kw">operator</span><span class="op">|(</span>Range<span class="op">&&</span> r, Self<span class="op">&&</span> self<span class="op">)</span> <span class="op">{</span></span>
<span id="cb15-7"><a href="#cb15-7"></a> <span class="cf">return</span> FWD<span class="op">(</span>self<span class="op">)(</span>FWD<span class="op">(</span>r<span class="op">))</span>;</span>
<span id="cb15-8"><a href="#cb15-8"></a> <span class="op">}</span></span>
<span id="cb15-9"><a href="#cb15-9"></a></span>
<span id="cb15-10"><a href="#cb15-10"></a> <span class="co">// support for C | D to produce a new Range Adaptor Closure Object</span></span>
<span id="cb15-11"><a href="#cb15-11"></a> <span class="co">// so that (R | C) | D and R | (C | D) can be equivalent</span></span>
<span id="cb15-12"><a href="#cb15-12"></a> <span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span> Lhs, <span class="kw">typename</span> Rhs<span class="op">></span></span>
<span id="cb15-13"><a href="#cb15-13"></a> <span class="kw">requires</span> derived_from<span class="op"><</span>Lhs, _RangeAdaptorClosure<span class="op">></span></span>
<span id="cb15-14"><a href="#cb15-14"></a> <span class="op">&&</span> derived_from<span class="op"><</span>Rhs, _RangeAdaptorClosure<span class="op">></span></span>
<span id="cb15-15"><a href="#cb15-15"></a> <span class="kw">friend</span> <span class="kw">constexpr</span> <span class="kw">auto</span> <span class="kw">operator</span><span class="op">|(</span>Lhs lhs, Rhs rhs<span class="op">)</span> <span class="op">{</span></span>
<span id="cb15-16"><a href="#cb15-16"></a> <span class="cf">return</span> _Pipe<span class="op"><</span>Lhs, Rhs<span class="op">>(</span>std<span class="op">::</span>move<span class="op">(</span>lhs<span class="op">)</span>, std<span class="op">::</span>move<span class="op">(</span>rhs<span class="op">))</span>;</span>
<span id="cb15-17"><a href="#cb15-17"></a> <span class="op">}</span></span>
<span id="cb15-18"><a href="#cb15-18"></a><span class="op">}</span>;</span></code></pre></div>
<p><code class="sourceCode cpp">_RangeAdaptor</code> is a CRTP template that is a base class of every range adaptor object (not range adaptor closure):</p>
<div class="sourceCode" id="cb16"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb16-1"><a href="#cb16-1"></a><span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span> Derived<span class="op">></span></span>
<span id="cb16-2"><a href="#cb16-2"></a><span class="kw">struct</span> _RangeAdaptor <span class="op">{</span></span>
<span id="cb16-3"><a href="#cb16-3"></a> <span class="co">// provides the partial overload</span></span>
<span id="cb16-4"><a href="#cb16-4"></a> <span class="co">// such that adaptor(args...)(range) is equivalent to adaptor(range, args...)</span></span>
<span id="cb16-5"><a href="#cb16-5"></a> <span class="co">// _Partial<Adaptor, Args...> is a _RangeAdaptorClosure</span></span>
<span id="cb16-6"><a href="#cb16-6"></a> <span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span><span class="op">...</span> Args<span class="op">></span></span>
<span id="cb16-7"><a href="#cb16-7"></a> <span class="kw">requires</span> <span class="op">(</span>Derived<span class="op">::</span>arity <span class="op">></span> <span class="dv">1</span><span class="op">)</span></span>
<span id="cb16-8"><a href="#cb16-8"></a> <span class="op">&&</span> <span class="op">(</span><span class="kw">sizeof</span><span class="op">...(</span>Args<span class="op">)</span> <span class="op">==</span> Derived<span class="op">::</span>arity <span class="op">-</span> <span class="dv">1</span><span class="op">)</span></span>
<span id="cb16-9"><a href="#cb16-9"></a> <span class="op">&&</span> <span class="op">(</span>constructible_from<span class="op"><</span>decay_t<span class="op"><</span>Args<span class="op">></span>, Args<span class="op">></span> <span class="op">&&</span> <span class="op">...)</span></span>
<span id="cb16-10"><a href="#cb16-10"></a> <span class="kw">constexpr</span> <span class="kw">auto</span> <span class="kw">operator</span><span class="op">()(</span>Args<span class="op">&&...</span> args<span class="op">)</span> <span class="kw">const</span> <span class="op">{</span></span>
<span id="cb16-11"><a href="#cb16-11"></a> <span class="cf">return</span> _Partial<span class="op"><</span>Derived, decay_t<span class="op"><</span>Args<span class="op">>...>(</span>FWD<span class="op">(</span>args<span class="op">)...)</span>;</span>
<span id="cb16-12"><a href="#cb16-12"></a> <span class="op">}</span></span>
<span id="cb16-13"><a href="#cb16-13"></a><span class="op">}</span>;</span></code></pre></div>
<p>The interesting point here is that every adaptor has to specify an <code class="sourceCode cpp">arity</code>, and the partial call must take all but one of those arguments. As we’ll see shortly, <code class="sourceCode cpp">transform</code> has arity <code class="sourceCode cpp"><span class="dv">2</span></code> and so this call operator is only viable for a single argument. As such, the library still implements every partial call, but it requires more input from the adaptor declaration itself.</p>
<p>The types <code class="sourceCode cpp">_Pipe<span class="op"><</span>T, U<span class="op">></span></code> and <code class="sourceCode cpp">_Partial<span class="op"><</span>D, Args<span class="op">...></span></code> are both <code class="sourceCode cpp">_RangeAdaptorClosure</code>s that provide call operators that accept a <code class="sourceCode cpp">viewable_range</code> and eagerly invoke the appropriate functions (both, in the case of <code class="sourceCode cpp">_Pipe</code>, and a <code class="sourceCode cpp">bind_back</code>, in the case of <code class="sourceCode cpp">_Partial</code>). Both types have appeared in other implementations already.</p>
<p>And with that, we can implement <code class="sourceCode cpp">join</code> and <code class="sourceCode cpp">transform</code> as follows:</p>
<table>
<thead>
<tr class="header">
<th><div style="text-align:center">
<strong><code class="sourceCode cpp">join</code></strong>
</div></th>
<th><div style="text-align:center">
<strong><code class="sourceCode cpp">transform</code></strong>
</div></th>
</tr>