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    <p>The <code>tellg()</code> function works by attempting to construct a sentry object and then checking for the <code>failbit</code> before returning a proper value. If the <code>failbit</code> is set, it returns -1. Details can be found <a href="http://www.cplusplus.com/reference/istream/istream/tellg/" rel="nofollow">here</a> or, if you prefer a more official source and don't mind a dry read, the ISO C++ standard (<code>27.6.1.3/36a-37</code> for C++03, <code>27.7.2.3/39-40</code> for C++11).</p> <p>The construction of the sentry first checks any of the <a href="http://www.cplusplus.com/reference/ios/ios/rdstate/" rel="nofollow">error flags</a> (like <code>eofbit</code>) and, if set, it sets the <code>failbit</code> and returns. See <a href="http://www.cplusplus.com/reference/istream/istream/sentry/" rel="nofollow">here</a> for detail (C++03 <code>27.6.1.1.2</code>, C++11 <code>27.7.2.1.3</code>),</p> <p>Hence a <code>tellg()</code> after the end of file flag has been set will fail. The fact that you're reading lines until <code>getline</code> returns false means that the stream's <code>eofbit</code> is being set, hence you've reached the end of the file.</p> <p>You can see the behavior with this following program:</p> <pre><code>#include &lt;iostream&gt; #include &lt;iomanip&gt; int main (void) { std::string line; while (std::getline (std::cin, line)) { if (line.length() &gt; 20) line = line.substr(0,17) + "..."; std::cout &lt;&lt; "tellg() returned " &lt;&lt; std::setw(5) &lt;&lt; std::cin.tellg() &lt;&lt; " after " &lt;&lt; line &lt;&lt; "\n"; } //std::cin.clear(); std::cout &lt;&lt; "tellg() returns: " &lt;&lt; std::cin.tellg() &lt;&lt; '\n'; return 0; } </code></pre> <p>When you run that and provide the file itself as input, you see:</p> <pre><code>tellg() returned 20 after #include &lt;iostream&gt; tellg() returned 39 after #include &lt;iomanip&gt; tellg() returned 40 after tellg() returned 58 after int main (void) { tellg() returned 80 after std::string l... tellg() returned 124 after while (std::g... tellg() returned 156 after if (line.... tellg() returned 202 after line ... tellg() returned 243 after std::cout... tellg() returned 291 after &lt;&lt; st... tellg() returned 333 after &lt;&lt; " ... tellg() returned 339 after } tellg() returned 363 after //std::cin.cl... tellg() returned 400 after std::cout &lt;&lt; ... tellg() returned 437 after &lt;&lt; std::c... tellg() returned 451 after return 0; tellg() returned 453 after } tellg() returned 454 after tellg() returns: -1 </code></pre> <p>If you uncomment the line in that code which clears the error state variables, it will work:</p> <pre><code>tellg() returned 20 after #include &lt;iostream&gt; tellg() returned 39 after #include &lt;iomanip&gt; tellg() returned 40 after tellg() returned 58 after int main (void) { tellg() returned 80 after std::string l... tellg() returned 124 after while (std::g... tellg() returned 156 after if (line.... tellg() returned 202 after line ... tellg() returned 243 after std::cout... tellg() returned 291 after &lt;&lt; st... tellg() returned 333 after &lt;&lt; " ... tellg() returned 339 after } tellg() returned 361 after std::cin.clea... tellg() returned 398 after std::cout &lt;&lt; ... tellg() returned 435 after &lt;&lt; std::c... tellg() returned 449 after return 0; tellg() returned 451 after } tellg() returned 452 after tellg() returns: 452 </code></pre> <hr> <p>And, as an aside, it looks like the bug you're referring to may be <a href="http://gcc.gnu.org/bugzilla/show_bug.cgi?id=2259" rel="nofollow">this one</a> (it's a little unclear since the post you linked to is sadly missing any detail - it would have been better had the poster bothered to <em>support</em> his assertion that it was a known bug by, for example, <em>linking</em> to it).</p> <p>If that's the case, the <em>first</em> thing you should notice is that it was fixed more than a decade ago so, unless you're using an absolutely <em>ancient</em> <code>gcc</code>, it's not going to be an issue now.</p>
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    1. POC++ tellg() doesn't work with getline()?
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    1. COI'm not actually calling close() on the input stream before calling tellg(). You're saying getline() sets the failbit after it runs through to the end of the file?
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    2. COMore specifically, `getline` will set the `failbit` when it extracts no characters, which will be typical at the end of file.
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    3. COEven if `getline` doesn't set failbit, it will set eofbit at end of file, and the sentry construction will set failbit because of that.
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