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POC++11 Magically Deleted Constructor in BST
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copied!<p><strong>SOLVED</strong>! See below</p> <p>So, I'm trying to learn C++11 by doing some simple data structures and playing around with them. I did something <em>similar</em> to the following BST example using raw pointers and <code>new</code> and <code>delete</code> and it worked fine. Then I wanted to do it in a way that was more leak-safe.</p> <pre><code>// tree.cpp // // #include <iostream> #include <memory> /* DECLARATIONS */ template <typename T> struct Tree { // members T data; std::unique_ptr<Tree<T> > left; std::unique_ptr<Tree<T> > right; // methods Tree (T arg); ~Tree () = default; void insert (Tree<T> child); void insert (T arg); void print (void); }; template <typename T> Tree<T>::Tree (T arg) { data = arg; left = nullptr; right = nullptr; } template <typename T> void Tree<T>::insert (Tree<T> child) { if (child.data < data) { if (left) { left->insert(child); } else { left = &child; } } else { if (right) { right->insert(child); } else { right = &child; } } } template <typename T> void Tree<T>::insert (T arg) { Tree<T> child (arg); this->insert(child); } template <typename T> void Tree<T>::print (void) { if (left) { left->print(); } std::cout << data; if (right) { right->print(); } } int main (void) { Tree<int> root (0); root.insert(3); root.insert(-3); root.insert(-2); root.insert(2); root.insert(11); root.print(); return 0; } </code></pre> <p>I don't undersatnd the error I'm getting from clang++, however.</p> <p><code>$ clang++ -std=c++11 tree.cpp</code></p> <pre><code>tree_new.cpp:50:16: error: call to deleted constructor of 'Tree<int>' this->insert(child); ^~~~~ tree_new.cpp:66:8: note: in instantiation of member function 'Tree<int>::insert' requested here root.insert(3); ^ tree_new.cpp:10:8: note: function has been explicitly marked deleted here struct Tree { ^ tree_new.cpp:18:24: note: passing argument to parameter 'child' here void insert (Tree<T> child); ^ tree_new.cpp:34:20: error: call to deleted constructor of 'Tree<int>' left->insert(child); ^~~~~ tree_new.cpp:50:9: note: in instantiation of member function 'Tree<int>::insert'requested here this->insert(child); ^ tree_new.cpp:66:8: note: in instantiation of member function 'Tree<int>::insert' requested here root.insert(3); ^ tree_new.cpp:10:8: note: function has been explicitly marked deleted here struct Tree { ^ tree_new.cpp:18:24: note: passing argument to parameter 'child' here void insert (Tree<T> child); ^ 2 errors generated. </code></pre> <p>Why does it say I explicitly deleted the constructor when I declared the <code>struct</code>? I even defined a constructor explicitly! Also, any comments on scoping/ownership failure would be appreciated. I'm pretty sure this won't work the way I did it anyways.</p> <p><strong>Solution</strong></p> <p>The following link from <a href="http://msdn.microsoft.com/en-us/library/vstudio/hh279676.aspx" rel="nofollow">MSDN</a> clarified how one can use <code>unique_ptr</code>s.</p> <p>Special thanks to BatchyX for his initial explanation of the problem (using a <code>unique_ptr</code> as a member implicitly (though the compiler says "explicitly"...) deletes the copy constructor for the class), and for noting that indeed Tree is still movable.</p> <p>Something mentioned in that MSDN article is that <code>std::move()</code> returns an rvalue of its argument.</p> <p>Here is the suitably modified code (excluding the obviously modified declarations). Note that there might be some optimizations possible still by using std::forward, but this at least seems to compile and run correctly.</p> <pre><code>template <typename T> void Tree<T>::insert (std::unique_ptr<Tree<T> >&& pchild) { if (pchild->data < data) { if (left) { // recurse, but must match on the rvalue signature left->insert(std::move(pchild)); } else { // invokes the move constructor for left instead of its copy constructor left = std::move(pchild); } } else { if (right) { right->insert(std::move(pchild)); } else { right = std::move(pchild); } } } template <typename T> void Tree<T>::insert (T arg) { // what is inside the insert(...) is an rvalue. this->insert(std::unique_ptr<Tree<T> >(new Tree<T> (arg))); } </code></pre>

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