C++:why base pointer and derived pointer point to the same address in vs2013?

Question

class A{
    int a;
};
class B : public  A{
    int b;
};
class C : public  B{
    int c;
};

int _tmain(int argc, _TCHAR* argv[]){
    C c;
    C* pc = &c;
    B* pb = &c;
    A* pa = &c;
    printf("%d\n", pc); //4344
    printf("%d\n", pb); //4344
    printf("%d\n", pa); //4344
    return 0;
}

All(pa,pb,pc) point to the same address "4344" , aren't they suppossed to be different?

-------------UPDATE-------------
If they are suppossed to be the same ,then when I change the code to this , pa would point to different address :

class A{
    int a;
};
class B {
    int b;
};
class C : public  B , public A{
    int c;
};

int _tmain(int argc, _TCHAR* argv[]){
    C c;
    C* pc = &c;
    B* pb = &c;
    A* pa = &c;
    printf("%p\n", pc); //4344
    printf("%p\n", pb); //4344
    printf("%p\n", pa); //4348
    return 0;
}

How to explain these?

using the wrong format for printf invokes undefined behavior [Correct format specifier to print pointer (address)?](http://stackoverflow.com/q/9053658/995714) — phuclv, Aug 30 '15 at 05:52
For an explanation see e.g. http://www.cs.bgu.ac.il/~spl121/Inheritance. Nicely explained. — Peter - Reinstate Monica, Aug 30 '15 at 05:57
size of pointers and int might not be the same, esp. in 64-bit systems, hence printing with `%d` may not result in the correct value. There's nothing unreadable in the hex address, as they are also digits just like decimal — phuclv, Aug 30 '15 at 05:57
While we're on this topic, using `%p` on a pointer that's not a `void *` also causes undefined behaviour. But we usually ignore that for practical reasons. — M.M, Aug 30 '15 at 06:07

score 1 · Accepted Answer · answered Aug 30 '15 at 05:17

1

No they are pointing to the same object i.e. c. Thus you will have same address. And you should not print addresses as %d but %p. https://10hash.com/c/stdio/#fprintf or even better http://pubs.opengroup.org/onlinepubs/009695399/functions/fprintf.html

answered Aug 30 '15 at 05:17

Shiv

1,912
1
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21

score 0 · Answer 2 · answered Aug 30 '15 at 05:19

0

You are assigning all pointers to the same address:

C* pc = &c;
B* pb = &c;
A* pa = &c;

I'd say that makes it very clear that they should be the same.

answered Aug 30 '15 at 05:19

Saeb Amini

23,054
9
78
76

2

"You are assigning all pointers to the same address": Not necessarily. The assignment to pb and pa involves a type conversion which principally could change the pointer's value. I am not sure whether the language guarantees that the numerical value is identical for conversion to base class ptrs in single inheritance (although of course "appending" extensions in a class layout is an obvious implementation, and probably necessary for preserving inherited member offsets). What happens with multiple inheritance? – Peter - Reinstate Monica Aug 30 '15 at 05:42

C++:why base pointer and derived pointer point to the same address in vs2013?

2 Answers2