sizeof and a class with virtual members

[Chameleon]

The code is:
-----------------------------------------------
#include <cstdio>

#define S(X) printf(#X ": %d\n", sizeof(X));

class Seekable
{
public:
virtual void seek(int to, char from);
virtual int tell();
virtual int size();
};
int main() {
S(Seekable);
return 0;
}
-----------------------------------------------
The output is:
-----------------------------------------------
Seekable: 4
-----------------------------------------------

How can be possible?
Compiler is Mingw.

[Barry]

Chameleon wrote:

The code is:
-----------------------------------------------
#include <cstdio>

#define S(X) printf(#X ": %d\n", sizeof(X));

class Seekable
{
public:
virtual void seek(int to, char from);
virtual int tell();
virtual int size();
};
int main() {
S(Seekable);
return 0;
}
-----------------------------------------------
The output is:
-----------------------------------------------
Seekable: 4
-----------------------------------------------

How can be possible?
Compiler is Mingw.

virtual function is often implemented with v-table.
compiler inserts a v-ptr into each instance of a class with at least one
virtual function.
sizeof (v-ptr) == 4, like as ordinary pointer.

[Chameleon]

O/H Barry Ýãñáøå:

Chameleon wrote:

>The code is:
-----------------------------------------------
#include <cstdio>

#define S(X) printf(#X ": %d\n", sizeof(X));

class Seekable
{
public:
virtual void seek(int to, char from);
virtual int tell();
virtual int size();
};
int main() {
S(Seekable);
return 0;
}
-----------------------------------------------
The output is:
-----------------------------------------------
Seekable: 4
-----------------------------------------------

How can be possible?
Compiler is Mingw.

virtual function is often implemented with v-table.
compiler inserts a v-ptr into each instance of a class with at least one
virtual function.
sizeof (v-ptr) == 4, like as ordinary pointer.

Holy shit!
Until now, I believe whole v-table is part of class data!
It is clear now!
v-table is static data for each class declaration.
The objects have only the pointer to these static data (v-table). Not
the whole v-table.
Thanks

[Default User]

Chameleon wrote:

The code is:
-----------------------------------------------
#include <cstdio>

#define S(X) printf(#X ": %d\n", sizeof(X));

You received your answer, but I'll point that the above is invalid. The
sizeof operator gives a result that is type size_t, which is an
unsigned integral type. The %d is for signed ints. size_t may or may
not even be an int.

#define S(X) printf(#X ": %d\n", (int)sizeof(X));

would work, or some variation. I didn't bother with a c++ cast, but you
could use one as well.

Brian

[Aragon]

On 2007-12-24 12:28:32 -0600, "Default User" <de***********@yahoo.comsaid:

Chameleon wrote:

>The code is:
-----------------------------------------------
#include <cstdio>

#define S(X) printf(#X ": %d\n", sizeof(X));

You received your answer, but I'll point that the above is invalid. The
sizeof operator gives a result that is type size_t, which is an
unsigned integral type. T

Or better yet,

std::cout << "Size: " << sizeof(X) << std::endl;

[eefacm]

On Dec 24, 11:03 am, Chameleon <[e-mail]:

The code is:
-----------------------------------------------
#include <cstdio>

#define S(X) printf(#X ": %d\n",sizeof(X));

This macro isn't safe for general use. The following example would be
very likely to cause problems:

int c = 5, d = 3;

S(c%d);

You should prefer:

#define S(X) printf("%s: %d\n", #X, static_cast<int>(sizeof(X)));

....which also reflects the comment elsewhere in this thread that
sizeof returns an unsigned type.