«

Why does one need to use two kinds of sizeof operator:
* sizeof unary-expression,
* sizeof (type-name)
?

Their behavior seem not to be different (see an example below).

------ C++ code ------
#include <iostream>
using namespace std;

int main()
{
int x = 100;

cout << sizeof ++x << endl; // expression sizeof
cout << x << endl;

x = 100;
cout << endl;
cout << sizeof (++x) << endl; // type sizeof
cout << x << endl;

return 0;
}
----------------------

------ Run ------

4
100

4
100

-----------------

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

Alex Vinokur wrote:

> Why does one need to use two kinds of sizeof operator:
> * sizeof unary-expression,
> * sizeof (type-name)
> ?

Because you might want to use it on a value to find out its size, or you
want to use it on a type to find out its size.

>
> Their behavior seem not to be different (see an example below).
>
> ------ C++ code ------
> #include <iostream>
> using namespace std;
>
> int main()
> {
> int x = 100;
>
> cout << sizeof ++x << endl; // expression sizeof
> cout << x << endl;
>
> x = 100;
> cout << endl;
> cout << sizeof (++x) << endl; // type sizeof

(++x) is not a type. int would be a type.

> cout << x << endl;
>
> return 0;
> }
> ----------------------
>
> ------ Run ------
>
> 4
> 100
>
> 4
> 100

"Rolf Magnus" <> wrote in message news:e6eihq$66v$03$...
> Alex Vinokur wrote:
[snip]
> >
> > int main()
> > {
> > int x = 100;
> >
> > cout << sizeof ++x << endl; // expression sizeof
> > cout << x << endl;
> >
> > x = 100;
> > cout << endl;
> > cout << sizeof (++x) << endl; // type sizeof
>
> (++x) is not a type. int would be a type.
[snip]

I think, ++x is parsed as typename in sizeof (++x).


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

Alex Vinokur wrote:
> "Rolf Magnus" <> wrote in message news:e6eihq$66v$03$...
> > Alex Vinokur wrote:
> [snip]
> > >
> > > int main()
> > > {
> > > int x = 100;
> > >
> > > cout << sizeof ++x << endl; // expression sizeof
> > > cout << x << endl;
> > >
> > > x = 100;
> > > cout << endl;
> > > cout << sizeof (++x) << endl; // type sizeof
> >
> > (++x) is not a type. int would be a type.
> [snip]
>
> I think, ++x is parsed as typename in sizeof (++x).

No it is a unary-expression because a primary-expression is also a
unary-expression and ( expression ) is a primary-expression and clearly
++x is an expression.

Markus Schoder wrote:
> Alex Vinokur wrote:
> > "Rolf Magnus" <> wrote in message news:e6eihq$66v$03$...
> > > Alex Vinokur wrote:
> > [snip]
> > > >
> > > > int main()
> > > > {
> > > > int x = 100;
> > > >
> > > > cout << sizeof ++x << endl; // expression sizeof
> > > > cout << x << endl;
> > > >
> > > > x = 100;
> > > > cout << endl;
> > > > cout << sizeof (++x) << endl; // type sizeof
> > >
> > > (++x) is not a type. int would be a type.
> > [snip]
> >
> > I think, ++x is parsed as typename in sizeof (++x).
>
> No it is a unary-expression because a primary-expression is also a
> unary-expression and ( expression ) is a primary-expression and clearly
> ++x is an expression.

What is the difference between
sizeof ++x
and
sizeof (++x)
?

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

Alex Vinokur wrote:

> What is the difference between
> sizeof ++x
> and
> sizeof (++x)
> ?

Nothing. sizeof takes an expression, and parens in expressions may be
optional.

sizeof(int) is the size of the typecast to int. (int)0.

--
Phlip
http://c2.com/cgi/wiki?ZeekLand <-- NOT a blog!!!

Phlip wrote:
> Alex Vinokur wrote:
>
> > What is the difference between
> > sizeof ++x
> > and
> > sizeof (++x)
> > ?
>
> Nothing. sizeof takes an expression, and parens in expressions may be
> optional.

So, why do we need two kinds of sizeof operator?

>
> sizeof(int) is the size of the typecast to int. (int)0.

[snip]

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

Alex Vinokur wrote:
> Phlip wrote:
> > Alex Vinokur wrote:
> >
> > > What is the difference between
> > > sizeof ++x
> > > and
> > > sizeof (++x)
> > > ?
> >
> > Nothing. sizeof takes an expression, and parens in expressions may be
> > optional.
>
> So, why do we need two kinds of sizeof operator?

One takes an expression and returns the size of the expression's type
and one takes a type directly (which must be put in parentheses). It is
mostly a matter of convenience.

Alex Vinokur schrieb:
> Phlip wrote:
>> Alex Vinokur wrote:
>>
>>> What is the difference between
>>> sizeof ++x
>>> and
>>> sizeof (++x)
>>> ?
>> Nothing. sizeof takes an expression, and parens in expressions may be
>> optional.
>
> So, why do we need two kinds of sizeof operator?

int x;

1) sizeof(int)
2) sizeof(x) (or sizeof x)

Which one do you think is redundant?

Thomas

Thomas J. Gritzan wrote:
[snip]
>
> int x;
>
> 1) sizeof(int)
> 2) sizeof(x) (or sizeof x)
>
> Which one do you think is redundant?
>

1) sizeof(int)
2) sizeof(x)
3) sizeof x // redundant

[snip]

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