const cast and pointers to pointers
<blockquote>
  <p><strong>Possible Duplicate:</strong><br>
  <a href="http://stackoverflow.com/questions/5055655/double-pointer-const-correctness-warnings-in-c">Double pointer const-correctness warnings in C</a>  </p>
</blockquote>



<p>Why does the cases number 1, 2 and 3 works fine in C++, but not number 4 ?</p>

<pre><code>//1
char* p1;
const char* p2 = p1;

//2
char** p3;
char * const * p4 = p3;

//3
char** p5;
const char* const * p6 = p5;

//4
char** p7;
const char** p8 = p7;
</code></pre>
