title: How to use an integer array as a key of an unordered_map
body: <p>I would like to use an integer array as a key of an unordered_map. The basic idea is that I have many different states of a problem which are represented as <code>int state[16]</code>. The values of the array are permutations of numbers from 0 to 15, like:</p>
  
  <pre><code>a= { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15}; 
  b= { 14, 1, 9, 6, 4, 8, 12, 5, 7, 2, 3, 0, 10, 11, 13, 15}; ...
  </code></pre>
  
  <p>and those will be the key in the unordered_map (the value will be a class with other stuff). How can I do that? Do I need to implement a new hash function to compare the values or I can use some provided by C++? 
  My goal is using this as a hash-table, is there any other better alternative?</p>