Returning Rvalue Reference and Temporary Materialization

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Consider the following functions. I'd like answers for C++17.

MyClass&& func() {
  return MyClass{};
}

int main() {
  MyClass&& myRef = func();
}

Questions:

  1. Is the expression func() an xvalue? Why?
  2. Why is myRef a dangling reference? Or, more specifically, why is func() returning a dangling reference? Wouldn't returning rvalue reference cause temporary materialization, and extend the temporary object's lifetime?
xvalue c++17 return rvalue-reference c++
1 Answers

func() is an xvalue because one of the rules of the language is that if a function is declared to have a return type of rvalue reference to object, then an expression consisting of calling that function is an xvalue . (C++17 expr.call/11).

Temporary materialization occurs any time a reference is bound to a prvalue.

The result of the function is myRef which is initialized by the prvalue func(). However if we consult the lifetime extension rules in class.temporary/6 it has:

The lifetime of a temporary bound to the returned value in a function return statement is not extended; the temporary is destroyed at the end of the full-expression in the return statement.

So the temporary object materialized by func() is destroyed when the return statement completes, with no extension.

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