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23. What is the size of an <empty> object?

Posted on 2015-01-24
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Last Modified: 2015-02-11
23. What is the size of an <empty> object?
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Question by:Nusrat Nuriyev
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LVL 86

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jkr earned 1000 total points
ID: 40568253
Give it a try ;o)

#include <iostream>

class Empty {

};

int main() {

  std::cout << "Size of Empty is " << sizeof(Empty) << std::endl;

  return 0;
}

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VC++ says:
Size of Empty is 1
g++ says:
Size of Empty is 1
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by:phoffric
phoffric earned 1000 total points
ID: 40568641
>> Size of Empty is 1
But why 1? Why not 0? Could it be 0 on some other platform?
From: http://www.stroustrup.com/bs_faq2.html#sizeof-empty
Why is the size of an empty class not zero?
To ensure that the addresses of two different objects will be different. For the same reason, "new" always returns pointers to distinct objects.
. . .
There is an interesting rule that says that an empty base class need not be represented by a separate byte:
	struct X : Empty {
		int a;
		// ...
	};

	void f(X* p)
	{
		void* p1 = p;
		void* p2 = &p->a;
		if (p1 == p2) cout << "nice: good optimizer";
	}

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This optimization is safe and can be most useful. It allows a programmer to use empty classes to represent very simple concepts without overhead. Some current compilers provide this "empty base class optimization".
BTW, from the link, you will find many other questions answered by one of the world's most authoritative authors on C++.
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by:jkr
jkr earned 1000 total points
ID: 40568656
>>But why 1? Why not 0? Could it be 0 on some other platform?

That's why I formulated exactly like that ;o)

My initial idea would have been "implementation dependant". Yet the ones I had available were not enough to make a point, so thank you for shedding in some light.
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LVL 32

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by:phoffric
phoffric earned 1000 total points
ID: 40568717
Those were my initial questions when I saw your result. So I looked to Stroustrup for an answer. Luckily I found it; otherwise, I would have to go through the wonderful C++ iso specification.
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