sizeof(EmptyStruct) in C and C++ (was: Base {}; sizeof(Base) == 1?)
 

"Alf P. Steinbach" <alfps@start.no> wrote in message news:4k9755Fb3nt6U1@individual.net...
> * moleskyca1@yahoo.com:
> > This may be stupid question, but why is sizeof(Base) == 1 in:
> >
> > int main(int argc, char* argv[])
> > {
> > class Base
> > {
> > };
> > cout << sizeof(Base) << endl;
> > return 0;
> > }
> >
> > I guess I want to know what the 1 byte is for? There is no vptr here,
> > so why 1 byte?I checked FAQ and couldn't find answer.
>
> Needs a unique address.
>

struct Empty {};

C: sizeof(Empty) == 0
C++: sizeof(Empty) > 0

Why doesn't C need a unique address?


--
Alex Vinokur
email: alex DOT vinokur AT gmail DOT com
http://mathforum.org/library/view/10978.html
http://sourceforge.net/users/alexvn

Re: sizeof(EmptyStruct) in C and C++ (was: Base {}; sizeof(Base) ==1?)
 

"Alex Vinokur" <alexvn@users.sourceforge.net> writes:
> "Alf P. Steinbach" <alfps@start.no> wrote in message
> news:4k9755Fb3nt6U1@individual.net...
>> * moleskyca1@yahoo.com:
>> > This may be stupid question, but why is sizeof(Base) == 1 in:
>> >
>> > int main(int argc, char* argv[])
>> > {
>> > class Base
>> > {
>> > };
>> > cout << sizeof(Base) << endl;
>> > return 0;
>> > }
>> >
>> > I guess I want to know what the 1 byte is for? There is no vptr here,
>> > so why 1 byte?I checked FAQ and couldn't find answer.
>>
>> Needs a unique address.
>>
>
> struct Empty {};
>
> C: sizeof(Empty) == 0
> C++: sizeof(Empty) > 0
>
> Why doesn't C need a unique address?

In C,
struct Empty {};

is a syntax error. (Some compilers might support that as an
extension; if so, it's up to the compiler to decide what
sizeof(struct Empty) should be.)

--
Keith Thompson (The_Other_Keith) kst-u@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.

Re: sizeof(EmptyStruct) in C and C++ (was: Base {}; sizeof(Base) == 1?)
 

Keith Thompson wrote:
> "Alex Vinokur" <alexvn@users.sourceforge.net> writes:
> > "Alf P. Steinbach" <alfps@start.no> wrote in message
> > news:4k9755Fb3nt6U1@individual.net...
> >> * moleskyca1@yahoo.com:
> >> > This may be stupid question, but why is sizeof(Base) == 1 in:
> >> >
> >> > int main(int argc, char* argv[])
> >> > {
> >> > class Base
> >> > {
> >> > };
> >> > cout << sizeof(Base) << endl;
> >> > return 0;
> >> > }
> >> >
> >> > I guess I want to know what the 1 byte is for? There is no vptr here,
> >> > so why 1 byte?I checked FAQ and couldn't find answer.
> >>
> >> Needs a unique address.
> >>
> >
> > struct Empty {};
> >
> > C: sizeof(Empty) == 0
> > C++: sizeof(Empty) > 0
> >
> > Why doesn't C need a unique address?
>
> In C,
> struct Empty {};
>
> is a syntax error. (Some compilers might support that as an
> extension; if so, it's up to the compiler to decide what
> sizeof(struct Empty) should be.)
>
> --
> Keith Thompson (The_Other_Keith) kst-u@mib.org <http://www.ghoti.net/~kst>
> San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
> We must do something. This is something. Therefore, we must do this.

What is the size of a in
int a[0];

Re: sizeof(EmptyStruct) in C and C++
 

swets wrote:

> What is the size of a in
> int a[0];

A zero size array is illegal according to ANSI C.


Igmar

Re: sizeof(EmptyStruct) in C and C++
 

What if I use it in a structure like -
struct A {
int a[0];
};

What is the size of such structure?

Thanks,
Sweta

Igmar Palsenberg wrote:
> swets wrote:
>
> > What is the size of a in
> > int a[0];
>
> A zero size array is illegal according to ANSI C.
>
>
> Igmar

Re: sizeof(EmptyStruct) in C and C++
 

Sweta writes:
> What if I use it in a structure like -
> struct A {
> int a[0];
> };
>
> What is the size of such structure?

Still not valid. Standard C does not allow 0-sized objects - and that
includes objects that are part of other objects.

--
Hallvard

Re: sizeof(EmptyStruct) in C and C++
 

Sweta wrote:
> What if I use it in a structure like -
> struct A {
> int a[0];
> };
>
> What is the size of such structure?

What isn't clear about 'a zero size array is illegal' ? Putting it in a
doesn't make it less of an array.


Igmar

Re: sizeof(EmptyStruct) in C and C++
 

On 2006-08-14 04:55:31 -0400, "Sweta" <swetag@gmail.com> said:
>
> Igmar Palsenberg wrote:
>> swets wrote:
>>
>>> What is the size of a in
>>> int a[0];
>>
>> A zero size array is illegal according to ANSI C.
>>
> What if I use it in a structure like -
> struct A {
> int a[0];
> };
>
> What is the size of such structure?

As Igmar said, "A zero size array is illegal according to ANSI C."

--
Clark S. Cox, III
clarkcox3@gmail.com