title: Accessing a std::map of std::map like an array
body: <p>I have initialize a std::map of a std::map like as below:</p>
  
  <pre><code>static std::map&lt;std::string, std::map&lt;std::string, float&gt;&gt; _ScalingMapFrequency = {
              {"mHz",    {{"mHz",     1.0}}},
              {"mHz",    {{"Hz",      1e-3}}},
  
              {"Hz",     {{"mHz",     1e+3}}},
              {"Hz",     {{"Hz",      1.0}}}};
  </code></pre>
  
  <p>And now I am trying to access the float values in the following way:</p>
  
  <pre><code>std::cout&lt;&lt;"  the scaling factor is     :"&lt;&lt;_ScalingMapFrequency["mHz"]["Hz"];
  </code></pre>
  
  <p>There is no problem when I compile and run the code but I am expecting to get "1e-3" instead I am always getting a "0". I need to access the std::map "_ScalingMapFrequency" as an array, that being the design decision. </p>
  
  <p>What mistake I am making ? Please give me some pointer and I would greatly appreciate. </p>