sizeof array element

In the code below, 'a' and 'r[0]' both refer to the same object (correct me if that is wrong).
sizeof(a) is 12 while sizeof(r[0]) is 1; why are they not the same size?

#include <iostream>
using namespace std;

class Base
{
};

class A: public Base
{
	int x;
	int y;
	int z;
};

class B: public Base
{
};

int main()
{
	A a;
	B b;
					// output:
	cout << sizeof(a) << endl;	// 12
	cout << sizeof(b) << endl;	// 1

	Base r[] = {a, b};
	cout << sizeof(r[0]) << endl;	// 1
	cout << sizeof(r[1]) << endl;	// 1
}

kbw (9488)

a is a polymorphic, i.e. it has more that one type. In this case, Base and A.

The size of the Base part, can't be zero, so it's one.
The size of the A part appears to be 12 on your system.

You're seeing object slicing, where the A part is sliced off, only the Base part is copied into r[0].
http://en.wikipedia.org/wiki/Object_slicing

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closed account (jyU4izwU)

Just a fast comment...
Do: int x,y,z;
It compiles faster.

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codewalker (394)

Just a fast comment...
Do: int x,y,z;
It compiles faster.

What? it absolutely shouldn't matter, but thats a bad coding style.

kbw (9488)

No, don't change your code in that way.

Disch (13742)

+1 @ not doing that.

The 1 nanosecond you might save in compile time is not worth the style tradeoff.

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