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POWhy does this std::sort predicate fail when the class is inside main()?
 

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  1. POWhy does this std::sort predicate fail when the class is inside main()?
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    <p>This is a much simplified repro which illustrates how <code>class Predicate</code> delcared outside <code>main()</code> works but when the exact code appears inline as <code>class InlinePredicate</code> the compiler can't match <code>std::sort</code>. The strange thing is that you can pass <em>anything</em> as the third argument to <code>std::sort</code> (say, integer 7) and you'll just get a compile error when it does not support the <code>operator ()</code> that <code>sort</code> expects. But when I pass <code>pred2</code> below it doesn't match at all:</p> <pre><code>#include &lt;string&gt; #include &lt;vector&gt; #include &lt;algorithm&gt; using namespace std; class Predicate { public: bool operator () (const pair&lt;string,int&gt;&amp; a, const pair&lt;string,int&gt;&amp; b) { return a.second &lt; b.second; } }; int main() { vector&lt;pair&lt;string, int&gt; &gt; a; Predicate pred; sort(a.begin(), a.end(), pred); class InlinePredicate { public: bool operator () (const pair&lt;string,int&gt;&amp; a, const pair&lt;string,int&gt;&amp; b) { return a.second &lt; b.second; } } pred2; sort(a.begin(), a.end(), pred2); return 0; } </code></pre> <blockquote> <p>repro.cc: In function ‘int main()’:</p> <p>repro.cc:30: error: no matching function for call to ‘sort(__gnu_cxx::__normal_iterator, std::allocator >, int>*, std::vector, std::allocator >, int>, std::allocator, std::allocator >, int> > > >, __gnu_cxx::__normal_iterator, std::allocator >, int>*, std::vector, std::allocator >, int>, std::allocator, std::allocator >, int> > > >, main()::InlinePredicate&amp;)’</p> </blockquote>
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    1. USBen Jackson
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    1. COAs a side note: your operator should probably be const `bool operator()(const pair<string,int>& a, const pair<string,int>& b) **const**`
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      1. USMartin York
 

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