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From: Phil Bouchard (philippe_at_[hidden])
Date: 2008-03-19 04:00:31

"Steven Watanabe" <watanabesj_at_[hidden]> wrote in message


> Which compilers? IMO, you shouldn't make the code even more obscure until
> it's known for certain to be a problem.

Well 0 is considered to be a null pointer, not a pointer to 0x0000. This
can potentially be replaced by "nullptr" in the next revision, which
shouldn't be assumed to be a pointer to 0x0000. Having a non-null constant
removes this nullness ambiguity.

I see there is a BOOST_PYTHON_OFFSETOF macro that exists and which uses this
technique. It is unfortunate offsetof() doesn't take a real
pointer-to-member address.

>> This address dissosiates
>> the pointer from an undefined behavior and is aligned for all types.
> 5.5/4:
> "If the dynamic type of the object does not contain the member to which
> the pointer refers, the behavior is
> undefined."

"operator ->" can be used in an address constant expression without being
considered to be a dereference because of "operator &".

An address constant expression is a pointer to an lvalue designating an
object of static storage duration, a string literal (_lex.string_), or a
function. The pointer shall be created explicitly, using the unary &
operator, or implicitly using a non-type template parameter of pointer
type, or using an expression of array (_conv.array_) or function
(_conv.func_) type. The subscripting operator [] and the class member
access . and -> operators, the & and * unary operators, and pointer casts
(except dynamic_casts, _expr.dynamic.cast_) can be used in the creation of
an address constant expression, but the value of an object shall not be
accessed by the use of these operators. [...]


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