-
Notifications
You must be signed in to change notification settings - Fork 22
/
p0848r3.html
725 lines (711 loc) · 43.9 KB
/
p0848r3.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
<!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="2019-07-16" />
<title>Conditionally Trivial Special Member Functions</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%;}
</style>
<style>
code.sourceCode > span { display: inline-block; line-height: 1.25; }
code.sourceCode > span { color: inherit; text-decoration: inherit; }
code.sourceCode > span:empty { height: 1.2em; }
.sourceCode { overflow: visible; }
code.sourceCode { white-space: pre; position: relative; }
div.sourceCode { margin: 1em 0; }
pre.sourceCode { margin: 0; }
@media screen {
div.sourceCode { overflow: auto; }
}
@media print {
code.sourceCode { white-space: pre-wrap; }
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 {
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; }
div#refs p { padding-left: 32px; text-indent: -32px; }
</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;
}
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; }
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/vnd.microsoft.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">Conditionally Trivial Special Member Functions</h1>
<table style="border:none;float:right">
<tr>
<td>Document #: </td>
<td>P0848R3</td>
</tr>
<tr>
<td>Date: </td>
<td>2019-07-16</td>
</tr>
<tr>
<td style="vertical-align:top">Project: </td>
<td>Programming Language C++<br>
CWG<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>
Casey Carter<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="#proposal"><span class="toc-section-number">3</span> Proposal<span></span></a></li>
<li><a href="#wording"><span class="toc-section-number">4</span> Wording<span></span></a></li>
<li><a href="#acknowledgments"><span class="toc-section-number">5</span> Acknowledgments<span></span></a></li>
<li><a href="#references"><span class="toc-section-number">6</span> References<span></span></a></li>
</ul>
</div>
<h1 id="revision-history" style="border-bottom:1px solid #cccccc"><span class="header-section-number">1</span> Revision History<a href="#revision-history" class="self-link"></a></h1>
<p><span class="citation" data-cites="P0848R0">[<a href="#ref-P0848R0" role="doc-biblioref">P0848R0</a>]</span> was presented to EWG at the San Diego meeting (November 2018). It proposed to change the rules for trivially copyable such that we only consider the <em>best viable candidate</em> amongst the copy constructors given a synthesized overload resolution. That is, depending on <code class="sourceCode cpp">C<span class="op"><</span>T<span class="op">></span></code>, we either consider only <code class="sourceCode cpp"><span class="pp">#2</span></code> (because <code class="sourceCode cpp"><span class="pp">#1</span></code> wouldn’t be viable) or only <code class="sourceCode cpp"><span class="pp">#1</span></code> (because it would be more constrained than <code class="sourceCode cpp"><span class="pp">#2</span></code> and hence a better match). However, EWG considered this to be confusing as trivially copyable is a property of a type and adding overload resolution simply adds more questions about context (e.g. do we consider accessibility?). EWG requested a new mechanism to solve this problem.</p>
<p><span class="citation" data-cites="P0848R1">[<a href="#ref-P0848R1" role="doc-biblioref">P0848R1</a>]</span> was also presented to EWG in San Diego, proposed to introduce an intermediate layer: a constructor for a class <code class="sourceCode cpp">X</code> of the form <code class="sourceCode cpp">X<span class="op">(</span>X <span class="kw">const</span><span class="op">&)</span></code> rather than being considered a copy constructor would instead become a <em>prospective copy constructor</em>, and a <em>copy constructor</em> would be any prospective copy constructor whose constraints are satisfied and is at least as constrained as every other prospective copy constructor. In this way, we don’t change the definition of trivially copyable at all - we change the definition for each of the special member functions.</p>
<p>During Core wording review in the Kona meeting (February 2019), a new direction was suggested that is somewhat of a compromise between the two other positions: we use the “constraints are satisfied and is at least as constrained as” rule on the special member functions, but we only apply it to the trivially copyable rules. This ensures that we maintain these as properties of the type rather than properties of an expression - and is the direction this paper takes.</p>
<h1 id="introduction" style="border-bottom:1px solid #cccccc"><span class="header-section-number">2</span> Introduction<a href="#introduction" class="self-link"></a></h1>
<p>For a complete motivation for this proposal, see <span class="citation" data-cites="P0848R0">[<a href="#ref-P0848R0" role="doc-biblioref">P0848R0</a>]</span>. In brief, it is important for certain class template instantiations to propagate the triviality from their template parameters - that is, we want <code class="sourceCode cpp">wrapper<span class="op"><</span>T<span class="op">></span></code> to be trivially copyable if and only if <code class="sourceCode cpp">T</code> is copyable. In C++17, this is possible, yet is extremely verbose. The introduction of Concepts provides a path to make this propagation substantially easier to write, but the current definition of trivially copyable doesn’t quite suffice for what we want.</p>
<p>Consider:</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">template</span> <span class="op"><</span><span class="kw">typename</span> T<span class="op">></span></span>
<span id="cb1-2"><a href="#cb1-2"></a><span class="kw">concept</span> C <span class="op">=</span> <span class="co">/* ... */</span>;</span>
<span id="cb1-3"><a href="#cb1-3"></a></span>
<span id="cb1-4"><a href="#cb1-4"></a><span class="kw">template</span> <span class="op"><</span><span class="kw">typename</span> T<span class="op">></span></span>
<span id="cb1-5"><a href="#cb1-5"></a><span class="kw">struct</span> X <span class="op">{</span></span>
<span id="cb1-6"><a href="#cb1-6"></a> <span class="co">// #1</span></span>
<span id="cb1-7"><a href="#cb1-7"></a> X<span class="op">(</span>X <span class="kw">const</span><span class="op">&)</span> <span class="kw">requires</span> C<span class="op"><</span>T<span class="op">></span> <span class="op">=</span> <span class="cf">default</span>;</span>
<span id="cb1-8"><a href="#cb1-8"></a> </span>
<span id="cb1-9"><a href="#cb1-9"></a> <span class="co">// #2</span></span>
<span id="cb1-10"><a href="#cb1-10"></a> X<span class="op">(</span>X <span class="kw">const</span><span class="op">&</span> <span class="op">)</span> <span class="op">{</span> <span class="co">/* ... */</span> <span class="op">}</span></span>
<span id="cb1-11"><a href="#cb1-11"></a><span class="op">}</span>;</span></code></pre></div>
<p>According to the current working draft, both <code class="sourceCode cpp"><span class="pp">#1</span></code> and <code class="sourceCode cpp"><span class="pp">#2</span></code> are copy constructors. The current definition for trivially copyable requires that <em>each</em> copy constructor be either deleted or trivial. That is, we always consider both copy constructors, regardless of <code class="sourceCode cpp">T</code> and <code class="sourceCode cpp">C<span class="op"><</span>T<span class="op">></span></code>, and hence no instantation of <code class="sourceCode cpp">X</code> is ever trivially copyable.</p>
<p>This paper suggests that those specializations <code class="sourceCode cpp">X<span class="op"><</span>T<span class="op">></span></code> for which <code class="sourceCode cpp">T</code> satisfies <code class="sourceCode cpp">C</code> should be considered trivially copyable - this very logically follows the model of constraints and would make it substantially easier to write class templates that are conditionally trivially copyable. Indeed, it would become actually easy to do such a thing rather than quite complex with multiple layers of conditional base classes.</p>
<h1 id="proposal" style="border-bottom:1px solid #cccccc"><span class="header-section-number">3</span> Proposal<a href="#proposal" class="self-link"></a></h1>
<p>The current relevant definitions in [class.prop] read:</p>
<blockquote>
<p>A <em>trivially copyable class</em> is a class:</p>
<ul>
<li>where each copy constructor, move constructor, copy assignment operator, and move assignment operator ([class.copy.ctor], [class.copy.assign]) is either deleted or trivial,</li>
<li>that has at least one non-deleted copy constructor, move constructor, copy assignment operator, or move assignment operator, and</li>
<li>that has a trivial, non-deleted destructor.</li>
</ul>
</blockquote>
<blockquote>
<p>A <em>trivial class</em> is a class that is trivially copyable and has one or more default constructors ([class.default.ctor]), all of which are either trivial or deleted and at least one of which is not deleted.</p>
</blockquote>
<p>There are two aspects of these definitions that need to be changed for this proposal: one that applies to five of the special member functions in the same way (the copy and move constructors and assignment operators and the default constructor), and one that applies specifically to the destructor.</p>
<p>For the five non-destructor special member functions, we introduce a new concept called an <em>eligible special member function</em> - which is a special member function that is:</p>
<ul>
<li>not deleted</li>
<li>has all of its constraints (if any) satisfied</li>
<li>no special member function of the same kind, with the same first parameter type (except for the default constructor), is more constrained</li>
</ul>
<p>And then simplify the definitions of <em>trivially copyable</em> and <em>trivial</em> to use eligible special members instead of just any special members.</p>
<p>For the destructor, we go in a slightly different direction. We introduce a sub-classification called a <em>prospective destructor</em> which is simply any function declared for a class <code class="sourceCode cpp">X</code> that is spelled <code class="sourceCode cpp"><span class="op">~</span>X<span class="op">()</span></code> with some constraints, and then redefine destructor to be the eligible destructor, requiring that there be only one.</p>
<p>Using the example from R0:</p>
<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">template</span> <span class="op"><</span><span class="kw">typename</span> T<span class="op">></span></span>
<span id="cb2-2"><a href="#cb2-2"></a><span class="kw">struct</span> optional <span class="op">{</span></span>
<span id="cb2-3"><a href="#cb2-3"></a> <span class="co">// #1</span></span>
<span id="cb2-4"><a href="#cb2-4"></a> optional<span class="op">(</span>optional <span class="kw">const</span><span class="op">&)</span></span>
<span id="cb2-5"><a href="#cb2-5"></a> <span class="kw">requires</span> TriviallyCopyConstructible<span class="op"><</span>T<span class="op">></span> <span class="op">&&</span> CopyConstructible<span class="op"><</span>T<span class="op">></span></span>
<span id="cb2-6"><a href="#cb2-6"></a> <span class="op">=</span> <span class="cf">default</span>;</span>
<span id="cb2-7"><a href="#cb2-7"></a> </span>
<span id="cb2-8"><a href="#cb2-8"></a> <span class="co">// #2</span></span>
<span id="cb2-9"><a href="#cb2-9"></a> optional<span class="op">(</span>optional <span class="kw">const</span><span class="op">&</span> rhs<span class="op">)</span></span>
<span id="cb2-10"><a href="#cb2-10"></a> <span class="kw">requires</span> CopyConstructible<span class="op"><</span>T<span class="op">></span></span>
<span id="cb2-11"><a href="#cb2-11"></a> <span class="op">:</span> engaged<span class="op">(</span>rhs<span class="op">.</span>engaged<span class="op">)</span></span>
<span id="cb2-12"><a href="#cb2-12"></a> <span class="op">{</span></span>
<span id="cb2-13"><a href="#cb2-13"></a> <span class="cf">if</span> <span class="op">(</span>engaged<span class="op">)</span> <span class="op">{</span></span>
<span id="cb2-14"><a href="#cb2-14"></a> <span class="kw">new</span> <span class="op">(</span>value<span class="op">)</span> T<span class="op">(</span>rhs<span class="op">.</span>value<span class="op">)</span>;</span>
<span id="cb2-15"><a href="#cb2-15"></a> <span class="op">}</span></span>
<span id="cb2-16"><a href="#cb2-16"></a> <span class="op">}</span></span>
<span id="cb2-17"><a href="#cb2-17"></a><span class="op">}</span>;</span></code></pre></div>
<p>For all specializations, we have two copy constructors: <code class="sourceCode cpp"><span class="pp">#1</span></code> and <code class="sourceCode cpp"><span class="pp">#2</span></code>. For <code class="sourceCode cpp">T<span class="op">=</span>unique_ptr<span class="op"><</span><span class="dt">int</span><span class="op">></span></code>, neither copy constructor has its constraints satisfied so there is no eligible copy constructor. For <code class="sourceCode cpp">T<span class="op">=</span>std<span class="op">::</span>string</code>, only <code class="sourceCode cpp"><span class="pp">#2</span></code> has its constraints satisfied, so only <code class="sourceCode cpp"><span class="pp">#2</span></code> is an eligible copy constructor. For <code class="sourceCode cpp">T<span class="op">=</span><span class="dt">int</span></code>, both <code class="sourceCode cpp"><span class="pp">#1</span></code> and <code class="sourceCode cpp"><span class="pp">#2</span></code> have their constraints satisfied. <code class="sourceCode cpp"><span class="pp">#1</span></code> is more constrained than <code class="sourceCode cpp"><span class="pp">#2</span></code>, so only <code class="sourceCode cpp"><span class="pp">#1</span></code> is an eligible copy constructor.</p>
<h1 id="wording" style="border-bottom:1px solid #cccccc"><span class="header-section-number">4</span> Wording<a href="#wording" class="self-link"></a></h1>
<p>Change 6.6.7 [class.temporary] to refer to the future definition of eligibility:</p>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized">3</a></span> When an object of class type X is passed to or returned from a function, if <span class="rm" style="color: #bf0303"><del>each copy constructor, move constructor, and destructor of X is either trivial or deleted,</del></span> <span class="addu"><code class="sourceCode cpp">X</code> has at least one one eligible copy or move constructor ([class.prop]), each such constructor is trivial, and the destructor of <code class="sourceCode cpp">X</code> is either trivial or deleted,</span> implementations are permitted to create a temporary object to hold the function parameter or result object. The temporary object is constructed from the function argument or return value, respectively, and the function’s parameter or return object is initialized as if by using the <span class="rm" style="color: #bf0303"><del>non-deleted</del></span> <span class="addu">eligible</span> trivial constructor to copy the temporary (even if that constructor is inaccessible or would not be selected by overload resolution to perform a copy or move of the object).</p>
</blockquote>
<p>And likewise for 7.6.1.2 [expr.call]:</p>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized">12</a></span> […] Passing a potentially-evaluated argument of class type having a non-trivial <span class="addu">eligible</span> copy constructor <span class="addu">([class.prop])</span>, a non-trivial <span class="addu">eligible</span> move constructor, or a non-trivial destructor, with no corresponding parameter, is conditionally-supported with implementation-defined semantics.</p>
</blockquote>
<p>Change 9.4.2 [dcl.fct.def.default] to account for prospective destructors that aren’t destructors:</p>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized">5</a></span> Explicitly-defaulted functions and implicitly-declared functions are collectively called <em>defaulted</em> functions, and the implementation shall provide implicit definitions for them ([class.ctor], [class.dtor], [class.copy.ctor], [class.copy.assign]), which might mean defining them as deleted. <span class="addu">A defaulted prospective destructor ([class.dtor]) that is not a destructor is defined as deleted. A defaulted special member function that is neither a prospective destructor nor an eligible special member function ([class.prop]) is defined as deleted.</span> A function is <em>user-provided</em> if it is user-declared and not explicitly defaulted or deleted on its first declaration.</p>
</blockquote>
<p>Insert at the beginning of 11.1 [class.prop] a definition of eligibility:</p>
<div class="add" style="color: #006e28">
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized">-1</a></span> Two special member functions are of the same kind if</p>
<ul>
<li><span class="marginalizedparent"><a class="marginalized">(-1.1)</a></span> they are both default constructors,</li>
<li><span class="marginalizedparent"><a class="marginalized">(-1.2)</a></span> they are both copy or move constructors with the same first parameter type, or</li>
<li><span class="marginalizedparent"><a class="marginalized">(-1.3)</a></span> they are both copy or move assignment operators with the same first parameter type and the same <em>cv-qualifier</em>s and <em>ref-qualifier</em>, if any.</li>
</ul>
</blockquote>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized">0</a></span> An <em>eligible special member function</em> is a special member function ([special])</p>
<ul>
<li><span class="marginalizedparent"><a class="marginalized">(0.1)</a></span> that is not deleted,</li>
<li><span class="marginalizedparent"><a class="marginalized">(0.2)</a></span> where its associated constraints ([temp.constr]), if any, are satisfied, and</li>
<li><span class="marginalizedparent"><a class="marginalized">(0.3)</a></span> where no special member function of the same kind is more constrained ([temp.constr.order]).</li>
</ul>
</blockquote>
</div>
<p>Change the definitions of <em>trivially copyable</em> and <em>trivial</em> in 11.1 [class.prop] (this flips the first two bullet points):</p>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized">1</a></span> A <em>trivially copyable class</em> is a class:</p>
<ul>
<li><span class="marginalizedparent"><a class="marginalized">(1.0)</a></span> <span class="rm" style="color: #bf0303"><del>where each copy constructor, move constructor, copy assignment operator, and move assignment operator ([class.copy.ctor], [class.copy.assign]) is either deleted or trivial,</del></span></li>
<li><span class="marginalizedparent"><a class="marginalized">(1.1)</a></span> that has at least one <span class="rm" style="color: #bf0303"><del>non-deleted</del></span> <span class="addu">eligible</span> copy constructor, move constructor, copy assignment operator, or move assignment operator <span class="addu">([class.copy.ctor], [class.copy.assign])</span>, <span class="rm" style="color: #bf0303"><del>and</del></span></li>
<li><span class="marginalizedparent"><a class="marginalized">(1.2)</a></span> <span class="addu">where each eligible copy constructor, move constructor, copy assignment operator, and move assignment operator is trivial, and</span></li>
<li><span class="marginalizedparent"><a class="marginalized">(1.3)</a></span> that has a trivial, non-deleted destructor.</li>
</ul>
</blockquote>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized">2</a></span> A <em>trivial class</em> is a class that is trivially copyable and has one or more <span class="addu">eligible</span> default constructors ([class.default.ctor]), all of which are <span class="addu">trivial.</span> <span class="rm" style="color: #bf0303"><del>either trivial or deleted and at least one of which is not deleted.</del></span></p>
</blockquote>
<p>Change 11.3.3 [special] to refer to prospective destructors, and make everything plural:</p>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized">1</a></span> <span class="rm" style="color: #bf0303"><del>The default</del></span> <span class="addu">Default</span> constructor<span class="addu">s</span> ([class.default.ctor]), copy constructor<span class="addu">s</span>, move constructor<span class="addu">s</span> ([class.copy.ctor]), copy assignment operator<span class="addu">s</span>, move assignment operator<span class="addu">s</span> ([class.copy.assign]), and <span class="addu">prospective</span> destructor<span class="addu">s</span> ([class.dtor]) are <em>special member functions</em>.</p>
</blockquote>
<p>Change the definition of destructor as follows.</p>
<p>Change 11.3.6 [class.dtor]:</p>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized">1</a></span> In a declaration of a <span class="addu">prospective</span> destructor, the declarator is a function declarator (9.2.3.5) of the form […] A <span class="addu">prospective</span> destructor shall take no arguments (9.2.3.5). Each <em>decl-specifier</em> of the <em>decl-specifier-seq</em> of a <span class="addu">prospective</span> destructor declaration (if any) shall be <code class="sourceCode cpp"><span class="kw">friend</span></code>, <code class="sourceCode cpp"><span class="kw">inline</span></code>, or <code class="sourceCode cpp"><span class="kw">virtual</span></code>.</p>
</blockquote>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized">a</a></span> <span class="addu">At the end of the definition of a class, overload resolution is performed among the prospective destructors declared in that class with an empty argument list to select the <em>destructor</em> for the class, also known as the <em>selected destructor</em>. The program is ill-formed if overload resolution fails. Destructor selection does not constitute a reference to ([dcl.fct.def.delete]) or odr-use ([basic.def.odr]) of the selected destructor, and in particular, the selected destructor may be deleted.</span></p>
</blockquote>
<p>Change 11.3.6 [class.dtor]:</p>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized">4</a></span> If a class has no user-declared <span class="addu">prospective</span> destructor, a <span class="addu">prospective</span> destructor is implicitly declared as defaulted (9.4). An implicitly-declared <span class="addu">prospective</span> destructor is an inline public member of its class.</p>
</blockquote>
<div class="add" style="color: #006e28">
<blockquote>
<p>An implicitly-declared prospective destructor for a class X will have the form</p>
<div class="sourceCode" id="cb3"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb3-1"><a href="#cb3-1"></a><span class="op">~</span>X<span class="op">()</span></span></code></pre></div>
</blockquote>
</div>
<p>Change 11.3.6 [class.dtor]:</p>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized">10</a></span> A <span class="addu">prospective</span> destructor can be declared <code class="sourceCode cpp"><span class="kw">virtual</span></code> (11.6.2) or pure <code class="sourceCode cpp"><span class="kw">virtual</span></code> (11.6.3)<span class="rm" style="color: #bf0303"><del>; if</del></span> <span class="addu">. If the destructor of a class is virtual and</span> any objects of that class or any derived class are created in the program, the destructor shall be defined. If a class has a base class with a virtual destructor, its destructor (whether user- or implicitly-declared) is virtual.</p>
</blockquote>
<p>Add to 13.8.2 [temp.explicit], immediately after paragraph 12:</p>
<blockquote>
<p><span class="marginalizedparent"><a class="marginalized">12</a></span> An explicit instantiation definition that names a class template specialization explicitly instantiates the class template specialization and is an explicit instantiation definition of only those members that have been defined at the point of instantiation.</p>
<p><span class="marginalizedparent"><a class="marginalized">12b</a></span> <span class="addu">An explicit instantiation of a prospective destructor shall name the selected destructor of the class.</span></p>
</blockquote>
<h1 id="acknowledgments" style="border-bottom:1px solid #cccccc"><span class="header-section-number">5</span> Acknowledgments<a href="#acknowledgments" class="self-link"></a></h1>
<p>Thanks to Gaby dos Reis, Daveed Vandevoorde, and Jonathan Wakely for helping bring us to this design. Thanks to Jens Maurer and Richard Smith for the lengthy discussions and wording wizardry.</p>
<h1 id="references" style="border-bottom:1px solid #cccccc"><span class="header-section-number">6</span> References<a href="#references" class="self-link"></a></h1>
<div id="refs" role="doc-bibliography">
<div id="ref-P0848R0">
<p>[P0848R0] Barry Revzin, Casey Carter. 2017. Conditionally Trivial Special Member Functions. <br />
<a href="https://wg21.link/p0848r0">https://wg21.link/p0848r0</a></p>
</div>
<div id="ref-P0848R1">
<p>[P0848R1] Barry Revzin, Casey Carter. 2019. Conditionally Trivial Special Member Functions. <br />
<a href="https://wg21.link/p0848r1">https://wg21.link/p0848r1</a></p>
</div>
</div>
</div>
</div>
</body>
</html>