casting of structs

[Servé Lau]

Consider the following code:

struct X { float f; };
struct Y { struct X x; };

void func(struct X *x) {}

int main(void) {
struct Y y;
func(&y);

return 0;
}

Ok, there's two structs. One struct X with some member and struct Y that has
a struct X as the first member. In OO terms we say that Y inherits from X.
Therefore, struct Y can safely be used as a struct X right? Is the above
legal code, can we call func() without casting Y * to X *?
My compiler accepts it without warning and I think it should.

[Christopher Benson-Manica]

"Servé Lau" <[e-mail] .com> spoke thus:

struct X { float f; };
struct Y { struct X x; };
(etc.) My compiler accepts it without warning and I think it should.

cat b.c

struct X { float f; };
struct Y { struct X x; };

void func(struct X *x) {}

int main(void) {
struct Y y;
func(&y);

return 0;
}

gcc -Wall -pedantic -O2 -ansi b.c

b.c: In function `main':
b.c:8: warning: passing arg 1 of `func' from incompatible pointer type

I don't think your compiler is doing you any favors by accepting your
code without issuing a diagnostic.

--
Christopher Benson-Manica | I *should* know what I'm talking about - if I
ataru(at)cybers pace.org | don't, I need to know. Flames welcome.

[Servé Lau]

"Christophe r Benson-Manica" <at***@nospam.c yberspace.org> wrote in message
news:bs******** **@chessie.cirr .com...

I don't think your compiler is doing you any favors by accepting your
code without issuing a diagnostic.

Why not? Is it not true that all pointers to Y can be used as a pointer to
X?

[Peter Pichler]

"Servé Lau" <[e-mail] .com> wrote in message
news:bs******** **@news3.tilbu1 .nb.home.nl...

In the context of...

struct X { float f; };
struct Y { struct X x; };
"Christophe r Benson-Manica" <at***@nospam.c yberspace.org> wrote in message
news:bs******** **@chessie.cirr .com...

I don't think your compiler is doing you any favors by accepting your
code without issuing a diagnostic.

Why not? Is it not true that all pointers to Y can be used as a pointer to
X?

No. A pointer to a structure can be portably /converted/ to a pointer to the
first element of the structure and back. Please note the word 'converted'.
On most PC-based platforms, the pointers will even be binary equivalent, but
it does not necessarily have to be the case.

If you want to simulate OOP inheritance in C, do something like:

In the context of...

struct X { /* fileds */ };
struct Y { struct X x; /* more fields */ };
....
type function (struct X *);
....
int main (void)
{
struct Y y;
func(&y.x); /* note .x */
return 0;
}

[Chris Torek]

In article <bs**********@n ews3.tilbu1.nb. home.nl>,
Servé Lau <[e-mail] .com> wrote:
[one "struct" contains another whole, so that &y.x has type "struct X *",
and:]

void func(struct X *x) {}

int main(void) {
struct Y y;
func(&y);

return 0;
}

Ok, there's two structs. One struct X with some member and struct Y that has
a struct X as the first member. In OO terms we say that Y inherits from X.
In "proper" OO terms the member named y.x (of type "struct X")
should be able to go anywhere, not just first.
Therefore, struct Y can safely be used as a struct X right?
In C89 and C99, only via conversions.
Is the above legal code, can we call func() without casting Y * to X *?
My compiler accepts it without warning and I think it should.

C89 and C99 both require diagnostics.

Plan 9 C, which is a different language from both C89 and C99,
allows this kind of call. It works even if "y.x" is not the
first member, too -- the call has the same effect as func(&y.x).
However, the definition for struct Y must read rather differently:

struct Y {
int any, stuff, you, like;
struct X; /* inherit all of struct X's members */
int more, things, ifdesired;
};

I am not quite sure what Plan 9 C says must occur if "struct X"
has members whose names conflict with those of "struct Y" (exclusive
of "struct X" of course). Moreover, what does it mean if you
try to inherit from the same datatype more than once? For
instance:

struct Point { int x, y; };

struct Rectangle {
Point; /* e.g., upper left corner */
int w, h; /* width and height */
};

is OK, but what about:

struct Rectangle {
Point;
double x; /* error? ok? */
};

and clearly:

struct Rectangle {
Point; /* upper left */
Point; /* lower right */
};

is right out. :-)
--
In-Real-Life: Chris Torek, Wind River Systems
Salt Lake City, UT, USA (40°39.22'N, 111°50.29'W) [phone]
email: forget about it http://web.torek.net/torek/index.html
Reading email is like searching for food in the garbage, thanks to spammers.

[Simon Biber]

"Servé Lau" <[e-mail] .com> wrote:

Consider the following code:

struct X { float f; };
struct Y { struct X x; };

void func(struct X *x) {}

int main(void) {
struct Y y;
func(&y);
This line is a constraint violation, a conforming compiler must
issue a diagnostic for the code. For example:
slau.c:8: warning: passing arg 1 of `func' from incompatible pointer type
return 0;
} Ok, there's two structs. One struct X with some member and struct Y
that has a struct X as the first member. In OO terms we say that Y
inherits from X. Therefore, struct Y can safely be used as a struct
X right?
Yes, it can, so long as you convert the pointer properly.
Is the above legal code, can we call func() without casting
Y * to X *? My compiler accepts it without warning and I think
it should.

No, you must use a cast to convert from Y* to X*. Any compiler that
accepts it without a diagnostic (either warning or error) is not
a standard-conforming compiler or not being invoked with the right
options (such as GCC's -ansi -pedantic).

--
Simon.

[Jack Klein]

On Fri, 26 Dec 2003 20:44:59 +0100, "Servé Lau" <[e-mail] .com>
wrote in comp.lang.c:

Consider the following code:

struct X { float f; };
struct Y { struct X x; };

void func(struct X *x) {}

int main(void) {
struct Y y;
func(&y);

return 0;
}

Ok, there's two structs. One struct X with some member and struct Y that has
a struct X as the first member. In OO terms we say that Y inherits from X.
There are no OO terms in C.
Therefore, struct Y can safely be used as a struct X right? Is the above
legal code, can we call func() without casting Y * to X *?
My compiler accepts it without warning and I think it should.

Either your compiler is broken, or you are using it in a
non-conforming mode.

C is a typed language. Regardless of location in memory, a pointer to
a struct X is not a pointer to a struct Y or a pointer to a float.

--
Jack Klein
Home: http://JK-Technology.Com
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[Christopher Benson-Manica]

Peter Pichler <pe***********@ tiscali.co.uk> spoke thus:

No. A pointer to a structure can be portably /converted/ to a pointer to the
first element of the structure and back.

FMI, can you quote the portion of the Standard that allows this?
Consider my curiousity piqued :)

--
Christopher Benson-Manica | I *should* know what I'm talking about - if I
ataru(at)cybers pace.org | don't, I need to know. Flames welcome.

[Peter Pichler]

Christopher Benson-Manica wrote :

Peter Pichler spoke thus:

No. A pointer to a structure can be portably /converted/ to a pointer to the first element of the structure and back.

FMI, can you quote the portion of the Standard that allows this?
Consider my curiousity piqued :)

Ehm, did I manage to make a fool of myself /again/? ;-)

No quote, just my simplified interpretation of 6.7.2.1:
....
13 Within a structure object, the non-bit-field members and the units in
which bit-fields reside have addresses that increase in the order in
which they are declared. A pointer to a structure object, suitably
^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^ ^^^^^^^^^^^
converted, points to its initial member (or if that member is a
bit-field,
^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^ ^^^^^^^^^
then to the unit in which it resides), and vice versa. There may be
^^^^^^^^^^^^^^
unnamed padding within a structure object, but not at its beginning.