Re: Structure mapping using reinterpret_cast.
Olivier wrote:
I would like to check if a certain technique can be used or whether
there is any entry in the standard that prohibits it explicitly. If I
recall correctly, the standard guarantees that two structures
containing the same starting data members can be mapped on one another
and these common members can be access without yielding undefined
behaviour.
Yes.
That is, given :
struct A { int a; int b; };
struct B { int c; };
the following code is guaranteed to work :
A a;
B &b = *reinterpret_cast<B *>(&a);
b.c = 12;
assert(a.a == b.c);
// In addition:
assert(&a.a == &b.c);
The only thing I would object to is the use of a reinterpret_cast, which by
definition yields implementation-defined behaviour, though the name
suggests that it is the right one.
I'd probably use this:
B& b = (B&)a;
Firstly, use a C-style cast to illustrate that I'm doing something hackish
here. Secondly, use a reference as target type, which you could do with a
reinterpret_cast, too.
By extension, if that is true, the following code should be guaranteed
to work :
[example using classes]
There are a few differences there:
1. Your baseclass has a ctor, therefore it's not a POD which would be
guaranteed to behave similar to C structs.
2. You are calling memberfunctions.
3. If your classes have virtual functions, there are no guarantees anyway.
If, as per your example, you want to treat the same set of data differently,
you can detach the operations on the data from the data itself. You just
create a structure similar to the vtable but store that next to the data
and not as integral and immutable part of it. That way, you can also change
the "type" (not in the C++ sense, but in the sense of effective behaviour)
at runtime.
// data container
struct data { int n; }
// behaviour container
struct operations { int (*foo)(A const& self); };
// container for data and behaviour
class object
{
data m_data;
operations const* m_ops;
public:
int foo() {
return (*m_ops.foo)(m_data);
}
};
Uli
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