-
Notifications
You must be signed in to change notification settings - Fork 1
Expand file tree
/
Copy pathuniform_initialization.html
More file actions
435 lines (356 loc) · 32.7 KB
/
uniform_initialization.html
File metadata and controls
435 lines (356 loc) · 32.7 KB
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
<!doctype html>
<html class="cpprefjp" lang="ja" itemscope="" itemtype="http://schema.org/WebPage">
<head>
<!-- Google tag (gtag.js) -->
<script async src="https://www.googletagmanager.com/gtag/js?id=G-NXNBNVBTJS"></script>
<script>
window.dataLayer = window.dataLayer || [];
function gtag(){dataLayer.push(arguments);}
gtag('js', new Date());
gtag('config', 'G-NXNBNVBTJS');
</script>
<meta charset="UTF-8">
<title>一様初期化 [N2672] - cpprefjp C++日本語リファレンス</title>
<meta name="viewport" content="width=device-width,initial-scale=1">
<meta name="keywords" content="
C++,標準ライブラリ,リファレンス,ドキュメント,STL,std,cpp11
">
<meta name="title" content="一様初期化 [N2672] - cpprefjp C++日本語リファレンス" />
<meta itemprop="name" content="一様初期化 [N2672] - cpprefjp C++日本語リファレンス" />
<meta property="og:title" content="一様初期化 [N2672] - cpprefjp C++日本語リファレンス" />
<meta property="og:url" content="https://cpprefjp.github.io/lang/cpp11/uniform_initialization.html" />
<meta property="og:site_name" content="cpprefjp - C++日本語リファレンス" />
<meta property="og:type" content="article" />
<meta property="og:description" content="「一様初期化 (uniform initialization)」は、コンストラクタの呼び出しを、リスト初期化と合わせて波カッコ `{ }`で記述する構文である。" />
<meta name="twitter:card" content="summary" />
<meta name="twitter:title" content="一様初期化 [N2672] - cpprefjp C++日本語リファレンス" />
<meta name="twitter:url" content="https://cpprefjp.github.io/lang/cpp11/uniform_initialization.html" />
<meta name="twitter:description" content="「一様初期化 (uniform initialization)」は、コンストラクタの呼び出しを、リスト初期化と合わせて波カッコ `{ }`で記述する構文である。" />
<link rel="alternate" type="application/atom+xml" title="Atom" href="https://cpprefjp.github.io/rss.xml" />
<link rel="apple-touch-icon" sizes="180x180" href="../../static/favicons/apple-touch-icon.png?cachebust=be86fa2321ebe02b6955b61b98b778e377bcbf95">
<link rel="icon" type="image/png" sizes="32x32" href="../../static/favicons/favicon-32x32.png?cachebust=be86fa2321ebe02b6955b61b98b778e377bcbf95">
<link rel="icon" type="image/png" sizes="16x16" href="../../static/favicons/favicon-16x16.png?cachebust=be86fa2321ebe02b6955b61b98b778e377bcbf95">
<link rel="manifest" href="../../manifest.json?cachebust=be86fa2321ebe02b6955b61b98b778e377bcbf95">
<meta name="theme-color" content="#f5f8fc">
<link rel="stylesheet" href="../../static/pygments/default.css?cachebust=be86fa2321ebe02b6955b61b98b778e377bcbf95">
<!-- <link rel="stylesheet" href="../../static/css/root.css"> -->
<link href="../../static/kunai/css/kunai-stage-0.css?cachebust=be86fa2321ebe02b6955b61b98b778e377bcbf95" rel="stylesheet">
<link href="../../static/kunai/css/kunai-stage-1.css?cachebust=be86fa2321ebe02b6955b61b98b778e377bcbf95" rel="stylesheet">
<link href="../../static/kunai/css/kunai-stage-2.css?cachebust=be86fa2321ebe02b6955b61b98b778e377bcbf95" rel="stylesheet">
<link href="../../static/kunai/css/kunai-stage-3.css?cachebust=be86fa2321ebe02b6955b61b98b778e377bcbf95" rel="stylesheet">
<script type="text/javascript" src="../../static/kunai/js/kunai-vendor.js?cachebust=be86fa2321ebe02b6955b61b98b778e377bcbf95"></script>
<script type="text/javascript" src="../../static/kunai/js/kunai.js?cachebust=be86fa2321ebe02b6955b61b98b778e377bcbf95"></script>
<script type="text/javascript">
document.addEventListener('DOMContentLoaded', function() {
var kn = new Kunai;
kn.cpprefjp();
});
</script>
</head>
<body>
<header data-kunai-mdinfo="{"meta": {"cpp": ["cpp11"]}, "sources": [{"id": "76190b4afdaa371810169b63ba9e39f19e691b1e", "source": "#include <iostream>\n#include <vector>\n#include <iterator>\n\nint main()\n{\n // \u30b3\u30f3\u30d1\u30a4\u30eb\u30a8\u30e9\u30fc\uff01\u95a2\u6570\u5ba3\u8a00\u69cb\u6587\u3068\u307f\u306a\u3055\u308c\u308b\u304c\u3001\u30d1\u30e9\u30e1\u30fc\u30bf\u540d\uff08std::cin\uff09\u304c\u540d\u524d\u7a7a\u9593\u4fee\u98fe\u4ed8\u304d\u306e\u305f\u3081\u3001\u30a8\u30e9\u30fc\n//std::vector<char> vec(std::istream_iterator<char>(std::cin),\n// std::istream_iterator<char>());\n\n // vec \u306f\u5f15\u6570 2 \u3064\u306e\u30b3\u30f3\u30b9\u30c8\u30e9\u30af\u30bf\u3067\u521d\u671f\u5316\u3055\u308c\u305f std::vector<char> \u578b\u306e\u5909\u6570\n std::vector<char> vec{std::istream_iterator<char>(std::cin),\n std::istream_iterator<char>()};\n}\n"}, {"id": "07427854853f1f23d01a05488e7f3af6a17ce473", "source": "#include <iostream>\n#include <initializer_list>\n\nstruct X {\n X()\n {\n std::cout << \"default constructor\" << std::endl;\n }\n\n X(std::initializer_list<int>)\n {\n std::cout << \"initializer-list constructor\" << std::endl;\n }\n};\n\nint main()\n{\n X x = {}; // \u300cdefault constructor\u300d\u304c\u51fa\u529b\u3055\u308c\u308b\n}\n"}, {"id": "0356d2baff8a7c50a4152a9a3e3c7bc6cf613756", "source": "#include <iostream>\n\nint f()\n{\n std::cout << 1 << std::endl;\n return 1;\n}\n\nint g()\n{\n std::cout << 2 << std::endl;\n return 2;\n}\n\nint h()\n{\n std::cout << 3 << std::endl;\n return 3;\n}\n\nstruct X {\n X(int, int, int) {}\n};\n\nint main()\n{\n X x1 { f(), g(), h() }; // 1, 2, 3\u306e\u9806\u3067\u51fa\u529b\u3055\u308c\u308b\n X x2(f(), g(), h()); // 1, 2, 3\u304c\u9806\u4e0d\u540c\u3067\u51fa\u529b\u3055\u308c\u308b\n}\n"}], "page_id": ["lang", "cpp11", "uniform_initialization"]}">
<nav class="navbar navbar-default" role="navigation">
<div class="container-fluid">
<div class="navbar-header">
<button type="button" class="navbar-toggle collapsed" data-toggle="collapse" data-target="#navbar-collapse">
<span class="sr-only">Toggle navigation</span>
<span class="icon-bar"></span>
<span class="icon-bar"></span>
<span class="icon-bar"></span>
</button>
<a class="navbar-brand" href="../../index.html">
<div class="title-wrapper clearfix">
<div class="title">cpprefjp - C++日本語リファレンス</div>
</div>
</a>
</div>
<div class="collapse navbar-collapse" id="navbar-collapse">
<ul class="nav navbar-nav navbar-right">
<li>
<div class="google-search">
<script>
(function() {
var cx = '013316413321391058734:ji_u66hl7hq';
var gcse = document.createElement('script');
gcse.type = 'text/javascript';
gcse.async = true;
gcse.src = (document.location.protocol == 'https:' ? 'https:' : 'http:') +
'//www.google.com/cse/cse.js?cx=' + cx;
var s = document.getElementsByTagName('script')[0];
s.parentNode.insertBefore(gcse, s);
})();
</script>
<div class="gcse-search"></div>
</div>
</li>
<li>
<a href="https://github.com/cpprefjp/site">GitHub Project</a>
</li>
</ul>
</div>
</div>
</nav>
</header>
<main id="main" role="main">
<div class="container-fluid">
<div class="row">
<div class="col-sm-9 col-sm-push-3" itemscope itemtype="http://schema.org/Article">
<div class="row">
<div class="col-sm-12 google-search-result">
<gcse:searchresults></gcse:searchresults>
</div>
</div>
<div class="row">
<div class="col-sm-12 content-header">
<ol class="breadcrumb">
<li itemscope itemtype="http://www.schema.org/SiteNavigationElement">
<span>
<a href="../../index.html" itemprop="url">
<i class="fa fa-fw fa-home"></i>
</a>
</span>
</li>
<li itemscope itemtype="http://www.schema.org/SiteNavigationElement">
<span>
<a href="../../lang.html" itemprop="url">
<span itemprop="name">言語機能</span>
</a>
</span>
</li>
<li itemscope itemtype="http://www.schema.org/SiteNavigationElement">
<span>
<a href="../../lang/cpp11.html" itemprop="url">
<span itemprop="name">C++11</span>
</a>
</span>
</li>
<li class="active" itemscope itemtype="http://www.schema.org/SiteNavigationElement">
<span>
<span itemprop="name">一様初期化 [N2672]</span>
</span>
</li>
</ol>
<div class="crsearch"></div>
</div>
</div>
<div class="row">
<div class="col-sm-12 edit-button">
<p class="text-right"><small>
最終更新日時(UTC):
<span itemprop="datePublished" content="2025-07-10T01:26:30">
2025年07月10日 01時26分30秒
</span>
<br/>
<span itemprop="author" itemscope itemtype="http://schema.org/Person">
<span itemprop="name">Koichi Murase</span>
</span>
が更新
</small></p>
<p class="text-right">
<a class="history" target="_blank" href="https://github.com/cpprefjp/site/commits/master/lang/cpp11/uniform_initialization.md">
<span class="fa fa-fw fa-clock-o fa-flip-horizontal"></span>履歴
</a>
<a class="edit" target="_blank" href="https://github.com/cpprefjp/site/edit/master/lang/cpp11/uniform_initialization.md">
<span class="fa fa-fw fa-pencil"></span>編集
</a>
</p>
</div>
</div>
<div class="row">
<div class="col-sm-12 content-body">
<h1 itemprop="name"><span class="token">一様初期化 [N2672]</span><span class="cpp cpp11" title="C++11で追加">(C++11)</span></h1>
<div itemprop="articleBody"><p></p>
<p>このページはC++11に採用された言語機能の変更を解説しています。</p>
<p>のちのC++規格でさらに変更される場合があるため<a href="#relative-page">関連項目</a>を参照してください。</p>
<p></p>
<h2>概要</h2>
<p>「一様初期化 (uniform initialization)」は、コンストラクタの呼び出しを、リスト初期化と合わせて波カッコ <code>{ }</code>で記述する構文である。</p>
<p><div class="codehilite"><pre><span></span><code><span class="k">struct</span><span class="w"> </span><span class="nc">X</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">X</span><span class="p">(</span><span class="kt">int</span><span class="p">)</span><span class="w"> </span><span class="p">{}</span>
<span class="p">};</span>
<span class="kt">int</span><span class="w"> </span><span class="nf">main</span><span class="p">()</span>
<span class="p">{</span>
<span class="w"> </span><span class="c1">// 従来のコンストラクタ呼び出し</span>
<span class="w"> </span><span class="n">X</span><span class="w"> </span><span class="n">x1</span><span class="p">(</span><span class="mi">0</span><span class="p">);</span>
<span class="w"> </span><span class="n">X</span><span class="w"> </span><span class="n">x2</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span>
<span class="w"> </span><span class="c1">// 一様初期化構文によるコンストラクタ呼び出し</span>
<span class="w"> </span><span class="n">X</span><span class="w"> </span><span class="n">x3</span><span class="w"> </span><span class="p">{</span><span class="mi">0</span><span class="p">};</span>
<span class="w"> </span><span class="n">X</span><span class="w"> </span><span class="n">x4</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">{</span><span class="mi">0</span><span class="p">};</span>
<span class="p">}</span>
</code></pre></div>
</p>
<p>この機能によって、まず<a class="cpprefjp-defined-word" data-desc="関数呼び出し式の評価結果となるオブジェクト・値">戻り値</a>の型が確定している文脈において、コンストラクタの引数を波カッコ内に列挙したものを<code>return</code>文で返せるようになる。その際、<code>return</code>文にはコンストラクタ呼び出しのために、<a class="cpprefjp-defined-word" data-desc="関数呼び出し式の評価結果となるオブジェクト・値">戻り値</a>の型を記述する必要はない：</p>
<p><div class="codehilite"><pre><span></span><code><span class="cp">#include <a href="../../reference/string.html"><string></a></span>
<span class="k">struct</span><span class="w"> </span><span class="nc">X</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">X</span><span class="p">(</span><span class="kt">int</span><span class="p">,</span><span class="w"> </span><span class="kt">double</span><span class="p">,</span><span class="w"> </span><span class="n"><a href="../../reference/string/basic_string.html">std::string</a></span><span class="p">)</span><span class="w"> </span><span class="p">{}</span>
<span class="p">};</span>
<span class="n">X</span><span class="w"> </span><span class="nf">createX</span><span class="p">()</span>
<span class="p">{</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="p">{</span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mf">3.14</span><span class="p">,</span><span class="w"> </span><span class="s">"hello"</span><span class="p">};</span><span class="w"> </span><span class="c1">// OK</span>
<span class="p">}</span>
</code></pre></div>
</p>
<p>これは、関数に引数を渡す場合も同様である。</p>
<p>また、丸カッコによる初期化は、状況によって意図せず関数宣言構文とみなされてしまうことがあるが、波カッコによる初期化を行うことで、意図通りに解釈される：</p>
<p><div class="codehilite"><pre><span></span><code><span class="k">struct</span><span class="w"> </span><span class="nc">X</span><span class="w"> </span><span class="p">{};</span>
<span class="kt">int</span><span class="w"> </span><span class="nf">main</span><span class="p">()</span>
<span class="p">{</span>
<span class="w"> </span><span class="n">X</span><span class="w"> </span><span class="n">x</span><span class="p">();</span><span class="w"> </span><span class="c1">// x は引数を持たず X を返す関数の宣言！（おそらく意図と異なる）</span>
<span class="w"> </span><span class="n">X</span><span class="w"> </span><span class="n">y</span><span class="p">{};</span><span class="w"> </span><span class="c1">// y はデフォルトコンストラクタで初期化した X 型の変数</span>
<span class="p">}</span>
</code></pre></div>
</p>
<p><div class="yata" id="76190b4afdaa371810169b63ba9e39f19e691b1e"><div class="codehilite"><pre><span></span><code><span class="cp">#include <a href="../../reference/iostream.html"><iostream></a></span>
<span class="cp">#include <a href="../../reference/vector.html"><vector></a></span>
<span class="cp">#include <a href="../../reference/iterator.html"><iterator></a></span>
<span class="kt">int</span><span class="w"> </span><span class="nf">main</span><span class="p">()</span>
<span class="p">{</span>
<span class="w"> </span><span class="c1">// コンパイルエラー！関数宣言構文とみなされるが、パラメータ名（<a href="../../reference/iostream/cin.html">std::cin</a>）が名前空間修飾付きのため、エラー</span>
<span class="c1">//<a href="../../reference/vector/vector.html">std::vector</a><char> vec(<a href="../../reference/iterator/istream_iterator.html">std::istream_iterator</a><char>(<a href="../../reference/iostream/cin.html">std::cin</a>),</span>
<span class="c1">// <a href="../../reference/iterator/istream_iterator.html">std::istream_iterator</a><char>());</span>
<span class="w"> </span><span class="c1">// vec は引数 2 つのコンストラクタで初期化された <a href="../../reference/vector/vector.html">std::vector</a><char> 型の変数</span>
<span class="w"> </span><span class="n"><a href="../../reference/vector/vector.html">std::vector</a></span><span class="o"><</span><span class="kt">char</span><span class="o">></span><span class="w"> </span><span class="n">vec</span><span class="p">{</span><span class="n"><a href="../../reference/iterator/istream_iterator.html">std::istream_iterator</a></span><span class="o"><</span><span class="kt">char</span><span class="o">></span><span class="p">(</span><span class="n"><a href="../../reference/iostream/cin.html">std::cin</a></span><span class="p">),</span>
<span class="w"> </span><span class="n"><a href="../../reference/iterator/istream_iterator.html">std::istream_iterator</a></span><span class="o"><</span><span class="kt">char</span><span class="o">></span><span class="p">()};</span>
<span class="p">}</span>
</code></pre></div>
</div></p>
<h2>仕様</h2>
<ul>
<li>波カッコによる初期化子リストを<code>auto</code>で受けた場合、そのオブジェクトは<code><a href="../../reference/initializer_list/initializer_list.html">std::initializer_list</a></code>型に推論される</li>
<li>
<p>関数テンプレートのパラメータとして波カッコの初期化子リストを渡して型推論させることはできない。波カッコの初期化子リストを受ける側で構築する型が確定しないためである</p>
<p><div class="codehilite"><pre><span></span><code><span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="k">class</span><span class="w"> </span><span class="nc">T</span><span class="o">></span>
<span class="kt">void</span><span class="w"> </span><span class="n">f</span><span class="p">(</span><span class="n">T</span><span class="p">)</span><span class="w"> </span><span class="p">{}</span>
<span class="kt">int</span><span class="w"> </span><span class="n">main</span><span class="p">()</span>
<span class="p">{</span>
<span class="w"> </span><span class="n">f</span><span class="p">({</span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="sc">'a'</span><span class="p">});</span><span class="w"> </span><span class="c1">// コンパイルエラー！Tの型を推論できない</span>
<span class="p">}</span>
</code></pre></div>
</p>
</li>
<li>
<p>関数のパラメータ型が確定している場合、波カッコによる初期化子リストを渡せる</p>
<p><div class="codehilite"><pre><span></span><code><span class="k">struct</span><span class="w"> </span><span class="nc">X</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">X</span><span class="p">(</span><span class="kt">int</span><span class="p">,</span><span class="w"> </span><span class="kt">char</span><span class="p">)</span><span class="w"> </span><span class="p">{}</span>
<span class="p">};</span>
<span class="kt">void</span><span class="w"> </span><span class="nf">f</span><span class="p">(</span><span class="n">X</span><span class="p">)</span><span class="w"> </span><span class="p">{}</span>
<span class="kt">int</span><span class="w"> </span><span class="nf">main</span><span class="p">()</span>
<span class="p">{</span>
<span class="w"> </span><span class="n">f</span><span class="p">({</span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="sc">'a'</span><span class="p">});</span><span class="w"> </span><span class="c1">// OK</span>
<span class="p">}</span>
</code></pre></div>
</p>
</li>
<li>
<p>関数の<a class="cpprefjp-defined-word" data-desc="関数呼び出し式の評価結果となるオブジェクト・値">戻り値</a>型が確定している場合、波カッコによる初期化子リストを<code>return</code>文で返せる。確定していない場合は返せない</p>
<p><div class="codehilite"><pre><span></span><code><span class="k">struct</span><span class="w"> </span><span class="nc">X</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">X</span><span class="p">(</span><span class="kt">int</span><span class="p">,</span><span class="w"> </span><span class="kt">char</span><span class="p">)</span><span class="w"> </span><span class="p">{}</span>
<span class="p">};</span>
<span class="kt">int</span><span class="w"> </span><span class="nf">main</span><span class="p">()</span>
<span class="p">{</span>
<span class="w"> </span><span class="k">auto</span><span class="w"> </span><span class="n">f</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">[]()</span><span class="w"> </span><span class="o">-></span><span class="w"> </span><span class="n">X</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="p">{</span><span class="mi">3</span><span class="p">,</span><span class="w"> </span><span class="sc">'a'</span><span class="p">};</span><span class="w"> </span><span class="p">};</span><span class="w"> </span><span class="c1">// OK</span>
<span class="c1">//auto g = []() { return {3, 'a'}; }; // コンパイルエラー！戻り値の型を推論できない</span>
<span class="p">}</span>
</code></pre></div>
</p>
</li>
<li>
<p><code><a href="../../reference/initializer_list/initializer_list.html">std::initializer_list</a></code>型を受け取るコンストラクタとデフォルトコンストラクタがある場合、空の初期化子リストが渡された際にはデフォルトコンストラクタが呼び出される</p>
<p><div class="yata" id="07427854853f1f23d01a05488e7f3af6a17ce473"><div class="codehilite"><pre><span></span><code><span class="cp">#include <a href="../../reference/iostream.html"><iostream></a></span>
<span class="cp">#include <a href="../../reference/initializer_list.html"><initializer_list></a></span>
<span class="k">struct</span><span class="w"> </span><span class="nc">X</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">X</span><span class="p">()</span>
<span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n"><a href="../../reference/iostream/cout.html">std::cout</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="s">"default constructor"</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n"><a href="../../reference/ostream/endl.html">std::endl</a></span><span class="p">;</span>
<span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="n">X</span><span class="p">(</span><span class="n"><a href="../../reference/initializer_list/initializer_list.html">std::initializer_list</a></span><span class="o"><</span><span class="kt">int</span><span class="o">></span><span class="p">)</span>
<span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n"><a href="../../reference/iostream/cout.html">std::cout</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="s">"initializer-list constructor"</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n"><a href="../../reference/ostream/endl.html">std::endl</a></span><span class="p">;</span>
<span class="w"> </span><span class="p">}</span>
<span class="p">};</span>
<span class="kt">int</span><span class="w"> </span><span class="nf">main</span><span class="p">()</span>
<span class="p">{</span>
<span class="w"> </span><span class="n">X</span><span class="w"> </span><span class="n">x</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">{};</span><span class="w"> </span><span class="c1">// 「default constructor」が出力される</span>
<span class="p">}</span>
</code></pre></div>
</div></p>
</li>
<li>
<p><code><a href="../../reference/initializer_list/initializer_list.html">std::initializer_list</a></code>型を受け取るコンストラクタと、その初期化子リストの要素型と同じ型のパラメータリストを受け取るコンストラクタでは、<code><a href="../../reference/initializer_list/initializer_list.html">std::initializer_list</a></code>型を受け取るコンストラクタが優先して呼び出される。そのような状況で非<code><a href="../../reference/initializer_list/initializer_list.html">std::initializer_list</a></code>のコンストラクタを呼び出す場合は、丸カッコでのコンストラクタ呼び出しが必要となる</p>
<p><div class="codehilite"><pre><span></span><code><span class="k">struct</span><span class="w"> </span><span class="nc">X</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">X</span><span class="p">(</span><span class="n"><a href="../../reference/initializer_list/initializer_list.html">std::initializer_list</a></span><span class="o"><</span><span class="kt">double</span><span class="o">></span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n"><a href="../../reference/iostream/cout.html">std::cout</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="mi">1</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n"><a href="../../reference/ostream/endl.html">std::endl</a></span><span class="p">;</span>
<span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="n">X</span><span class="p">(</span><span class="kt">double</span><span class="w"> </span><span class="n">d</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n"><a href="../../reference/iostream/cout.html">std::cout</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="mi">2</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n"><a href="../../reference/ostream/endl.html">std::endl</a></span><span class="p">;</span>
<span class="w"> </span><span class="p">}</span>
<span class="p">};</span>
<span class="n">X</span><span class="w"> </span><span class="n">x1</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">{</span><span class="mf">3.0</span><span class="p">};</span><span class="w"> </span><span class="c1">// 「1」が出力される</span>
</code></pre></div>
</p>
</li>
</ul>
<h3><a href="#evaluation-order" id="evaluation-order">評価順序</a></h3>
<ul>
<li>
<p>波カッコによるコンストラクタ呼び出しで渡す引数は、先頭から順番に評価されることが保証される</p>
<p><div class="yata" id="0356d2baff8a7c50a4152a9a3e3c7bc6cf613756"><div class="codehilite"><pre><span></span><code><span class="cp">#include <a href="../../reference/iostream.html"><iostream></a></span>
<span class="kt">int</span><span class="w"> </span><span class="nf">f</span><span class="p">()</span>
<span class="p">{</span>
<span class="w"> </span><span class="n"><a href="../../reference/iostream/cout.html">std::cout</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="mi">1</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n"><a href="../../reference/ostream/endl.html">std::endl</a></span><span class="p">;</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="mi">1</span><span class="p">;</span>
<span class="p">}</span>
<span class="kt">int</span><span class="w"> </span><span class="nf">g</span><span class="p">()</span>
<span class="p">{</span>
<span class="w"> </span><span class="n"><a href="../../reference/iostream/cout.html">std::cout</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="mi">2</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n"><a href="../../reference/ostream/endl.html">std::endl</a></span><span class="p">;</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="mi">2</span><span class="p">;</span>
<span class="p">}</span>
<span class="kt">int</span><span class="w"> </span><span class="nf">h</span><span class="p">()</span>
<span class="p">{</span>
<span class="w"> </span><span class="n"><a href="../../reference/iostream/cout.html">std::cout</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="mi">3</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n"><a href="../../reference/ostream/endl.html">std::endl</a></span><span class="p">;</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="mi">3</span><span class="p">;</span>
<span class="p">}</span>
<span class="k">struct</span><span class="w"> </span><span class="nc">X</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">X</span><span class="p">(</span><span class="kt">int</span><span class="p">,</span><span class="w"> </span><span class="kt">int</span><span class="p">,</span><span class="w"> </span><span class="kt">int</span><span class="p">)</span><span class="w"> </span><span class="p">{}</span>
<span class="p">};</span>
<span class="kt">int</span><span class="w"> </span><span class="nf">main</span><span class="p">()</span>
<span class="p">{</span>
<span class="w"> </span><span class="n">X</span><span class="w"> </span><span class="n">x1</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="n">f</span><span class="p">(),</span><span class="w"> </span><span class="n">g</span><span class="p">(),</span><span class="w"> </span><span class="n">h</span><span class="p">()</span><span class="w"> </span><span class="p">};</span><span class="w"> </span><span class="c1">// 1, 2, 3の順で出力される</span>
<span class="w"> </span><span class="n">X</span><span class="w"> </span><span class="n">x2</span><span class="p">(</span><span class="n">f</span><span class="p">(),</span><span class="w"> </span><span class="n">g</span><span class="p">(),</span><span class="w"> </span><span class="n">h</span><span class="p">());</span><span class="w"> </span><span class="c1">// 1, 2, 3が順不同で出力される</span>
<span class="p">}</span>
</code></pre></div>
</div></p>
</li>
</ul>
<h2><a href="#relative-page" id="relative-page">関連項目</a></h2>
<ul>
<li><a href="initializer_lists.html">C++11 初期化子リスト</a></li>
</ul>
<h2>参照</h2>
<ul>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2477.pdf" target="_blank">N2477 Uniform initialization design choices</a></li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2532.pdf" target="_blank">N2532 Uniform initialization design choices (Revision 2)</a></li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2575.pdf" target="_blank">N2575 Initializer Lists - Alternative Mechanism and Rationale</a></li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2640.pdf" target="_blank">N2640 Initializer Lists - Alternative Mechanism and Rationale (v. 2)</a></li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2672.htm" target="_blank">N2672 Initializer List proposed wording</a></li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1030" target="_blank">CWG Issue 1030. Evaluation order in initializer-lists used in aggregate initialization</a></li>
</ul></div>
</div>
</div>
</div>
<div id="sidebar" class="col-sm-3 col-sm-pull-9">
</div>
</div>
</div>
</main>
<footer class="footer navbar navbar-default">
<div class="container-fluid">
<p><small>
本サイトの情報は、
<a href="https://creativecommons.org/licenses/by/4.0/deed.ja" rel="nofollow">クリエイティブ・コモンズ 表示 4.0 非移植 ライセンス(CC BY)</a>
の下に提供されています。
</small></p>
</div>
</footer>
</body>
</html>