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    <p>Hash_map and hash_set are non-standard, <a href="http://msdn.microsoft.com/en-us/library/bb983026.aspx" rel="nofollow noreferrer">unordered_map</a> and <a href="http://msdn.microsoft.com/en-us/library/bb982739.aspx" rel="nofollow noreferrer">unordered_set</a> are the most likely soon to be standard versions. Without having a reproducer, I don't think this is going to get far though. Under the hood, they are the same data structures, so they should have similar performance.</p> <hr> <p>I compiled the provided sample under MS Visual Studio 2008 v9.0.30729.1, as Visual C++ -> Win32 -> Console Application (though I rolled my own Timer class because I wasn't sure what you were using). Under debug, I got times of 1000 ms, but compiling under release was 50 ms.</p> <pre><code>#include &lt;vector&gt; #include &lt;iostream&gt; #include &lt;map&gt; #include &lt;stdio.h&gt; #include &lt;stdlib.h&gt; #include &lt;time.h&gt; #include &lt;windows.h&gt; typedef struct { LARGE_INTEGER start; LARGE_INTEGER stop; } stopWatch; class CStopWatch { private: stopWatch timer; LARGE_INTEGER frequency; double LIToSecs( LARGE_INTEGER &amp; L); public: CStopWatch(); void startTimer( ); void stopTimer( ); double getElapsedTime(); }; double CStopWatch::LIToSecs( LARGE_INTEGER &amp; L) { return ((double)L.QuadPart /(double)frequency.QuadPart) ; } CStopWatch::CStopWatch(){ timer.start.QuadPart=0; timer.stop.QuadPart=0; QueryPerformanceFrequency( &amp;frequency ) ; } void CStopWatch::startTimer( ) { QueryPerformanceCounter(&amp;timer.start) ; } void CStopWatch::stopTimer( ) { QueryPerformanceCounter(&amp;timer.stop) ; } double CStopWatch::getElapsedTime() { LARGE_INTEGER time; time.QuadPart = timer.stop.QuadPart - timer.start.QuadPart; return LIToSecs( time) ; } using namespace std; int runIntersectionTest() { std::map&lt;int,int&gt; theMap; vector&lt;int&gt; set1; vector&lt;int&gt; set2; // Create 100,000 values for set1 for ( int i = 0; i &lt; 100000; i++ ) { int value = 1000000000 + i; set1.push_back(value); } // Create 1,000 values for set2 for ( int i = 0; i &lt; 1000; i++ ) { int random = rand() % 200000 + 1; random *= 10; int value = 1000000000 + random; set2.push_back(value); } // Now intersect the two sets by populating the map for ( vector&lt;int&gt;::iterator iterator = set1.begin(); iterator != set1.end(); iterator++ ) { int value = *iterator; theMap[value] = 1; } int intersectionSize = 0; for ( vector&lt;int&gt;::iterator iterator = set2.begin(); iterator != set2.end(); iterator++ ) { int value = *iterator; map&lt;int,int&gt;::iterator foundValue = theMap.find(value); if ( foundValue != theMap.end() ) { theMap[value] = 2; intersectionSize++; } } return intersectionSize; } int main(int argc, char* argv[]) { srand ( time(NULL) ); int tests = 2; while(tests--){ CStopWatch timer; timer.startTimer(); int intersectionSize = runIntersectionTest(); timer.stopTimer(); cout &lt;&lt; "Found " &lt;&lt; intersectionSize &lt;&lt; " values in the intersection, in " &lt;&lt; timer.getElapsedTime() &lt;&lt; "s\r\n"; } getchar(); return 0; } </code></pre> <p>(I would try with unordered_map but my version doesn't have it). I suspect there is some problem in your setup for C++.</p>
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    1. CONote: boost offers implementation of both.
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    2. COI figured something out: if I attach the debugger to either RELEASE or DEBUG builds (e.g. hit F5 in the IDE), then I get horrible times.
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