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copied!<p><a href="http://en.cppreference.com/w/cpp/memory/unique_ptr" rel="nofollow noreferrer"><code>std::unique_ptr</code></a> is not copyable, and any class that contains a <code>unique_ptr</code> is also not copyable, meaning <code>struct Tree</code> is not copyable. The argument to:</p> <pre><code>void Tree<T>::insert (Tree<T> child) { </code></pre> <p>is taking its argument by value. And:</p> <pre><code>template <typename T> void Tree<T>::insert (T arg) { Tree<T> child (arg); this->insert(child); } </code></pre> <p>requires the copy constructor. To correct this, make <code>struct Tree</code> moveable.</p> <hr> <p>Note <code>Tree</code> is <em>not</em> moveable (in contrast to <a href="https://stackoverflow.com/users/22985/batchyx">BatchyX</a>'s comment) due to the presence of:</p> <pre><code>~Tree () = default; </code></pre> <p>which is a <em>user-declared</em> destructor and from section <em>12.8 Copying and moving class objects</em> (point 9) of the c++11 standard (draft n3337):</p> <blockquote> <p>If the definition of a class X does not explicitly declare a move constructor, one will be implicitly declared as defaulted if and only if</p> <ul> <li>X does not have a user-declared copy constructor,</li> <li>X does not have a user-declared copy assignment operator,</li> <li>X does not have a user-declared move assignment operator,</li> <li><strong>X does not have a user-declared destructor</strong>, and</li> <li>the move constructor would not be implicitly defined as deleted.</li> </ul> </blockquote> <p>(I was uncertain about the implicit generation of the move members and asked <a href="https://stackoverflow.com/questions/16897845/move-member-function-generation">this</a> question to be certain). To make it moveable either:</p> <ul> <li>remove the user-declared destructor, or</li> <li>define a move constructor and move assignment operator</li> </ul>

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