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
Consider:
int* p = false; // Well-formed?
int* q = !1; // What about this?
>From 6.8.2 [basic.fundamental] paragraph 6:
"As described below, bool values behave as integral types."
From 7.3.12 [conv.ptr] paragraph 1: "A null pointer constant is an integral constant expression rvalue of integer type that evaluates to zero."
From 7.7 [expr.const] paragraph 1: "An integral constant-expression can involve only literals, enumerators, const variables or static members of integral or enumeration types initialized with constant expressions, ..."
In 5.13.2 [lex.icon] : No mention of true or false as an integer literal.
From 5.13.7 [lex.bool] : true and false are Boolean literals.
So the definition of q is certainly valid, but the validity of p depends on how the sentence in 7.7 [expr.const] is parsed. Does it mean
If the latter, then (3.0 < 4.0) is a constant expression, which I don't think we ever wanted. If the former, though, we have the anomalous notion that true and false are not constant expressions.
Now, you may argue that you shouldn't be allowed to convert false to a pointer. But what about this?
static const bool debugging = false;
// ...
int table[debugging? n+1: n];
Whether the definition of table is well-formed hinges on whether
false is an integral constant expression.
I think that it should be, and that failure to make it so was just an oversight.
Rationale (04/99): A careful reading of 7.7 [expr.const] indicates that all types of literals can appear in integral constant expressions, but floating-point literals must immediately be cast to an integral type.