title: Deleting mapping from mapping in Solidity
body: <p>I have something like this:</p>
  <pre><code>mapping (address =&gt; mapping(string =&gt; uint)) m_Map;
  </code></pre>
  <p>It can be accessed as
  <code>m_Map[strCampaignName][addrRecipient]</code>, campaign can have multiple recipients...</p>
  <p>Now at some point (ICO failed), I need to remove that campaign with all recipients. I don't think a simple
  <code>delete m_Map[strCampaignName]</code> will work.
  If I use
  <code>m_Map[strCampaignName] = null</code>, I think data will not be deleted.
  If I iterate through a list of all recipients, I will run out of gas.</p>
  <p>How should this situation be handled?
  Min: I want m_Map[strCampaignName] to be empty,
  Max: I want to stop wasting memory on it.</p>