Is it possible to use return value move semantics with a protected copy constructor in C++?

Question

I am using the CGAL library and writing a function that I hope to use move semantics as described in this question:

PointTree preparePointTree(const vector<PlyPoint>& pointCloud) {
  PointTree tree;
  kd_tree::prepare<...>(pointCloud, tree);
  return tree;
}

and I'm calling it like:

PointTree tree = preparePointTree(pointCloud);

Unfortunately, it does not compile; GCC complains that:

return tree;  // <-- error: 'CGAL::Kd_tree<...>' is private within this context

with note:

private:
  // protected copy constructor
  Kd_tree(const Tree& tree)   // <-- note: declared private here

which is from this code.

So the issue seems to be that returning tree is illegal if the copy-constructor is unavailable, even though I don't want to copy, but to move.

I suppose that's because because move semantics depend on the call site (the call to preparePointTree()), and the function implementation by itself is treated as "has to compile even in the non-move case".

So, what now?

Is there something that I, or the CGAL library, can do to allow moving but not copying?

What's the reasoning behind this?

Answer 1

From cppreference on move constructors:

If no user-defined move constructors are provided for a class type (struct, class, or union), and all of the following is true:

• there are no user-declared copy constructors;
• there are no user-declared copy assignment operators;
• there are no user-declared move assignment operators;
• there is no user-declared destructor.

then the compiler will declare a move constructor as a non-explicit inline public member of its class with the signature T::T(T&&).

The Kd_tree class provides a user-declared copy constructor, so it doesn't get a move constructor automatically. Since it doesn't have a move constructor you cannot move it.