how to allocate memory to a member dynamically

[raghu]

Hello ,

This is Raghu. I have a problem in declaring a structure.
Consider
struct hai{
int id;
char sex;
int age;
};
here when a variable is instianted for this structure then immediately
for all members memory is allocated. But I need to allocate memory for
age only if sex is M else the memory should not allocate.
Think you understand the query.
and awaiting for your reply
bye take care
with smile
Raghu

[Andrew Poelstra]

On Tue, 2006-10-24 at 04:21 -0700, raghu wrote:
<formalities snipped>

Consider
struct hai{
int id;
char sex;
int age;
};
here when a variable is instianted for this structure then immediately
for all members memory is allocated. But I need to allocate memory for
age only if sex is M else the memory should not allocate.

<formalities snipped>

There's no way to do this. You could pull it off using pointers, but
then you'd still have to allocate memory for the pointers itself
(which, chances are, is just as large as an int would be).

If you really need to do this, you should create separate structs
for men and women. In an OO language, you could do this a little
more elegantly than in C.

--
Andrew Poelstra <http://www.wpsoftware. net/projects/>

[Andrew Poelstra]

On Tue, 2006-10-24 at 07:04 -0700, Ancient_Hacker wrote:

Well, it's hardly worth the effort. If you're allocating with
malloc() there's typically a minium allocation of 8 to 16 bytes, so
saving the "age" field isn much of a savings. In fact often no savings
at all, if malloc does 16-byte alignment for speed.

Also, most people don't live more than 127 years, so you could store
the gender and age in one byte. Either with a union, or by some
convention, such as a signed char where -1 implies female, positive
implies male and gives their age.

Remember to quote context. Yours seems to be the cleverest solution to
the OP's problem, but few can remember what that problem was!

(It was, essentially, "can I only allocate the .age member of a struct
if the .sex member is 'M'"?)
--
Andrew Poelstra <http://www.wpsoftware. net/projects/>

[Michal Nazarewicz]

"Ancient_Hacker " <gr**@comcast.n etwrites:

What you're trying to do can be expressed in better-thought-out
languages such as Pascal, which have the concept of a "variant record",
your structure would look like:

hai = record
id: integer;
case
sex: boolean of
true: ( age: integer );
false: ( );
end;

But in C you don't have that option.

The "case" doesn't do anything in your example. It's "the same"
structure as in C99 would be:

#v+
struct hai {
int id;
char sex;
union {
int age;
};
};
#v-

"case" insinde of a "record" in Pascal is very similar to "union" and
still your record hai will consume at least sizeof(integer) +
sizeof(boolean) + sizeof(integer) bytes no mether the value of sex.

--
Best regards, _ _
.o. | Liege of Serenly Enlightened Majesty of o' \,=./ `o
..o | Computer Science, Michal "mina86" Nazarewicz (o o)
ooo +--<mina86*tlen.pl >--<jid:mina86*jab ber.org>--ooO--(_)--Ooo--

[Richard Bos]

Andrew Poelstra <ap*******@fals e.sitewrote:

On Tue, 2006-10-24 at 07:04 -0700, Ancient_Hacker wrote:

Well, it's hardly worth the effort. If you're allocating with
malloc() there's typically a minium allocation of 8 to 16 bytes, so
saving the "age" field isn much of a savings. In fact often no savings
at all, if malloc does 16-byte alignment for speed.

Also, most people don't live more than 127 years, so you could store
the gender and age in one byte. Either with a union, or by some
convention, such as a signed char where -1 implies female, positive
implies male and gives their age.

Remember to quote context. Yours seems to be the cleverest solution to
the OP's problem, but few can remember what that problem was!

Clever, but also wrong. The signed char option works, but is rather
brittle and hard to maintain. The union option simply does not work. You
can't store two things in a byte with a union. You can store _either_
one _or_ the other, and writing to either destroys the other.

What might work, or might not, but is at least correct C, is to use
bitfields. 1 bit for the sex, 7 bits for the age. Both unsigned, of
course. Unfortunately there's no guarantee that you can use unsigned
chars for bitfields (only signed and unsigned int, and in C99 _Bool -
and implementation-defined types, so you _may_ be able to use unsigned
char, but it's not portable), so there's no guarantee that you save any
memory this way, over a simple non-bitfield unsigned char for both.

Richard

[Ancient_Hacker]

Michal Nazarewicz wrote:

"case" insinde of a "record" in Pascal is very similar to "union" and
still your record hai will consume at least sizeof(integer) +
sizeof(boolean) + sizeof(integer) bytes no mether the value of sex.

Ah, not exactly. If the record has a "discrimina ted" variant part,
meaning there's a variable in the record that indicates which variant
this is, as in this example, then when you make a new instance of the
record with new(), you can specify which variant record size you want,
for instance in the above example: new( Rec, true ) would allocate
only enough space for a record with the sex=true option.