title: boost::copy_file copy_option to skip over existing destination file?
body: <p>I would like to copy a file to an other, and I would like to use Boost::copy_file. It has a paramether called copy_option which can be:</p>
  
  <pre><code>  BOOST_SCOPED_ENUM_START(copy_option)
      {none, fail_if_exists = none, overwrite_if_exists};
    BOOST_SCOPED_ENUM_END
  </code></pre>
  
  <p>I have found another question regarding the <code>overwrite_if_exists</code> behaviour here: <a href="https://stackoverflow.com/questions/829468/how-to-perform-boostfilesystem-copy-file-with-overwrite">how to perform boost::filesystem copy_file with overwrite</a></p>
  
  <p>My problem however is that I don't know how to use the <code>fail_if_exists = none</code> option. I would like to <strong>skip</strong> the copy operation, if the target file already exists.</p>
  
  <p>I know its possible with <code>if ( !exists(path) )</code> but I want to understand how does <code>copy_option</code> work. </p>
  
  <p>How can I use <code>fail_if_exists = none</code> inside Boost::copy_file function?</p>
  
  <p><strong>Update</strong>: corrected the code, <a href="http://www.boost.org/doc/libs/1_46_1/libs/filesystem/v3/doc/reference.html#copy_option" rel="nofollow noreferrer">the one on boost doc website</a> is kind of broken.</p>