title: execution policy in c++
body: <p>I was going through copy_If definition in cpprefrence , I saw that there are some constructs which are dependent upon execution _policy .I searched about it ,but I didn't get any good explanation . can somebody help me understand what is this and how it can be useful ?</p>
  
  <p>Link : <a href="http://en.cppreference.com/w/cpp/algorithm/copy" rel="noreferrer">http://en.cppreference.com/w/cpp/algorithm/copy</a></p>
  
  <p>Example :</p>
  
  <pre><code>template&lt; class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class UnaryPredicate &gt;
  ForwardIt2 copy_if( ExecutionPolicy&amp;&amp; policy, ForwardIt1 first, ForwardIt1 last,
                    ForwardIt2 d_first,
                    UnaryPredicate pred )
  </code></pre>