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<h1 itemprop="name"><span class="token">if文とswitch文の条件式と初期化を分離 [P0305R1]</span><span class="cpp cpp17" title="C++17で追加">(C++17)</span></h1>
<div itemprop="articleBody"><p>
</p>
<p>このページはC++17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>従来<code>for</code>文しか使用できなかった初期化を<code>if</code>文と<code>switch</code>文でも使えるようになった。</p>
<p><code>for</code>文は下記のように初期化と条件式を同時に書くことができる。
初期化で宣言した変数は<code>for</code>文のスコープのみで有効であり<code>for</code>文の外では参照できない特徴がある。</p>
<p><div class="codehilite"><pre><span></span><code><span class="k">for</span><span class="w"> </span><span class="p">(</span><span class="k">auto</span><span class="w"> </span><span class="n">i</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="n">i</span><span class="w"> </span><span class="o"><</span><span class="w"> </span><span class="n">n</span><span class="p">;</span><span class="w"> </span><span class="n">i</span><span class="o">++</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="c1">//i はこのスコープ内で有効</span>
<span class="w"> </span><span class="err">…</span>
<span class="p">}</span>
<span class="c1">//この時点で i は無効</span>
</code></pre></div>
</p>
<p>C++17では<code>if</code>文と<code>switch</code>文においても、<code>for</code>文との一貫性を持った初期化を記述できるようになった。</p>
<p><div class="codehilite"><pre><span></span><code><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="n">status_code</span><span class="w"> </span><span class="n">c</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">bar</span><span class="p">();</span><span class="w"> </span><span class="n">c</span><span class="w"> </span><span class="o">!=</span><span class="w"> </span><span class="n">SUCCESS</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="c1">//ステータスコード c が成功ではなかったら、何かして、処理を終了する</span>
<span class="w"> </span><span class="err">…</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">c</span><span class="p">;</span>
<span class="p">}</span><span class="w"> </span><span class="k">else</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="c1">//ステータスコード c が成功だったら、何かして、処理を続行する</span>
<span class="w"> </span><span class="err">…</span>
<span class="p">}</span>
<span class="c1">//この時点で c は無効</span>
<span class="k">switch</span><span class="w"> </span><span class="p">(</span><span class="n">Foo</span><span class="w"> </span><span class="n">gadget</span><span class="p">{</span><span class="n">args</span><span class="p">};</span><span class="w"> </span><span class="k">auto</span><span class="w"> </span><span class="n">s</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">gadget</span><span class="p">.</span><span class="n">status</span><span class="p">())</span><span class="w"> </span><span class="p">{</span>
<span class="k">case</span><span class="w"> </span><span class="no">OK</span><span class="p">:</span>
<span class="w"> </span><span class="c1">//正常</span>
<span class="w"> </span><span class="n">gadget</span><span class="p">.</span><span class="n">zip</span><span class="p">();</span>
<span class="w"> </span><span class="k">break</span><span class="p">;</span>
<span class="k">case</span><span class="w"> </span><span class="no">Warn</span><span class="p">:</span>
<span class="w"> </span><span class="c1">//異常だが、続行</span>
<span class="w"> </span><span class="n">gadget</span><span class="p">.</span><span class="n">log</span><span class="p">();</span>
<span class="w"> </span><span class="k">break</span><span class="p">;</span>
<span class="k">case</span><span class="w"> </span><span class="no">Bad</span><span class="p">:</span>
<span class="w"> </span><span class="c1">//異常なので、中断</span>
<span class="w"> </span><span class="k">throw</span><span class="w"> </span><span class="n">BadFoo</span><span class="p">(</span><span class="n">s</span><span class="p">.</span><span class="n">message</span><span class="p">());</span>
<span class="p">}</span>
<span class="c1">//この時点で gadget, s は無効</span>
</code></pre></div>
</p>
<h2>仕様</h2>
<p>詳細な文法は下記の通り。</p>
<p><pre><code>init-statement:
expression-statement
simple-declaration
selection statement:
if ( init-statement opt condition ) statement
if ( init-statement opt condition ) statement else statement
switch ( init-statement opt condition ) statement
</code></pre></p>
<p>初期化<code>init-statement</code>は省略可能なため、従来の<code>if</code>文や<code>switch</code>文の書き方も文法違反にはならない。</p>
<p>この機能と、等しい意味を持つ従来記法は下記の通りである。</p>
<p>初期化付き<code>if</code>文。</p>
<p><div class="codehilite"><pre><span></span><code><span class="c1">//新しい</span>
<span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="w"> </span><span class="n">init</span><span class="o">-</span><span class="n">statement</span><span class="w"> </span><span class="n">condition</span><span class="w"> </span><span class="p">)</span><span class="w"> </span><span class="n">statement</span>
<span class="c1">//従来</span>
<span class="p">{</span>
<span class="w"> </span><span class="n">init</span><span class="o">-</span><span class="n">statement</span>
<span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="w"> </span><span class="n">condition</span><span class="w"> </span><span class="p">)</span><span class="w"> </span><span class="n">statement</span>
<span class="p">}</span>
</code></pre></div>
</p>
<p>初期化付き<code>if-else</code>文。</p>
<p><div class="codehilite"><pre><span></span><code><span class="c1">//新しい</span>
<span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="w"> </span><span class="n">init</span><span class="o">-</span><span class="n">statement</span><span class="w"> </span><span class="n">condition</span><span class="w"> </span><span class="p">)</span><span class="w"> </span><span class="n">statement</span><span class="w"> </span><span class="k">else</span><span class="w"> </span><span class="n">statement</span>
<span class="c1">//従来</span>
<span class="p">{</span>
<span class="w"> </span><span class="n">init</span><span class="o">-</span><span class="n">statement</span>
<span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="w"> </span><span class="n">condition</span><span class="w"> </span><span class="p">)</span><span class="w"> </span><span class="n">statement</span><span class="w"> </span><span class="k">else</span><span class="w"> </span><span class="n">statement</span>
<span class="p">}</span>
</code></pre></div>
</p>
<p>初期化付き<code>switch</code>文。</p>
<p><div class="codehilite"><pre><span></span><code><span class="c1">//新しい</span>
<span class="k">switch</span><span class="w"> </span><span class="p">(</span><span class="w"> </span><span class="n">init</span><span class="o">-</span><span class="n">statement</span><span class="w"> </span><span class="n">condition</span><span class="w"> </span><span class="p">)</span><span class="w"> </span><span class="n">statement</span>
<span class="c1">//従来</span>
<span class="p">{</span>
<span class="w"> </span><span class="n">init</span><span class="o">-</span><span class="n">statement</span>
<span class="w"> </span><span class="k">switch</span><span class="w"> </span><span class="p">(</span><span class="w"> </span><span class="n">condition</span><span class="w"> </span><span class="p">)</span><span class="w"> </span><span class="n">statement</span>
<span class="p">}</span>
</code></pre></div>
</p>
<h2>例</h2>
<p>異常があれば関数を脱出する1行の<code>if</code>文がより短く記述できる。</p>
<p><div class="codehilite"><pre><span></span><code><span class="n">status_code</span><span class="w"> </span><span class="nf">foo</span><span class="p">()</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">n</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">get_value</span><span class="p">();</span>
<span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="n">status_code</span><span class="w"> </span><span class="n">c</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">bar</span><span class="p">(</span><span class="n">n</span><span class="p">);</span><span class="w"> </span><span class="n">c</span><span class="w"> </span><span class="o">!=</span><span class="w"> </span><span class="n">status_code</span><span class="o">::</span><span class="n">SUCCESS</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">c</span><span class="p">;</span><span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="n">status_code</span><span class="w"> </span><span class="n">c</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">do_more_stuff</span><span class="p">();</span><span class="w"> </span><span class="n">c</span><span class="w"> </span><span class="o">!=</span><span class="w"> </span><span class="n">status_code</span><span class="o">::</span><span class="n">SUCCESS</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">c</span><span class="p">;</span><span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">status_code</span><span class="o">::</span><span class="n">SUCCESS</span><span class="p">;</span>
<span class="p">}</span>
</code></pre></div>
</p>
<p>また、ロックを取得してからリストにアクセスする場合なども短く記述できる。</p>
<p><div class="codehilite"><pre><span></span><code><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="n"><a href="../../reference/mutex/lock_guard.html">std::lock_guard</a></span><span class="o"><</span><span class="n"><a href="../../reference/mutex/mutex.html">std::mutex</a></span><span class="o">></span><span class="w"> </span><span class="n">lock</span><span class="p">(</span><span class="n">mx</span><span class="p">);</span><span class="w"> </span><span class="n">shared_flag</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="n">unsafe_ping</span><span class="p">();</span><span class="w"> </span><span class="n">shared_flag</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="nb">false</span><span class="p">;</span><span class="w"> </span><span class="p">}</span>
</code></pre></div>
</p>
<h3>出力</h3>
<p><pre><code></code></pre></p>
<h2>この機能が必要になった背景・経緯</h2>
<p>変数の宣言あるいは初期化を行い、<code>if</code>文や<code>switch</code>文のスコープ内だけで変数を使用するケースは多数あるにも関わらず、
<code>if</code>文や<code>switch</code>文で行える宣言は限定的だった。
具体的には、単一かつ配列ではない変数しか宣言できず、その変数の初期値が条件になってしまうので、変数を宣言したうえで別の式を条件にすることはできなかった。</p>
<p><div class="codehilite"><pre><span></span><code><span class="c1">// ポインターを受け取ってnullチェックをするパターンは、C++14まででもよく見かけた。</span>
<span class="c1">// しかし、この条件を反転(nullptrのとき真)することさえ不可能であった。</span>
<span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="k">auto</span><span class="o">*</span><span class="w"> </span><span class="n">p</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">get_some_object</span><span class="p">())</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="p">...</span><span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="c1">// pがnullptrでなければ真</span>
</code></pre></div>
</p>
<p>C++14まででも初期化付きの条件分岐と同様の意味を持つ記述は不可能ではないが、外側にスコープを追加する必要があり冗長な記述になっていた:</p>
<p><div class="codehilite"><pre><span></span><code><span class="n">status_code</span><span class="w"> </span><span class="nf">foo</span><span class="p">()</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">n</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">get_value</span><span class="p">();</span>
<span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">status_code</span><span class="w"> </span><span class="n">c</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">bar</span><span class="p">(</span><span class="n">n</span><span class="p">);</span>
<span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="n">c</span><span class="w"> </span><span class="o">!=</span><span class="w"> </span><span class="n">status_code</span><span class="o">::</span><span class="n">SUCCESS</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">c</span><span class="p">;</span><span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">status_code</span><span class="w"> </span><span class="n">c</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">do_more_stuff</span><span class="p">();</span>
<span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="n">c</span><span class="w"> </span><span class="o">!=</span><span class="w"> </span><span class="n">status_code</span><span class="o">::</span><span class="n">SUCCESS</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">c</span><span class="p">;</span><span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">status_code</span><span class="o">::</span><span class="n">SUCCESS</span><span class="p">;</span>
<span class="p">}</span>
</code></pre></div>
</p>
<p>見方を変えると<code>for</code>文で可能な初期化が、なぜか<code>if</code>文や<code>switch</code>文ではできない、文法に一貫性がない問題と見ることもできる。</p>
<h2>検討されたほかの選択肢</h2>
<h3>従来の機能を使う</h3>
<p>仕様で説明したようにこの機能を使わなくても、従来の機能を使って<code>{ init; if (cond) E; }</code>と書けば同じ効果を得られる。
しかし従来の記述が冗長である問題は解決しない。</p>
<p>従来の記述を面倒に思い、時にプログラマは外側のスコープを省略して書いてしまう:</p>
<p><div class="codehilite"><pre><span></span><code><span class="c1">//{ このカッコを省略してしまった</span>
<span class="k">auto</span><span class="w"> </span><span class="n">it</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">m</span><span class="p">.</span><span class="n">find</span><span class="p">(</span><span class="mi">10</span><span class="p">);</span>
<span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="n">it</span><span class="w"> </span><span class="o">!=</span><span class="w"> </span><span class="n">m</span><span class="p">.</span><span class="n">end</span><span class="p">())</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">it</span><span class="o">-></span><span class="n">size</span><span class="p">();</span><span class="w"> </span><span class="p">}</span>
<span class="c1">//} このカッコを省略してしまった</span>
<span class="c1">// "it" は不要なのにここでも使えてしまう、すなわちリークしてしまう</span>
</code></pre></div>
</p>
<p>大抵はカッコを省略しても同じ効果を得られるし、変数がリークしても問題にならないが、オブジェクトの生存期間が大事な場合もある。
例えばミューテックスのロックがリークすると予測しづらいバグが発生しやすい。</p>
<p>新しい記法を使えば、初期化に使用した変数のスコープは<code>if</code>文または <code>switch</code>文の内部に限定されるため、
変数の名前をつけるときに外側のスコープを意識して長い名前をつける必要がない利点もある。</p>
<h3>ライブラリを使う</h3>
<p>従来記法で同じ意味の記述が可能である以上、ライブラリによる実現は不可能ではないと思われる。
しかしコードが見づらくなる問題があるだろう。</p>
<h3>他言語の記法を取り入れる</h3>
<p>従来と異なる記法、例えば <code>with (init) if (cond) E;</code> のような記法を取り入れる方法も考えられる。
新しいキーワードの導入により過去のC++コードとの互換性に問題が生じる可能性があるし、
新しい概念を教えたり広めたりするコストが掛かる。さらに<code>for</code>文と文法の一貫性を取れる機会が失われてしまう問題がある。</p>
<h2><a href="#relative-page" id="relative-page">関連項目</a></h2>
<ul>
<li><a href="../cpp20/range-based_for_statements_with_initializer.html">C++20 初期化式をともなう範囲for文</a></li>
<li><a href="../cpp23/extend_init_statement_to_allow_alias_declaration.html">C++23 初期化文での型の別名宣言を許可</a></li>
<li><a href="../cpp26/structured_binding_declaration_as_a_condition.html">C++26 条件式での構造化束縛の使用を許可</a></li>
</ul>
<h2>参照</h2>
<ul>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0305r1.html" target="_blank">P0305R1 Selection statements with initializer.</a></li>
</ul></div>
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