title: Add unique_ptr of derived class to vector of base class unique_ptr
body: <p>I have a <code>std::unique_ptr&lt;Derived&gt;</code> that I want to add to a <code>std::vector&lt;std::unique_ptr&lt;Base&gt;&gt;</code>.</p>
  
  <pre><code>std::unique_ptr&lt;Derived&gt; derivedObject;
  std::vector&lt;std::unique_ptr&lt;Base&gt;&gt; vec;
  
  vec.push_back(derivedObject) // Invalid arguments
  </code></pre>