title: What does the "::" mean in "::tolower"?
body: <p>I've seen code like this:</p>
  
  <pre><code>std::string str = "wHatEver";
  std::transform(str.begin(), str.end(), str.begin(), ::tolower);
  </code></pre>
  
  <p>And I have a question: what does mean <strong><code>::</code></strong> before tolower?</p>
  
  <p>and <code>std::tolower</code> not works, but <code>::tolower</code> works OK</p>