C++: What is the size of an object of an empty class?


Question

I was wondering what could be the size of an object of an empty class. It surely could not be 0 bytes since it should be possible to reference and point to it like any other object. But, how big is such an object?

I used this small program:

#include <iostream>
using namespace std;

class Empty {};

int main()
{
    Empty e;
    cerr << sizeof(e) << endl;
    return 0;
}

The output I got on both Visual C++ and Cygwin-g++ compilers was 1 byte! This was a little surprising to me since I was expecting it to be of the size of the machine word (32 bits or 4 bytes).

Can anyone explain why the size of 1 byte? Why not 4 bytes? Is this dependent on compiler or the machine too? Also, can someone give a more cogent reason for why an empty class object will not be of size 0 bytes?

1
101
3/7/2009 10:36:18 AM

Accepted Answer

Quoting Bjarne Stroustrup's C++ Style and Technique FAQ, the reason the size is non-zero is "To ensure that the addresses of two different objects will be different." And the size can be 1 because alignment doesn't matter here, as there is nothing to actually look at.

120
6/6/2017 5:48:27 PM

The standard states that all most derived objects have sizeof() >= 1:

Unless it is a bit-field (class.bit), a most derived object shall have a non-zero size and shall occupy one or more bytes of storage. Base class sub-objects may have zero size. ISO/IEC FDIS 14882:1998(E) intro.object


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