Can explicitly defaulted / deleted functions overload on ref qualifiers?

Question

INTRO

• Ref qualifiers : A way to dissambiguate the rl-valuness of the implied object. As a quick example, take the following class

  class example
  {
    int member;
  public:
    // ...
    int&       value() &;
    //                 ^
    int&&      value() &&;
    //                 ^^
    int const& value() const&;
    //                      ^
  };
  

  The use of this C++11 feature (syntax marked with ^), allows us to control the version of value() that will be called with

  • l-values
  • temporaries
  • const l-values

  Practically the ref qualification applies to the classe's *this

• Defaulted / Deleted functions : Specify a special member function as having the compiler generated (default) definition or inaccessible (delete). As an example take

  struct type {
    type(const type&)            = delete;
    type& operator=(const type&) = delete;
  };
  

  The above struct, achieves being non copyable with extremely clear semantics

QUESTIONs

• Is it possible / valid to combine these features ?
• Which are the cases where it's explicitly forbidden or bad style ?
• Is there any use case / pattern for such a combination ? (Eg creating conditional interfaces based rl-valueness quick and easy)

Answer 1

Yes, but there's not much use, as constructors and destructors can't be ref-qualified.

You can ref-qualify assignment operators:

struct S {
  S &operator=(const S &) && = default;
};

int main() {
  S s;
  S() = s;  // OK
  s = S();  // error - LHS must be an rvalue
}

However, I'm somewhat at a loss to imagine what this would be useful for.