title: What is shared_ptr's aliasing constructor for?
body: <p>In this page (<a href="http://www.cplusplus.com/reference/memory/shared_ptr/">http://www.cplusplus.com/reference/memory/shared_ptr/</a>), paragraph 5, it says:</p>
  
  <blockquote>
    <p>Additionally, shared_ptr objects can share ownership over a pointer while at the same time pointing to another object. This ability is known as aliasing (see constructors), and is commonly used to point to member objects while owning the object they belong to. Because of this, a shared_ptr may relate to two pointers:</p>
    
    <ul>
    <li><p>A stored pointer, which is the pointer it is said to point to, and the one it dereferences with operator*.</p></li>
    <li><p>An owned pointer (possibly shared), which is the pointer the ownership group is in charge of deleting at some point, and for which it counts as a use.</p></li>
    </ul>
    
    <p>Generally, the stored pointer and the owned pointer refer to the same object, <strong>but alias shared_ptr objects (those constructed with the alias constructor and their copies) may refer to different objects</strong>.</p>
  </blockquote>
  
  <p>Then I read this page (<a href="http://www.cplusplus.com/reference/memory/shared_ptr/shared_ptr/">http://www.cplusplus.com/reference/memory/shared_ptr/shared_ptr/</a>) about the aliasing constructor of shared_ptr. But I still think this "aliasing" behavior confusing. <strong>Why is it here? What is it for? In what situation would I want this feature?</strong></p>