This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 116c. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2025-03-10

3004. Pointer arithmetic on array of unknown bound

(From submission #680.)

Consider:

  extern const int arr[];
  constexpr const int *p = arr + N;  // #1
  constexpr int arr[2] = {0, 1};     // #2
  constexpr int k = *p;              // #3

What is the outcome here? Accept for N == 0 and otherwise #1 is non-constant (clang)? Always accept #1, but reject #3 for out-of-bounds accesses (gcc)? Always accept #1, but always reject #3, even for in-bound accesses (EDG)? Rejecct #2 (MSVC)?

Further, we miss a specification when a both a declaration of type "array of unknown bound of T" as well as one of the type "array of N T" is reachable. We should take the latter as the type.

Possible resolution:

1. Add a bullet after 7.7 [expr.const] bullet 10.15 as follows:

   • ...
   • a reinterpret_cast (7.6.1.10 [expr.reinterpret.cast]);
   • pointer arithmetic (7.6.6 [expr.add]) where one (possibly converted) operand points to the first element of an array of unknown bound and the other (possibly converted) operand is of integral type with non-zero value
   • ...

2. Change in 6.8.1 [basic.types.general] paragraph 6 as follows:

   A class type (such as “class X”) can be incomplete at one point in a translation unit and complete later on; the type “class X” is the same type at both points. The declared type of an array object variable can be an array of incomplete class type C and therefore incomplete; if the class type is completed later on wherever a definition of C is reachable in the translation unit, the array type becomes is complete; the array type at those two points is the same type. The declared type of an array object variable v can be an array of unknown bound and therefore be incomplete at one point in a translation unit and is complete later on wherever a declaration of v with a known bound is reachable; the array types at those two points (“array of unknown bound of T” and “array of N T”) are different types.