This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 108i. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2022-07-16


330. Qualification conversions and pointers to arrays of pointers

Section: 7.3.6  [conv.qual]     Status: CD4     Submitter: Roger Orr     Date: 2 Jan 2002

[Moved to DR as N4261 at the November, 2014 meeting.]

Section 7.3.6 [conv.qual] covers the case of multi-level pointers, but does not appear to cover the case of pointers to arrays of pointers. The effect is that arrays are treated differently from simple scalar values.

Consider for example the following code: (from the thread "Pointer to array conversion question" begun in comp.lang.c++.moderated)

  int main()
  {
     double *array2D[2][3];

     double       *       (*array2DPtr1)[3] = array2D;     // Legal
     double       * const (*array2DPtr2)[3] = array2DPtr1; // Legal
     double const * const (*array2DPtr3)[3] = array2DPtr2; // Illegal
  }
and compare this code with:-
  int main()
  {
     double *array[2];

     double       *       *ppd1 = array; // legal
     double       * const *ppd2 = ppd1;  // legal
     double const * const *ppd3 = ppd2;  // certainly legal (4.4/4)
  }

The problem appears to be that the pointed to types in example 1 are unrelated since nothing in the relevant section of the standard covers it - 7.3.6 [conv.qual] does not mention conversions of the form "cv array of N pointer to T" into "cv array of N pointer to cv T"

It appears that reinterpret_cast is the only way to perform the conversion.

Suggested resolution:

Artem Livshits proposed a resolution :-

"I suppose if the definition of "similar" pointer types in 7.3.6 [conv.qual] paragraph 4 was rewritten like this:

T1 is cv1,0 P0 cv1,1 P1 ... cv1,n-1 Pn-1 cv1,n T

and

T2 is cv1,0 P0 cv1,1 P1 ... cv1,n-1 Pn-1 cv1,n T

where Pi is either a "pointer to" or a "pointer to an array of Ni"; besides P0 may be also a "reference to" or a "reference to an array of N0" (in the case of P0 of T2 being a reference, P0 of T1 may be nothing).

it would address the problem.

In fact I guess Pi in this notation may be also a "pointer to member", so 7.3.6 [conv.qual]/{4,5,6,7} would be nicely wrapped in one paragraph."

Additional note, February, 2014:

Geoffrey Romer: LWG plans to resolve US 16/LWG 2118, which concerns qualification-conversion of unique_ptr for array types, by effectively punting the issue to core: unique_ptr<T[]> will be specified to be convertible to unique_ptr<U[]> only if T(*)[] is convertible to U(*)[]. LWG and LEWG have jointly decided to adopt the same approach for shared_ptr<T[]> and shared_ptr<T[N]> in the Fundamentals TS. This will probably substantially raise the visibility of core issue 330, which concerns the fact that array types support only top-level qualification conversion of the element type, so it'd be nice if CWG could bump up the priority of that issue.

See also issue 1865.

Proposed resolution (October, 2014):

The resolution is contained in paper N4261.