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POWhy does string extraction from a stream set the eof bit?
 

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  1. POWhy does string extraction from a stream set the eof bit?
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    <p>Let's say we have a stream containing simply:</p> <pre><code>hello </code></pre> <p>Note that there's no extra <code>\n</code> at the end like there often is in a text file. Now, the following simple code shows that the <code>eof</code> bit is set on the stream after extracting a single <code>std::string</code>.</p> <pre><code>int main(int argc, const char* argv[]) { std::stringstream ss("hello"); std::string result; ss &gt;&gt; result; std::cout &lt;&lt; ss.eof() &lt;&lt; std::endl; // Outputs 1 return 0; } </code></pre> <p>However, I can't see why this would happen according to the standard (I'm reading C++11 - <em>ISO/IEC 14882:2011(E)</em>). <code>operator&gt;&gt;(basic_stream&lt;...&gt;&amp;, basic_string&lt;...&gt;&amp;)</code> is defined as behaving like a <em>formatted input function</em>. This means it constructs a <code>sentry</code> object which proceeds to eat away whitespace characters. In this example, there are none, so the <code>sentry</code> construction completes with no problems. When converted to a <code>bool</code>, the <code>sentry</code> object gives <code>true</code>, so the extractor continues to get on with the actual extraction of the string.</p> <p>The extraction is then defined as:</p> <blockquote> <p>Characters are extracted and appended until any of the following occurs:</p> <ul> <li><code>n</code> characters are stored;</li> <li>end-of-file occurs on the input sequence;</li> <li><code>isspace(c,is.getloc())</code> is true for the next available input character <em>c</em>.</li> </ul> <p>After the last character (if any) is extracted, is.width(0) is called and the sentry object k is destroyed. If the function extracts no characters, it calls <code>is.setstate(ios::failbit)</code>, which may throw <code>ios_base::failure</code> (27.5.5.4).</p> </blockquote> <p>Nothing here actually causes the <code>eof</code> bit to be set. Yes, extraction stops if it hits the end-of-file, but it doesn't set the bit. In fact, the <code>eof</code> bit should only be set if we do another <code>ss &gt;&gt; result;</code>, because when the <code>sentry</code> attempts to gobble up whitespace, the following situation will occur:</p> <blockquote> <p>If <code>is.rdbuf()-&gt;sbumpc()</code> or <code>is.rdbuf()-&gt;sgetc()</code> returns <code>traits::eof()</code>, the function calls <code>setstate(failbit | eofbit)</code></p> </blockquote> <p>However, this is definitely not happening yet because the <code>failbit</code> isn't being set.</p> <p>The consequence of the <code>eof</code> bit being set is that the only reason the evil-idiom <code>while (!stream.eof())</code> doesn't work when reading files is because of the extra <code>\n</code> at the end and <em>not</em> because the <code>eof</code> bit isn't yet set. My compiler is happily setting the <code>eof</code> bit when the extraction stops at the end of file.</p> <p>So should this be happening? Or did the standard mean to say that <code>setstate(eofbit)</code> should occur?</p> <hr> <p>To make it easier, the relevant sections of the standard are:</p> <ul> <li>21.4.8.9 Inserters and extractors [string.io]</li> <li>27.7.2.2 Formatted input functions [istream.formatted]</li> <li>27.7.2.1.3 Class <code>basic_istream::sentry</code> [istream::sentry]</li> </ul>
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    1. USJoseph Mansfield
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    1. COThe evil idiom is evil because it also fails to account for a situation where the input stream doesn't even exist, like when you open a program without an open `stdin`.
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    2. CO+1 Well formed question, includes references to the standard and minimal examples.
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      1. USThomas Matthews
 

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