sizeof class ?

[Sameer]

Hi,
I have a class.
class C1
{
int i ;
char c ;
} ;

let me say the size of int is 4 and char is 1. Then if I say sizeof(C1)
it should be 5, why is it showing 8 ?

 

[Ian Collins]

Hi,
I have a class.
class C1
{
int i ;
char c ;
} ;

let me say the size of int is 4 and char is 1. Then if I say sizeof(C1)
it should be 5, why is it showing 8 ?

Because your compiler aligns data on 4 byte boundaries, which is typical
on a 32 bit system.

 

[void]

so concise! Right !

 

[Ian Collins]

so concise! Right !

? please quite some context in your reply.

 

[Sameer]

So how can I correct this? Is it only possible that the size of a class
is only multiple of size of int ?

 

[Jim Langston]

So how can I correct this? Is it only possible that the size of a class
is only multiple of size of int ?

What do you mean by correcting it? It is correct. What is it about the
size that you don't like?

It may be possible to compact the class/structure by a compiler specific
switch, but this can cause problems. There are some systems that an
unaligned value can cause the CPU to not read it correctly. It can also
slow down operation.

I would suggest you just leave it alone and don't worry about the 3 extra
bytes.

If you really must compact it for some reason, look at your compilers
switches and such. If you're using a microsoft compiler look at #pragma
pack

 

[Sameer]

That question (How to make the size fo class exactly as the total size
of variables defined?) was asked with me in 2 interviews. Thats why i
asked.

 

[skishorev]

first, what is the size of an empty class? what the empty class
contains?

 

[Rolf Magnus]

first, what is the size of an empty class?

Anything bigger than zero.

what the empty class contains?

Padding bytes that ensure that the size is not zero.

 

[Marcus Kwok]

Anything bigger than zero.

....as long as it is not an empty base class for a derived class. In
that case, it can be zero. However, on its own, I agree it must be
nonzero, so that distinct objects can be distinguished.


#include <iostream>

class Base { };

class Derived : public Base {
int x;
};

class OneInt {
int x;
};

int main()
{
std::cout << "sizeof int: " << sizeof(int) << '\n';
std::cout << "sizeof Base: " << sizeof Base << '\n';
std::cout << "sizeof Derived: " << sizeof Derived << '\n';
std::cout << "sizeof OneInt: " << sizeof OneInt << '\n';
}


/*
Output on my machine:
sizeof int: 4
sizeof Base: 1
sizeof Derived: 4
sizeof OneInt: 4
*/