Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.c++ > #10778 > unrolled thread

sizeof class

Back to article view | Back to comp.lang.c++

Contents

  sizeof class asit <lipun4u@gmail.com> - 2011-10-10 11:22 -0700
    Re: sizeof class Gerald Breuer <Gerald.Breuer@googlemail.com> - 2011-10-10 20:30 +0200
      Re: sizeof class asit <lipun4u@gmail.com> - 2011-10-10 12:13 -0700
        Re: sizeof class Ian Collins <ian-news@hotmail.com> - 2011-10-11 08:23 +1300
          Re: sizeof class Victor Bazarov <v.bazarov@comcast.invalid> - 2011-10-10 15:28 -0400
        Re: sizeof class Gerald Breuer <Gerald.Breuer@Use-Author-Supplied-Address.invalid> - 2011-10-10 22:25 +0200
        Re: sizeof class Markus Wichmann <nullplan@gmx.net> - 2011-10-10 22:48 +0200

#10778 — sizeof class

Please go through the following code...


#include <iostream>

using namespace std;

class Empty {};

class Derived1 : public Empty {};

class Derived2 : virtual public Empty {};

class Derived3 : public Empty
{
        char c;
};

class Derived4 : virtual public Empty
{
        char c;
};

class Dummy
{
        char c;
};

int main()
{
        cout<<"Sizeof(Empty) : "<<sizeof(Empty)<<endl;
        cout<<"Sizeof(Derived1) : "<<sizeof(Derived1)<<endl;
        cout<<"Sizeof(Derived2) : "<<sizeof(Derived2)<<endl;
        cout<<"Sizeof(Derived3) : "<<sizeof(Derived3)<<endl;
        cout<<"Sizeof(Derived4) : "<<sizeof(Derived4)<<endl;
        cout<<"Sizeof(Dummy) : "<<sizeof(Dummy)<<endl;

        return 0;
}


Why sizeof Derived4 is 8 bytes ?

[toc] | [next] | [standalone]

#10779

Am 10.10.2011 20:22, schrieb asit:

> Why sizeof Derived4 is 8 bytes ?

In this case the compiler inserted a four bytes pointer (you're on
a 32-bit-environment) to the virtual base-class. That's usual with
virtual base-classes - but implementation defined.

[toc] | [prev] | [next] | [standalone]

#10780

Hence the size should be 5 bytes(4 for v-pointer + 1 for char). Then why its is of 8 bytes ?

[toc] | [prev] | [next] | [standalone]

#10781

On 10/11/11 08:13 AM, asit wrote:

What are you replying to?  Please keep context.

> Hence the size should be 5 bytes(4 for v-pointer + 1 for char). Then why its is of 8 bytes ?

The size of a class with a virtual base is implementation defined.

-- 
Ian Collins

[toc] | [prev] | [next] | [standalone]

#10782

On 10/10/2011 3:23 PM, Ian Collins wrote:
> On 10/11/11 08:13 AM, asit wrote:
>
> What are you replying to? Please keep context.
>
>> Hence the size should be 5 bytes(4 for v-pointer + 1 for char). Then
>> why its is of 8 bytes ?
>
> The size of a class with a virtual base is implementation defined.

I am not even sure that implementation-definedness is limited to classes 
with virtual bases.  To OP: have you tried printing out the value sizeof 
yields for a class with a single 'char' data member?

V
-- 
I do not respond to top-posted replies, please don't ask

[toc] | [prev] | [next] | [standalone]

#10784

Am 10.10.2011 21:13, schrieb asit:

> Hence the size should be 5 bytes(4 for v-pointer + 1 for char). Then why its is of 8 bytes ?

That's also implementation-defined. But usually your compiler
adds some padding so that each elment in an array of objects
is properly aligned. I'm pretty sure the implementation of
your compiler has a #pragma pack(xyz) directive.

[toc] | [prev] | [next] | [standalone]

#10785

On 10.10.2011 21:13, asit wrote:
> Hence the size should be 5 bytes(4 for v-pointer + 1 for char). Then why its is of 8 bytes ?

Can you say padding? The alignment of the structure resulting from the
class definition would be four bytes, but it has a size of five bytes.
To enforce alignment in aggregate types, the size is adjusted to be
divisible by four.

HTH,
Markus

[toc] | [prev] | [standalone]

Back to top | Article view | comp.lang.c++

csiph-web