Title
Explicit casts to reference types
Status
ready
Section
7.6.1.7 [expr.dynamic.cast]
Submitter
Richard Smith

Created on 2014-07-07.00:00:00 last changed 2 weeks ago

Messages

Date: 2024-11-19.11:51:56

CWG 2024-11-19

Resolved by the resolution of issue 2879.

Date: 2015-05-15.00:00:00

Notes from the May, 2015 meeting:

CWG reaffirmed the status quo for dynamic_cast but felt that const_cast should be changed to permit binding an rvalue reference to types that have associated memory (class and array types).

Date: 2015-05-25.00:00:00

The specification of dynamic_cast in 7.6.1.7 [expr.dynamic.cast] paragraph 2 (and const_cast in 7.6.1.11 [expr.const.cast] is the same) says that the operand of a cast to an lvalue reference type must be an lvalue, so that

  struct A { virtual ~A(); }; A &&make_a();

  A &&a = dynamic_cast<A&&>(make_a());   // ok
  const A &b = dynamic_cast<const A&>(make_a()); // ill-formed

The behavior of static_cast is an odd hybrid:

  struct B : A { }; B &&make_b();
  A &&c = static_cast<A&&>(make_b()); // ok
  const A &d = static_cast<const A&>(make_b()); // ok
  const B &e = static_cast<const B&>(make_a()); // ill-formed

(Binding a const lvalue reference to an rvalue is permitted by 7.6.1.9 [expr.static.cast] paragraph 4 but not by paragraphs 2 and 3.)

There is implementation divergence on the treatment of these examples.

Also, const_cast permits binding an rvalue reference to a class prvalue but not to any other kind of prvalue, which seems like an unnecessary restriction.

Finally, 7.6.1.9 [expr.static.cast] paragraph 3 allows binding an rvalue reference to a class or array prvalue, but not to other kinds of prvalues; those are covered in paragraph 4. This would be less confusing if paragraph 3 only dealt with binding rvalue references to glvalues and left all discussion of prvalues to paragraph 4, which adequately handles the class and array cases as well.

History
Date User Action Args
2024-11-19 11:51:56adminsetmessages: + msg7902
2024-11-19 11:51:56adminsetstatus: drafting -> ready
2015-05-25 00:00:00adminsetmessages: + msg5492
2015-05-25 00:00:00adminsetstatus: open -> drafting
2014-07-07 00:00:00admincreate