This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 115a. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2024-07-24


484. Can a base-specifier name a cv-qualified class type?

Section: 11.7  [class.derived]     Status: CD1     Submitter: Richard Corden     Date: 21 Oct 2004

[Voted into WP at the October, 2006 meeting.]

Issue 298, recently approved, affirms that cv-qualified class types can be used as nested-name-specifiers. Should the same be true for base-specifiers?

Rationale (April, 2005):

The resolution of issue 298 added new text to 11.3 [class.name] paragraph 5 making it clear that a typedef that names a cv-qualified class type is a class-name. Because the definition of base-specifier simply refers to class-name, it is already the case that cv-qualified class types are permitted as base-specifiers.

Additional notes (June, 2005):

It's not completely clear what it means to have a cv-qualified type as a base-specifier. The original proposed resolution for issue 298 said that “the cv-qualifiers are ignored,” but that wording is not in the resolution that was ultimately approved.

If the cv-qualifiers are not ignored, does that mean that the base-class subobject should be treated as always similarly cv-qualified, regardless of the cv-qualification of the derived-class lvalue used to access the base-class subobject? For instance:

    typedef struct B {
        void f();
        void f() const;
        int i;
    } const CB;

    struct D: CB { };

    void g(D* dp) {
        dp->f();    // which B::f?
        dp->i = 3;  // permitted?
    }

Proposed resolution (October, 2005):

  1. Change 11.3 [class.name] paragraph 5 as indicated:

  2. A typedef-name (9.2.4 [dcl.typedef]) that names a class type, or a cv-qualified version thereof, is also a class-name, but class-name. If a typedef-name that names a cv-qualified class type is used where a class-name is required, the cv-qualifiers are ignored. A typedef-name shall not be used as the identifier in a class-head.
  3. Delete 9.2.4 [dcl.typedef] paragraph 8:

  4. [Note: if the typedef-name is used where a class-name (or enum-name) is required, the program is ill-formed. For example,

        typedef struct {
            S();     // error: requires a return type because S is
                      // an ordinary member function, not a constructor
        } S;
    

    end note]