title: C++ unordered_map fail when used with a vector as key
body: <p>Background: I am comming from the Java world and I am fairly new to C++ or Qt.</p>
  <p>In order to play with unordered_map, I have written the following simple program:</p>
  <pre><code>#include &lt;QtCore/QCoreApplication&gt;
  #include &lt;QtCore&gt;
  #include &lt;iostream&gt;
  #include &lt;stdio.h&gt;
  #include &lt;string&gt;
  #include &lt;unordered_map&gt;
  
  using std::string;
  using std::cout;
  using std::endl;
  typedef std::vector&lt;float&gt; floatVector;
  
  int main(int argc, char *argv[]) {
      QCoreApplication a(argc, argv);
      
      floatVector c(10);
      floatVector b(10);
      
      for (int i = 0; i &lt; 10; i++) {
          c[i] = i + 1;
          b[i] = i * 2;
      }
      
      std::unordered_map&lt;floatVector, int&gt; map;
      
      map[b] = 135;
      map[c] = 40;
      map[c] = 32;
    
      std::cout &lt;&lt; &quot;b -&gt; &quot; &lt;&lt; map[b] &lt;&lt; std::endl;
      std::cout &lt;&lt; &quot;c -&gt; &quot; &lt;&lt; map[c] &lt;&lt; std::endl;
      std::cout &lt;&lt; &quot;Contains? -&gt; &quot; &lt;&lt; map.size() &lt;&lt; std::endl;
      
      return a.exec();
  }
  </code></pre>
  <p>Unfortunately, I am running into the folowing error which isn't inspiring. There is not even a line number.</p>
  <blockquote>
  <p>:-1: error: collect2: ld returned 1 exit status</p>
  </blockquote>
  <p>Any idea of the origin of the problem?</p>