This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 110b. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2023-01-15


616. Definition of “indeterminate value”

Section: Clause 3  [intro.defs]     Status: CD3     Submitter: Bjarne Stroustrup     Date: 2 February 2007

[Moved to DR at the April, 2013 meeting.]

The C++ Standard uses the phrase “indeterminate value” without defining it. C99 defines it as “either an unspecified value or a trap representation.” Should C++ follow suit?

In addition, 7.3.2 [conv.lval] paragraph 1 says that applying the lvalue-to-rvalue conversion to an “object [that] is uninitialized” results in undefined behavior; this should be rephrased in terms of an object with an indeterminate value.

Proposed resolution (October, 2012):

  1. Change 7.3.2 [conv.lval] paragraphs 1 and 2 as follows (including changing the running text of paragraph 2 into bullets):

  2. A glvalue (7.2.1 [basic.lval]) of a non-function, non-array type T can be converted to a prvalue.53 If T is an incomplete type, a program that necessitates this conversion is ill-formed. If the object to which the glvalue refers is not an object of type T and is not an object of a type derived from T, or if the object is uninitialized, a program that necessitates this conversion has undefined behavior. If T is a non-class type, the type of the prvalue is the cv-unqualified version of T. Otherwise, the type of the prvalue is T.54

    When an lvalue-to-rvalue conversion occurs in an unevaluated operand or a subexpression thereof (Clause 7 [expr]) the value contained in the referenced object is not accessed. In all other cases, the result of the conversion is determined according to the following rules:

  3. Change 7.6.1.5 [expr.ref] paragraph 4 second bullet as follows:

  4. Change 7.6.4 [expr.mptr.oper] paragraph 6 as follows:

  5. ...The result of a .* expression whose second operand is a pointer to a data member is of the same value category (7.2.1 [basic.lval]) as its first operand an lvalue if the first operand is an lvalue and an xvalue otherwise. The result of a .* expression whose second operand is a pointer to a member function...

This resolution also resolves issues 129, 240, 312, 623, and 1013.

(See also issue 1213.)

Additional note (August, 2012):

It was observed that the phrase in the fourth bullet of the change to 7.3.2 [conv.lval] paragraph 2 that reads “is not a local variable” should probably be changed to “does not have automatic storage duration,” because objects with static storage duration are zero-initialized and thus cannot have an indeterminate value. The issue was returned to "review" status for discussion of this point.