what is the size of empty class?

Question

2.00/5 (1 vote)

See more:

Hi ,
what is the size of empty class in c++ and if the class contain only one VTABLE then what is the size?

Please explain?

Regards,
Ranjith

Posted 23-Dec-12 20:25pm

ranjithkumar81

Add a Solution

Comments

Philippe Mori 25-Dec-12 9:45am

Why not try it first or use Google?

For example, a better question would be why on my compiler an empty class is not really empty... At least, it would show us that you have tried something.

4 solutions

Solution 4

In c++ sizeof operator will never returns zero. so i can say it will be a non zero which depends on the compiler your are using.

jkchan
http://cgmath.blogspot.com/[^]

Posted 23-Dec-12 23:26pm

jk chan

Comments

Philippe Mori 25-Dec-12 9:40am

It is not that the operator result cannot be 0. It is that the class cannot be empty and still be conformant to C++ spécifications (each time you call new, it should give a different address provide that previous addresses are still in use).

ranjithkumar81 26-Dec-12 0:20am

ok,

class Test
{
public:
virtual void function()
{
}
};

what is the size of Test class in 32bit machine and 64bit machine?

Solution 3

Your time would be much better spent studying the language in detail rather than randomly posting questions here. You could start by reading everything on Bjarne Stroustrup's website[^].

Posted 23-Dec-12 22:27pm

Richard MacCutchan

Solution 2

Is is non-zero, you may see the reason, for instance, here: "Empty Classes" at MSDN[^].

Posted 23-Dec-12 21:13pm

CPallini

Comments

ranjithkumar81 24-Dec-12 3:36am

#include<iostream>

using namespace std;

class empty{
public:

virtual void function()
{
}

};

int main()
{

printf ("Size of empty is= %d\n", sizeof(empty));
return 0;
}

I am getting size of empty class is one byte and in a class contain one virtual function ,the size is 8 bytes.how?
any one can explain?

Philippe Mori 25-Dec-12 9:37am

At least one pointer is required for the vtable. There are also alignment issues.

The actual size depend on the compiler options and the platforms.

CPallini 24-Dec-12 4:21am

You get 8 because, when you add the virtual function, a vtable for the class must be created (and each instance of the class must hold a pointer to such vtable, and your machine is possibly a 64 bit one).

ranjithkumar81 24-Dec-12 5:33am

My question is not related to 64 bit machine and 32 bit machine?

for example:

class Base{
public:
int a;
};

void main()
{
printf("Size of class %d",sizeof(Base));

}

its print 4. because int ocoupy 4 bytes in memory likewise ,i am asking about the VTABLE.

CPallini 24-Dec-12 5:43am

Your test just tells us pointer size is 64 bits on your (64 bit) machine while int size is 32 bits.

Add a Solution

Add your solution here

Treat my content as plain text, not as HTML

Preview 0

…

Existing Members

Sign in to your account

...or Join us

Download, Vote, Comment, Publish.

Your Email
Password
Forgot your password?

Your Email
This email is in use. Do you need your password?
Optional Password

I have read and agree to the Terms of Service and Privacy Policy
Please subscribe me to the CodeProject newsletters

When answering a question please:

Read the question carefully.
Understand that English isn't everyone's first language so be lenient of bad spelling and grammar.
If a question is poorly phrased then either ask for clarification, ignore it, or edit the question and fix the problem. Insults are not welcome.
Don't tell someone to read the manual. Chances are they have and don't get it. Provide an answer or move on to the next question.

Let's work to help developers, not make them feel stupid.

This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

Pablo Aliskevicius · Accepted Answer · 2012-12-23T20:37:00

Solution 1

C++

int main()
{
    class empty {};
    printf ("Size of empty is %d", sizeof(empty));
    return 0;
}

About VTable, virtual destructors and similar - google is your friend.

Hope this helps,
Pablo.

Posted 23-Dec-12 20:37pm

Pablo Aliskevicius