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

2024-12-19

519. Null pointer preservation in void* conversions

Section: 7.3.12  [conv.ptr]     Status: CD1     Submitter: comp.std.c++     Date: 19 May 2005

[Voted into WP at April, 2006 meeting.]

The C standard says in 6.3.2.3, paragraph 4:

Conversion of a null pointer to another pointer type yields a null pointer of that type. Any two null pointers shall compare equal.

C++ appears to be incompatible with the first sentence in only two areas:

    A *a = 0;
    void *v = a;

C++ (7.3.12 [conv.ptr] paragraph 2) says nothing about the value of v.

    void *v = 0;
    A *b = (A*)v; // aka static_cast<A*>(v)

C++ (7.6.1.9 [expr.static.cast] paragraph 10) says nothing about the value of b.

Suggested changes:

  1. Add the following sentence to 7.3.12 [conv.ptr] paragraph 2:

    2. The null pointer value is converted to the null pointer value of the destination type.

  2. Add the following sentence to 7.6.1.9 [expr.static.cast] paragraph 10:

    3. The null pointer value (7.3.12 [conv.ptr]) is converted to the null pointer value of the destination type.

Proposed resolution (October, 2005):

  1. Add the indicated words to 7.3.12 [conv.ptr] paragraph 2:

    2. An rvalue of type “pointer to cv T,” where T is an object type, can be converted to an rvalue of type “pointer to cv void”. The result of converting a “pointer to cv T” to a “pointer to cv void” points to the start of the storage location where the object of type T resides, as if the object is a most derived object (6.7.2 [intro.object]) of type T (that is, not a base class subobject). The null pointer value is converted to the null pointer value of the destination type.

  2. Add the indicated words to 7.6.1.9 [expr.static.cast] paragraph 11:

    3. An rvalue of type “pointer to cv1 void” can be converted to an rvalue of type “pointer to cv2 T,” where T is an object type and cv2 is the same cv-qualification as, or greater cv-qualification than, cv1. The null pointer value is converted to the null pointer value of the destination type. A value of type pointer to object converted to “pointer to cv void” and back, possibly with different cv-qualification, shall have its original value...