CWG 2024-06-26

Implementations are required to diagnose undefined behavior in constant expressions. The issue is kept in review status to allow time for submitting a paper to EWG to make the &a[5] case well-defined. See also C23 6.5.3.2p3.

Proposed resolution (approved by CWG 2024-06-26):

Add a new paragraph to C.7.4 [diff.expr] as follows:

Affected subclause: 7.6.2.2 [expr.unary.op]
Change: In certain contexts, taking the address of a dereferenced null or past-the-end pointer value is well-defined in C (and yields the original pointer value), but results in undefined behavior in C++. For example:

  void f() {
    char *p = 0;
    char *p2 = &*p;   // well-defined in C, undefined behavior in C++
    char *p3 = &p[0]; // well-defined in C, undefined behavior in C++
    int a[5];
    int *q = &a[5];   // well-defined in C, undefined behavior in C++
  }

Rationale: Consistent treatment of lvalues in C++.
Effect on original feature: Well-formed and well-defined C code exhibits undefined behavior in C++.
Difficulty of converting: Syntactic transformation to pointer arithmetic and possible addition of a check for null pointer values.
How widely used: Occasionally.

C supports the following, C++ does not (see issues 232 and 2823):

  void f() {
    char *p = 0;
    char *p2 = &*p;       // OK in C, undefined behavior in C++
    int a[5];
    int *q = &a[5];       // OK in C, undefined behavior in C++
  }

This incompatibility should be documented in Annex C.