Unseen C++ syntax
<p>A friend sent me this and I really don't know what it is, inside the loop. Whatever it be it calls the std::set constructor half million times.... any help appreciated. I would expect a compiler error, but it actually compiles in g++ 4.4 and 4.5 and the behavior is different to copy construction...</p>

<pre><code>#include &lt;stdio.h&gt;
#include &lt;stdlib.h&gt;
#include &lt;boost/unordered_map.hpp&gt;
#include &lt;set&gt;
#include &lt;string&gt;

typedef boost::unordered_map&lt;int, std::set&lt;int&gt; &gt; mymap;

int main () {
    mymap map;
    for ( int i = 0 ; i &lt; 1000 ; i++ )     
    {
        std::set&lt;int&gt;  map[i] ;
    }
    return 1;
};
</code></pre>
