what is the size of empty class in C++,java?

Question

What is the size of an empty class in C++ and Java? Why is it not zero? sizeof(); returns 1 in the case of C++.

Answer 1

It's defined by the C++ standard as "a nonzero value", because an allocated object must have a nonzero size in order to have a distinct address. A class that inherits from an empty class, however, is not required to increase in size, barring the usual increase of a vtable if there are virtual functions involved.

Answer 2

Short Answer:

The standard explicitly says that a class can not have zero size.

Long Answer:

Because each object needs to have a unique address (also defined in the standard) you can't really have zero sized objects.
Imagine an array of zero sized objects. Because they have zero size they would all line up on the same address location. So it is easier to say that objects can not have zero size.

Note:

Even though an object has a non zero size, if it actually takes up zero room it does not need to increase the size of derived class:

Example:

#include <iostream>

class A {};
class B {};
class C: public A, B {};

int main()
{
     std::cout << sizeof(A) << "\n";
     std::cout << sizeof(B) << "\n";
     std::cout << sizeof(C) << "\n";  // Result is not 3 as intuitively expected.
}

g++ ty.cpp
./a.out
1
1
1

Answer 3

Because every C++ object needs to have a separate address, it isn't possible to have a class with zero size (other than some special cases related to base classes). There is more information in C++: What is the size of an object of an empty class? .

Answer 4

Because an object has to have an address in memory, and to have an address in memory, it has to occupy "some" memory. So, it is usually, in C++, the smallest possible amount, i.e. 1 char (but that might depend on the compiler). In Java, I wouldn't be so sure.. it might have some default data (more than just a placeholder like in C++), but it would be surprising if it was much more than in C++.

Answer 5

I don't know if there is a sizeof() operator in java. What you can do is create an instance of the empty class (have it serializable), send it through a PipedOutputStream and read it as byte array - byteArray.length gives you the size.

Alternatively, write out the instance to a file using DataOutputStream, close the File, open it and file.length() will give you the size of the Object. Hope this helps, - M.S.

Answer 6

C++ requires that a normal instantiation of it have a size of at least 1 (could be larger, though I don't know of a compiler that does that). It allows, however, an "empty base class optimization", so even though the class has a minimum size of 1, when it's used as a base class it does not have to add anything to the size of the derived class.

I'd guess Java probably does pretty much the same. The reason C++ requires a size of at least 1 is that it requires each object to be unique. Consider, for example, an array of objects with size zero. All the objects would be at the same address, so you'd really only have one object. Allowing it to be zero sounds like a recipe for problems...