title: Convert hex std::string to unsigned char
body: <p>I have a hex string and want to convert this to a hex unsigned char array!</p>
  
  <pre><code>std::string hex = "0c45a1bf"
  
  unsigned char hexCh = ""
      [0] = "0c"
      [1] = "45"
      [2] = "a1"
      [3] = "bf"
  </code></pre>
  
  <p>I want this bevavior shown in hexCh!</p>
  
  <p>Best way over stringstream and std::hex? Have you an implementation?!</p>
  
  <p>Thx</p>