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POstringstream operator>> fails as function, but works as instance?
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  1. POstringstream operator>> fails as function, but works as instance?
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    copied!<p>I'm writing simple code that will extract a bunch of name, int pairs from a file. I'm modifying existing code that just uses:</p> <pre><code>string chrom; unsigned int size; while ( cin &gt;&gt; chrom &gt;&gt; size ) { // save values } </code></pre> <p>But I want to use another (similar) input file that has the same first two columns, but are followed by other data (that will be ignored). So I write:</p> <pre><code>string chrom; unsigned int size; string line; while ( getline(cin, line) ) { if( stringstream(line) &gt;&gt; chrom &gt;&gt; size ) { // save values } } </code></pre> <p>But this fails to compile, giving the typical obscene std lib template spew:</p> <pre><code> error: no match for "operator&gt;&gt;" in "std::basic_stringstream&lt;char, std::char_traits&lt;char&gt;, std::allocator&lt;char&gt; &gt;(((const std::basic_string&lt;char, std::char_traits&lt;char&gt;, std::allocator&lt;char&gt; &gt;&amp;)((const std::basic_string&lt;char, std::char_traits&lt;char&gt;, std::allocator&lt;char&gt; &gt;*)(&amp; line))), std::operator|(_S_out, _S_in)) &gt;&gt; chrom" istream:131: note: candidates are: std::basic_istream&lt;_CharT, _Traits&gt;&amp; std::basic_istream&lt;_CharT, _Traits&gt;::operator&gt;&gt;(std::basic_istream&lt;_CharT, _Traits&gt;&amp; (*)(std::basic_istream&lt;_CharT, _Traits&gt;&amp;)) [with _CharT = char, _Traits = std::char_traits&lt;char&gt;] [...another dozen lines...] </code></pre> <p>Right. line isn't a std::string, but some variation of std::basic_string, etc. However, explicitly instantiating the stringstream works. </p> <pre><code>string chrom; unsigned int size; string line; while ( getline(genome, line) ) { stringstream ss(line); if ( ss &gt;&gt; chrom &gt;&gt; size ) { // save values } // Discard remainder of line } </code></pre> <p>Why? What is wrong with the first case? The <a href="http://www.cplusplus.com/doc/tutorial/basic_io/" rel="nofollow noreferrer">example basic_io</a> at the always helpful cplusplus.com works, why doesn't my code? </p> <p>Update: Another point of reference: the temporary stringstream works when the first value extracted is an int instead of a string:</p> <pre><code>unsigned int chrom; // works as int... unsigned int size; string line; while ( getline(cin, line) ) { if( stringstream(line) &gt;&gt; chrom &gt;&gt; size ) { // save values } } </code></pre>
 

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