Re: Order of Variable
On Sep 10, 4:51 am, "Alf P. Steinbach /Usenet" <alf.p.steinbach
+use...@gmail.com> wrote:
* shaanxxx, on 10.09.2010 05:36:
i have following class
Class A
{
public :
int a;
int b;
int c;
}
Class B
{
public :
int a;
int b;
}
can i take pointer of type *B and point to class A Object and
successfully modify Data member a and b of A . What standard has to
say about it ?
In C++98 you're formally in UB-land when you try that on
non-POD types such as above.
The types above are POD structs. (Or would be if the keyword
"Class" was written without the capital---as it is, the posted
code isn't legal C++.)
For POD types the standard supports it via the discussion of
layout-compatible types in =A79.2, and in particular support of
reinterpret_cast from first member in struct to struct and
vice versa in =A79.2/17.
I'm not sure that this is covered by that paragraph. That
paragraph makes guarantees about the address of a (which
indirectly does guarantee access), but says nothing about
b. There's also a guarantee if the classes are in a union,
e.g.:
union AB
{
A a;
B b;
};
Of course, in practice, given these two guarantees, I can't see
how an implementation could make access to b not work.
This support is there because it was/is a common technique in
C. In C++ you simply use inheritance instead. No
reinterpret_cast required then. :-)
B * ptr = reinterpret_cast<B*>( new A );
Here you're on shaky ground because you need to deallocate via
an A* pointer.
Also this problem is solved by inheritance, combined with
a virtual destructor.
Are Data members laid in memory in order in which they are
declared ?
As long as there's no intervening access specifier, yes. Later
members are then at higher addresses than earlier members.
However, there can be padding between them (although not
before the first member of POD).
And in theory (but only in theory), the amount of padding could
vary arbitrarily.
--
James Kanze