On Sunday, 9 December 2012 at 07:39:29 UTC, js.mdnq wrote:
On Sunday, 9 December 2012 at 07:24:57 UTC, Jonathan M Davis
wrote:
On Sunday, December 09, 2012 07:54:25 js.mdnq wrote:
Why can't a struct inside a class access the members of that
class without a this pointer? It would seem natural to me
that a
nested struct should probably be special in that it is really
just a special container to reduce clutter in the class.
Without a this pointer, there are no class members to access,
because they
have to be associated with a specific instance of the class
unless they're
static. Non-static, nested structs have access to their
enclosing scope, but
then you can't create them separate from the enclosing class.
However, if you
declare a nested class to be static, it has no access to the
class' members
and is not associated with a specific instance of the class.
It's just that
it's declared inside the class instead of outside of it.
- Jonathan M Davis
NOOOOOOOOO!!!! It does have a this pointer!! If the sub-struct
is a true sub struct then it is inline inside the memory of the
class! That is, any member inside a struct can easily be gotten
from the pointer to the class(object) as one just has to add a
simple(and static) offset.
That is why nested struct inside a class actually does not have a
context pointer -its fields addresses are calculated as offsets
from class this pointer, not from struct this pointer.
If the struct is only used inside the class then there should
be no problem.
Problems comes when nested struct is created inside a function
without creating a class.
It makes no sense to have a struct inside a class behave
exactly as that outside as it offers no benefit to do so(or
maybe it does, but very little). Hence, we can redefine the way
structs behave inside classes to make them more useful.
Perhaps the situation can be improved.
In any case, take this example:
class A {
public:
string Name;
struct B { public: int x; alias x this; void func(A _a) {
writeln(_a.Name, x, y); }}
B x;
B y;
}
...
A a;
What is the address of A?
Meaning a, it is null.
What is the address of x inside A? (i.e., the struct inside A?)
x does not exists
Is it not a simple static offset from the address of A? i.e.,
knowing the address of a lets us know the address of x. Knowing
the address of x also lets us know the address of a! (Same goes
for y)
Yes, it is a CT-known offset which varies from one class to
another. Note, you can insert nested struct into another class.
This is why a nested struct(using my semantics) contains the
this pointer! (because it is a simple offset from the this
pointer which can be computed at compile time)!
That's why nested struct does not have this pointer - it is POD
structure.
http://dpaste.dzfl.pl/76e8ec0a
