Title
cv-qualifiers on function types
Status
cd1
Section
9.3.4.6 [dcl.fct]
Submitter
Nathan Sidwell

Created on 2001-06-29.00:00:00 last changed 161 months ago

Messages

Date: 2002-10-15.00:00:00

[Moved to DR at October 2002 meeting.]

Date: 2003-04-15.00:00:00

Notes from April 2003 meeting:

Nathan Sidwell pointed out that some libraries use the inability to add const to a type T as a way of testing that T is a function type. He will get back to us if he has a proposal for a change.

Date: 2002-04-15.00:00:00

Notes from 4/02 meeting:

Yes, the difference is intentional. There is no way to add cv-qualifiers other than those cases.

Date: 2001-10-15.00:00:00

Proposed resolution (10/01):

In 9.3.4.6 [dcl.fct] paragraph 4, replace

The effect of a cv-qualifier-seq in a function declarator is not the same as adding cv-qualification on top of the function type, i.e., it does not create a cv-qualified function type. In fact, if at any time in the determination of a type a cv-qualified function type is formed, the program is ill-formed. [Example:
  typedef void F();
  struct S {
    const F f;          // ill-formed
  };
-- end example]
by
The effect of a cv-qualifier-seq in a function declarator is not the same as adding cv-qualification on top of the function type. In the latter case, the cv-qualifiers are ignored. [Example:
  typedef void F();
  struct S {
    const F f;          // ok; equivalent to void f();
  };
-- end example]

Strike the last bulleted item in 13.10.3 [temp.deduct] paragraph 2, which reads

Attempting to create a cv-qualified function type.

Nathan Sidwell comments (18 Dec 2001 ): The proposed resolution simply states attempts to add cv qualification on top of a function type are ignored. There is no mention of whether the function type was introduced via a typedef or template type parameter. This would appear to allow

  void (const *fptr) ();
but, that is not permitted by the grammar. This is inconsistent with the wording of adding cv qualifiers to a reference type, which does mention typedefs and template parameters, even though
  int &const ref;
is also not allowed by the grammar.

Is this difference intentional? It seems needlessly confusing.

Date: 2001-06-29.00:00:00

This concerns the inconsistent treatment of cv qualifiers on reference types and function types. The problem originated with GCC bug report c++/2810. The bug report is available at http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view&pr=2810&database=gcc

9.3.4.3 [dcl.ref] describes references. Of interest is the statement (my emphasis)

Cv-qualified references are ill-formed except when the cv-qualifiers are introduced through the use of a typedef or of a template type argument, in which case the cv-qualifiers are ignored.

Though it is strange to ignore 'volatile' here, that is not the point of this defect report. 9.3.4.6 [dcl.fct] describes function types. Paragraph 4 states,

In fact, if at any time in the determination of a type a cv-qualified function type is formed, the program is ill-formed.

No allowance for typedefs or template type parameters is made here, which is inconsistent with the equivalent reference case.

The GCC bug report was template code which attempted to do,

    template <typename T> void foo (T const &);
    void baz ();
    ...
    foo (baz);

in the instantiation of foo, T is `void ()' and an attempt is made to const qualify that, which is ill-formed. This is a surprise.

Suggested resolution:

Replace the quoted sentence from paragraph 4 in 9.3.4.6 [dcl.fct] with

cv-qualified functions are ill-formed, except when the cv-qualifiers are introduced through the use of a typedef or of a template type argument, in which case the cv-qualifiers are ignored.

Adjust the example following to reflect this.

History
Date User Action Args
2008-10-05 00:00:00adminsetstatus: wp -> cd1
2003-04-25 00:00:00adminsetmessages: + msg865
2003-04-25 00:00:00adminsetstatus: dr -> wp
2002-11-08 00:00:00adminsetmessages: + msg773
2002-11-08 00:00:00adminsetstatus: ready -> dr
2002-05-10 00:00:00adminsetmessages: + msg615
2002-05-10 00:00:00adminsetstatus: review -> ready
2001-11-09 00:00:00adminsetmessages: + msg553
2001-11-09 00:00:00adminsetstatus: open -> review
2001-06-29 00:00:00admincreate