object question

[junw2000]

how much memory is allocated when creating an object of an empty class?

Thanks.

Jack

[AB]

how much memory is allocated when creating an object of an empty class?

Try this...

class Empty
{
} ;

Empty e ;

cout<<"\nSizeof "<<typeid(e).name()<<" = "<<sizeof(Empty) ;

Output is 1 when using MSVC 8.0

[mast2as]

[e-mail] wrote:

how much memory is allocated when creating an object of an empty class?

Thanks.

I imaging that this would be a good way of getting the size of a
class/instance of a class -

class DoNothing
{
public:
DoNothing() {}
//int a;
};

int main()
{
DoNothing donothing;
std::cout << sizeof( donothing ) << std::endl;
std::cout << sizeof( DoNothing ) << std::endl;
}

[Alf P. Steinbach]

* ju******@gmail.com:

how much memory is allocated when creating an object of an empty class?

Depends on the compiler and the context.

As a base class object it can occupy zero bytes, otherwise more.

--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

[tarun.mohapatra]

Alf P. Steinbach wrote:

* ju******@gmail.com:

how much memory is allocated when creating an object of an empty class?

It purely depends upon the compiler. But in most of the compilers it
will be 1 byte(Size of char). Emplty class object takes the minimum
space to identify it as an entity in memory.

If it will not take any space the in the below example
Class A{
};

A a, b;

&a and &b can be same. Which is pratically impossible(two
differentities can't reside in same place)

[anubhav.anand]

ta*************@gmail.com wrote:

Alf P. Steinbach wrote:

* ju******@gmail.com:

how much memory is allocated when creating an object of an empty class?
>

It purely depends upon the compiler. But in most of the compilers it
will be 1 byte(Size of char). Emplty class object takes the minimum
space to identify it as an entity in memory.

If it will not take any space the in the below example
Class A{
};

A a, b;

Here you are not talking about size of a and b, &a and &b means
"address of".
&a and &b will not be same in above case, but this is nothing to do
with size
Size of a and b will be same but they will have different address. But
yes, size depends upon compliers and usually 1 byte.

&a and &b can be same. Which is pratically impossible(two
differentities can't reside in same place)

[Jakob Bieling]

an***********@gmail.com wrote:

ta*************@gmail.com wrote:

>Class A{
};

A a, b;

>&a and &b can be same. Which is pratically impossible(two
differentities can't reside in same place)

Here you are not talking about size of a and b, &a and &b means
"address of".
&a and &b will not be same in above case, but this is nothing to do
with size
Size of a and b will be same but they will have different address. But
yes, size depends upon compliers and usually 1 byte.

You misunderstood him. If 'a' and 'b' *would* have a size of 0 *and*
they are allocated right next to each other, so that a + sizeof(a) == b,
then &a == &b, because sizeof(a) == 0. Since &a == &b is invalid for
different objects, sizeof(a) cannot be 0 in this case.

So in the above example, &a and &b (address of 'a' and address of
'b') could be the same, if sizeof(a) could be 0, which is what tarun
said.

regards
--
jb

(reply address in rot13, unscramble first)

[raxitsheth2000]

Replying to Alf P. Steinbach
I dont't think logically Object of size 0 is possible.

Even for emptyclass it would required some min. byte (which is on most
platform 1 byte,)
--
Raxit Sheth

[Sumit RAJAN]

ra************@yahoo.co.in wrote:

Replying to Alf P. Steinbach

Please quote what you are replying to.

I dont't think logically Object of size 0 is possible.

Even for emptyclass it would required some min. byte (which is on most
platform 1 byte,)

If you look carefully at Alf's reply, you will find that he says it
*can* be zero in case of a *base* class. Otherwise, the only requirement
is that it have a non-zero size.

Google for "Empty Base Class Optimization" in this regard.
--
Sumit RAJAN <su*********@geoconcept.com>

[KMS]

Hi,

It depends on compiler and it will take 1 byte in most of the
compilers.

Reason:

Class can be empty but there are possibilities that there can be an
empty class with virtual function, in which case there will be a
virtual table and it need a pointer to store address of virtual
table.So we need a byte to store the address of virtual table.

I remember that'z what even Bjarne Stroustroup said in his book C++
Programming Language.

Thanks
Suresh K.M
ju******@gmail.com wrote:

how much memory is allocated when creating an object of an empty class?

Thanks.

Jack

[Rolf Magnus]

Jakob Bieling wrote:

an***********@gmail.com wrote:

>ta*************@gmail.com wrote:

>>Class A{
};

A a, b;

>>&a and &b can be same. Which is pratically impossible(two
differentities can't reside in same place)

>Here you are not talking about size of a and b, &a and &b means
"address of".
&a and &b will not be same in above case, but this is nothing to do
with size
Size of a and b will be same but they will have different address. But
yes, size depends upon compliers and usually 1 byte.

You misunderstood him. If 'a' and 'b' *would* have a size of 0 *and*
they are allocated right next to each other, so that a + sizeof(a) == b,
then &a == &b, because sizeof(a) == 0. Since &a == &b is invalid for
different objects, sizeof(a) cannot be 0 in this case.

Nobody forces the compiler to put a and b directly next to each other. So
that's not really a good example.

The example of an array is better. In:

A array[5];

each array member must have a distinct address, but that's not possible if
the members have a size of zero. All elements would be at the same address,
or depending on your view, actually not be at any address at all. After
all, an address marks some storage location, and an object with a size of
zero does not occupy any storage and thus can't have a location there.

[Alf P. Steinbach]

* ta*************@gmail.com:

Alf P. Steinbach wrote:

>* ju******@gmail.com:

>>how much memory is allocated when creating an object of an empty class?

It purely depends upon the compiler.

No, I'm sorry, that's incorrect.

As I wrote (but you snipped all I wrote, don't do that) it depends on
the compiler but also on the context.

Quoting myself:

<q>
Depends on the compiler and the context.

As a base class object it can occupy zero bytes, otherwise more.
</q>

It purely depends upon the compiler. But in most of the compilers it
will be 1 byte(Size of char). Emplty class object takes the minimum
space to identify it as an entity in memory.

If it will not take any space the in the below example
Class A{
};

A a, b;

&a and &b can be same. Which is pratically impossible(two
differentities can't reside in same place)

Right.

--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

[Ron Natalie]

ju******@gmail.com wrote:

how much memory is allocated when creating an object of an empty class?

All objects must occupy at least one byte.
Your compiler is free to generate anything that size or
greater.

[Ron Natalie]

an***********@gmail.com wrote:

ta*************@gmail.com wrote:

>Alf P. Steinbach wrote:

>>* ju******@gmail.com:
how much memory is allocated when creating an object of an empty class?

It purely depends upon the compiler. But in most of the compilers it
will be 1 byte(Size of char). Emplty class object takes the minimum
space to identify it as an entity in memory.

If it will not take any space the in the below example
Class A{
};

A a, b;

Here you are not talking about size of a and b, &a and &b means
"address of".
&a and &b will not be same in above case, but this is nothing to do
with size
Size of a and b will be same but they will have different address. But
yes, size depends upon compliers and usually 1 byte.

A better example would be
A array[2];

The size of an element in the array can't be zero or else
the array indexing rules wouldn't work.

[Frederick Gotham]

posted:

how much memory is allocated when creating an object of an empty class?

Here's a link to Bjarne's answer:
http://www.research.att.com/~bs/bs_f...l#sizeof-empty
--

Frederick Gotham

[Jakob Bieling]

Rolf Magnus <ra******@t-online.dewrote:

Jakob Bieling wrote:

>an***********@gmail.com wrote:

>>ta*************@gmail.com wrote:

>>>Class A{
};

A a, b;

>>>&a and &b can be same. Which is pratically impossible(two
differentities can't reside in same place)

>>Here you are not talking about size of a and b, &a and &b means
"address of".
&a and &b will not be same in above case, but this is nothing to do
with size
Size of a and b will be same but they will have different address.
But yes, size depends upon compliers and usually 1 byte.

You misunderstood him. If 'a' and 'b' *would* have a size of 0
*and* they are allocated right next to each other, so that a +
sizeof(a) == b, then &a == &b, because sizeof(a) == 0. Since &a ==
&b is invalid for different objects, sizeof(a) cannot be 0 in this
case.

Nobody forces the compiler to put a and b directly next to each
other. So that's not really a good example.

Right, assume tarun was referring to the possibility of this event.
I should have made clearer that the "*and* they are allocated right next
to each other" also belongs to the "if this happens" part.

regards
--
jb

(reply address in rot13, unscramble first)

[Default User]

ra************@yahoo.co.in wrote:

Replying to Alf P. Steinbach

When you reply, use the standard quoting mechanism. Google now provides
that by default. See the FAQ for other tips:
<http://www.parashift.com/c++-faq-lite/how-to-post.html#faq-5.4>

Brian

[Default User]

KMS wrote:

Hi,

Please don't top-post. Your reply belongs following or interspersed
with properly trimmed quotes. See the newsgroup FAQ:
<http://www.parashift.com/c++-faq-lite/how-to-post.html#faq-5.4>


Brian

[Jakob Bieling]

Jakob Bieling <ar****************@rot13.comwrote:

Right, assume tarun was referring to the possibility of this event.

*I assumed
--
jb

(reply address in rot13, unscramble first)

[junw2000]

Frederick Gotham wrote:

posted:

how much memory is allocated when creating an object of an empty class?

Here's a link to Bjarne's answer:
http://www.research.att.com/~bs/bs_f...l#sizeof-empty

Thank you all.
For the followinf code:

#include <iostream>
class A{

};

int main(){
A a[5];
std::cout<<"sizeof(a):"<<sizeof(a)<<std::endl;
}

THe output is:

sizeof(a):5

I use gcc.