Re: [boost] Review Requests: shifted_ptr, list, stack & roofof

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

"Steven Watanabe" <watanabesj_at_[hidden]> wrote in message 
news:47DFF458.7010409_at_providere-consulting.com...

[...]

> 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 &".

5.19/4:
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. [...]

-Phil