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Question about sizeof a class.

P: n/a
Hi,
lets say I have the following class:

class AAA{};

if I do:
sizeof(AAA)
it gives me 1. Why is the size 1 ?

Regards,

Ab.

May 10 '06 #1
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19 Replies


P: n/a
Abubakar wrote:
Hi,
lets say I have the following class:

class AAA{};

if I do:
sizeof(AAA)
it gives me 1. Why is the size 1 ?


Because the minimum size of a class is required to be 1 by the C++ standard.

-cd
May 10 '06 #2

P: n/a
And why does the standard says that it should be 1?

Ab.

"Carl Daniel [VC++ MVP]" <cp*****************************@mvps.org.nospam >
wrote in message news:uD**************@TK2MSFTNGP03.phx.gbl...
Abubakar wrote:
Hi,
lets say I have the following class:

class AAA{};

if I do:
sizeof(AAA)
it gives me 1. Why is the size 1 ?
Because the minimum size of a class is required to be 1 by the C++

standard.
-cd

May 10 '06 #3

P: n/a
Abubakar wrote:
And why does the standard says that it should be 1?


Because you should always be able to take the address of an object, and the
addresses of 2 distincts objects should always be different. This is
fundamental to be able to write generic code (template or otherwise) that
use pointers. The only way to fullfill those requirements is to always have
sizeof(something) >=1

Arnaud
MVP - VC
May 10 '06 #4

P: n/a
So that the object can exist, have its place in memory, and be
identified as unique from other objects of the same class.
And why does the standard says that it should be 1?
Because the minimum size of a class is required to be 1 by the C++
standard.

May 10 '06 #5

P: n/a
Ok, got it. Thanks all.

Ab.

"Arnaud Debaene" <ad******@club-internet.fr> wrote in message
news:#4**************@TK2MSFTNGP04.phx.gbl...
Abubakar wrote:
And why does the standard says that it should be 1?
Because you should always be able to take the address of an object, and

the addresses of 2 distincts objects should always be different. This is
fundamental to be able to write generic code (template or otherwise) that
use pointers. The only way to fullfill those requirements is to always have sizeof(something) >=1

Arnaud
MVP - VC

May 11 '06 #6

P: n/a
>> And why does the standard says that it should be 1?

Because you should always be able to take the address of an object, and
the addresses of 2 distincts objects should always be different. This is
fundamental to be able to write generic code (template or otherwise) that
use pointers. The only way to fullfill those requirements is to always
have sizeof(something) >=1


This in not always true taking unioun(s), or object containment, or
threading into account. Two different objects of the union can have same
address, or contained object can have the same address as a container
object.

After all, why should this be so fundamental for the generic code writing
(that uses pointers)? I believe generic code could survive without it.

I'd rather thought of "sizeof(something) >=1" as of heritage.
--
Vladimir Nesterovsky
May 11 '06 #7

P: n/a
Vladimir Nesterovsky wrote:
After all, why should this be so fundamental for the generic code writing
(that uses pointers)? I believe generic code could survive without it.
I'd rather thought of "sizeof(something) >=1" as of heritage.


What is the size of an array of zero-sized classes? How does one go
about accessing the 5th element of an array of such a class? How does
one go about incrementing a pointer to an array of such a class? How
does one compare two pointers to of such a class, to determine if they
are pointing to the same object? What does 'new' when applied to such a
class?

-n
May 12 '06 #8

P: n/a

"Vladimir Nesterovsky" <vl******@nesterovsky-bros.com> a écrit dans le
message de news: um*************@TK2MSFTNGP04.phx.gbl...
And why does the standard says that it should be 1?
Because you should always be able to take the address of an object, and
the addresses of 2 distincts objects should always be different. This is
fundamental to be able to write generic code (template or otherwise) that
use pointers. The only way to fullfill those requirements is to always
have sizeof(something) >=1


This in not always true taking unioun(s), or object containment, or
threading into account. Two different objects of the union can have same
address, or contained object can have the same address as a container
object.


- union is precisely the explicit exception tat allow 2 objects to overlap
(though I believe it is undefined to apply an unin to anything but primitive
types).
- One could argue that in the container/contained case, the contained object
*is* a sub-part of the container, and therefore thay can be seen as being
only one object (I agree this is dubious, especially in case of private
members).
- Concerning threading.... What's your point? My assertion has a meaning
only at one given point in time. Besides, the C++ language as, for now, no
notion of threads so the point is moot from a language definition point of
view.

After all, why should this be so fundamental for the generic code writing
(that uses pointers)? I believe generic code could survive without it.

I'd rather thought of "sizeof(something) >=1" as of heritage.

Heritage of what? Are you speaking about empty base optimization?

Arnaud
MVP - VC
May 12 '06 #9

P: n/a
>> After all, why should this be so fundamental for the generic code writing
(that uses pointers)? I believe generic code could survive without it.

I'd rather thought of "sizeof(something) >=1" as of heritage. Heritage of what?


The issue could be resolved either way, but obviously not now.
Are you speaking about empty base optimization?


I'm speaking about the fact that many contemporary std:basic_string
implementations have "allocator" instance field, which mostly is an instance
of an empty class, and just wastes the space.
--
Vladimir Nesterovsky
May 12 '06 #10

P: n/a
On Fri, 12 May 2006 17:46:49 +0200, "Vladimir Nesterovsky"
<vl******@nesterovsky-bros.com> wrote:
Are you speaking about empty base optimization?


I'm speaking about the fact that many contemporary std:basic_string
implementations have "allocator" instance field, which mostly is an instance
of an empty class, and just wastes the space.


See this article for a way around that problem:

The "Empty Member" C++ Optimization
http://www.cantrip.org/emptyopt.html

--
Doug Harrison
Visual C++ MVP
May 12 '06 #11

P: n/a
>>> Are you speaking about empty base optimization?

I'm speaking about the fact that many contemporary std:basic_string
implementations have "allocator" instance field, which mostly is an
instance
of an empty class, and just wastes the space.


See this article for a way around that problem:

The "Empty Member" C++ Optimization
http://www.cantrip.org/emptyopt.html


This is clever solution! But say, why should I be so smart just not to waste
a space?
--
Vladimir Nesterovsky
May 13 '06 #12

P: n/a
On Sat, 13 May 2006 08:46:37 +0200, "Vladimir Nesterovsky"
<vl******@nesterovsky-bros.com> wrote:
This is clever solution! But say, why should I be so smart just not to waste
a space?


Because empty classes have sizeof == 1. :)

--
Doug Harrison
Visual C++ MVP
May 14 '06 #13

P: n/a
Why do we need empty classes in the first place? Any article or example that
could explain this to me?

Ab.

"Doug Harrison [MVP]" <ds*@mvps.org> wrote in message
news:c8********************************@4ax.com...
On Sat, 13 May 2006 08:46:37 +0200, "Vladimir Nesterovsky"
<vl******@nesterovsky-bros.com> wrote:
This is clever solution! But say, why should I be so smart just not to wastea space?


Because empty classes have sizeof == 1. :)

--
Doug Harrison
Visual C++ MVP

May 15 '06 #14

P: n/a
> Why do we need empty classes in the first place? Any article or example
that
could explain this to me?


from the url supplied by Doug in this thread:
http://www.cantrip.org/emptyopt.html

"They typically declare typedefs or member functions, and you can replace
them with your own classes (which might not be empty) to handle special
needs."

--

Kind regards,
Bruno van Dooren
br**********************@hotmail.com
Remove only "_nos_pam"
May 15 '06 #15

P: n/a
On Mon, 15 May 2006 11:14:12 +0500, "Abubakar" <em**********@yahoo.com>
wrote:
Why do we need empty classes in the first place? Any article or example that
could explain this to me?


They are useful in a number of contexts:

1. A class can define an interface, but its implementation may be in terms
of another facility defined at a lower level. For example, the class
template std::allocator is defined in terms of the global operators new and
delete. It requires no state of its own, but because it is a class, it can
have data members, and those members may be unique to each instance.

2. It is possible to define class templates that contain nothing but
typedefs that ease (somewhat) the creation of classes that must also define
those typedefs; the latter can derive publicly from the former. An example
would be std::iterator.

3. It is possible to define classes other classes can derive from in order
to indicate type categories. Examples would be std::output_iterator_tag,
std::input_iterator_tag, etc.

4. I once needed a standard way to create a unique type U<T> from a given
type T, because using T directly would interfere with the user's use of a
related facility, and an empty class template provided a way to do that.

5. Classes can be template arguments.

6. Classes have type_info, and map keys can be derived from type_info.

None of the above are possible with namespaces, which are similar to empty
classes that contain nothing but public static members and whose ctors,
dtor, and assignment operator are all private and unimplemented. (Well, I
suppose the first half of (1) can be done with namespaces.)

--
Doug Harrison
Visual C++ MVP
May 15 '06 #16

P: n/a
Can u recommend some book(s) that best explains stl, one that is all about
stl? Some 1000 page kind of book. I wanna read all about stl :)

Ab.

"Doug Harrison [MVP]" <ds*@mvps.org> wrote in message
news:4v********************************@4ax.com...
On Mon, 15 May 2006 11:14:12 +0500, "Abubakar" <em**********@yahoo.com>
wrote:
Why do we need empty classes in the first place? Any article or example thatcould explain this to me?
They are useful in a number of contexts:

1. A class can define an interface, but its implementation may be in terms
of another facility defined at a lower level. For example, the class
template std::allocator is defined in terms of the global operators new

and delete. It requires no state of its own, but because it is a class, it can
have data members, and those members may be unique to each instance.

2. It is possible to define class templates that contain nothing but
typedefs that ease (somewhat) the creation of classes that must also define those typedefs; the latter can derive publicly from the former. An example
would be std::iterator.

3. It is possible to define classes other classes can derive from in order
to indicate type categories. Examples would be std::output_iterator_tag,
std::input_iterator_tag, etc.

4. I once needed a standard way to create a unique type U<T> from a given
type T, because using T directly would interfere with the user's use of a
related facility, and an empty class template provided a way to do that.

5. Classes can be template arguments.

6. Classes have type_info, and map keys can be derived from type_info.

None of the above are possible with namespaces, which are similar to empty
classes that contain nothing but public static members and whose ctors,
dtor, and assignment operator are all private and unimplemented. (Well, I
suppose the first half of (1) can be done with namespaces.)

--
Doug Harrison
Visual C++ MVP

May 16 '06 #17

P: n/a

Abubakar a écrit :
Can u recommend some book(s) that best explains stl, one that is all about
stl? Some 1000 page kind of book. I wanna read all about stl :)


"The C++ Standard Library", by Josuttis, is generally viewed as the
reference on the subject. But as all references, it isn't really ideal
as a teaching tool IMHO.

Arnaud
MVP - VC

May 16 '06 #18

P: n/a
Abubakar wrote:
Can u recommend some book(s) that best explains stl, one that is all about
stl? Some 1000 page kind of book. I wanna read all about stl :)


I recommend Effective STL by Scott Meyers.

Tom
May 16 '06 #19

P: n/a
Thank you all for the book suggestions and the answers.

Regards,

Ab.
"Tamas Demjen" <td*****@yahoo.com> wrote in message
news:O#**************@TK2MSFTNGP05.phx.gbl...
Abubakar wrote:
Can u recommend some book(s) that best explains stl, one that is all about stl? Some 1000 page kind of book. I wanna read all about stl :)


I recommend Effective STL by Scott Meyers.

Tom

May 17 '06 #20

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