title: How to add valid key without specifying value to a std::map?
body: <p>I have a std::map, and I would like to add a valid key to iterate over it later, but without giving any value (it will be given later on in the course of the iterations). </p>
  
  <p>This is how I do it for now :</p>
  
  <pre><code>std::vector&lt;std::string&gt; valid_keys;
  //Fill... Then :
  std::map&lt;std::string, float&gt; map;
  for(size_t i = 0 ; i &lt; valid_keys.size() ; ++i) {
      /*I don't want to do that because in fact I don't use a float type*/ 
      map[valid_keys[i]] = 0.f; //&lt;- 
  }
  //Using :
  for(std::map&lt;std::string, float&gt;::iterator it = map.begin() ; it != map.end() ; ++it) {
      it-&gt;second = 0; //Dummy
  }
  </code></pre>
  
  <p>How can I do that, please ?</p>
  
  <p>Thanks aforehand.</p>