Does inheriting from an empty class increase that class's size?

Asked Apr 23 '15 at 15:40

Active Apr 23 '15 at 15:46

Viewed 86 times

Given:

class A
{
};

class B : A // inherit from an empty class
{
    int * x;
};

class C
{
    int * x;
};

Is sizeof(B) == sizeof(C) according to standards?

While sizeof(A) == 1, the 1 is a "dummy" value because sizeof cannot return 0 in this case

According to standards, is the dummy value carried forward during inheritance from a non empty class.

On a practical note, the reason for the question is that I am using void * as a pointer to several classes, and it would be clearer if they all shared a base class, but this is performance critical code and adding one byte, would actually add 8 because of alignment with a 64 bit pointer in the child class. That will significantly add to the total memory footprint and I will potentially have a large number of them.

asked Apr 23 '15 at 15:40

Glenn Teitelbaum

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You're describing [empty base optimization](http://en.cppreference.com/w/cpp/language/ebo). – Drew Dormann Apr 23 '15 at 15:42
http://en.cppreference.com/w/cpp/language/ebo – David G Apr 23 '15 at 15:42
Thank you, didn't know it had a name – Glenn Teitelbaum Apr 23 '15 at 16:09

Does inheriting from an empty class increase that class's size?

0 Answers0