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

[Jens Thoms Toerring]

raghu <ra*********@gm ail.comwrote:

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.

Then you can't use the structure like this - a structure _is_ the
set of its components and when you have the compiler create a new
instance of the structure it will give you memory for all its
members. The only way out is to have a pointer in the structure
that you assign memory to manually under certain conditions only,
like in (required error checking left out):

struct hai2 {
int id;
char sex;
int *age;
} person;

/* Assign some value to person.sex here */

if ( person.sex == 'M' ) {
person.age = malloc( sizeof *person.age );
*person.age = 17;
}

Of course, you also have to take care of deallocating the memory before
the structure goes out of scope.

Obvioulsy, this looks stupid in this example (the int pointer probably
requires at least as much memory as an integer), but there are several
situations where such an approach makes a lot of sense.

Regards, Jens
--
\ Jens Thoms Toerring ___ [e-mail]
\______________ ____________ http://toerring.de

[mark_bluemel]

raghu wrote:

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

I suppose you could do something like this (error handling omitted for
brevity) :-

char sex;
int age;
struct hai *hp;
....
<processing to get the data to work with>
....
if (sex == 'M') {
hp = malloc(sizeof(s truct hai));
hp->id = <whatever>;
hp->sex = sex;
hp->age = age;
} else { /* I assume you've validate that sex is 'M' or 'F' */
hp = malloc(sizeof(s truct hai) - sizeof(int));
hp->id = <whatever>;
hp->sex = sex;
}

It's ugly and almost guaranteed to cause you grief in later times, but
it does (in some sense) meet your requirement.

However, your requirement is nonsense. Saving 1 machine word per
"struct hai" is not worth bothering with unless you are hugely
memory-constrained, IMNSHO...

[mark_bluemel]

mark_blue...@po box.com wrote:

..., your requirement is nonsense. Saving 1 machine word per
"struct hai" is not worth bothering with unless you are hugely
memory-constrained, IMNSHO...

As Jens points out, if there is more than just a single word, such as a
nested structure, the presence or absence of which is dependant on
other data in the structure, then a pointer-based solution may be
appropriate...

[raghu]

Okay but the problem is i have number of such type of members in a
structure. And instead of a byte I want to check the bit positions. If
i followed ur approach then i have to waste the memory of 4 bytes for
each member. But i don't want to happen like that.
-Raghu
Jens Thoms Toerring wrote:

raghu <ra*********@gm ail.comwrote:

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.

Then you can't use the structure like this - a structure _is_ the
set of its components and when you have the compiler create a new
instance of the structure it will give you memory for all its
members. The only way out is to have a pointer in the structure
that you assign memory to manually under certain conditions only,
like in (required error checking left out):

struct hai2 {
int id;
char sex;
int *age;
} person;

/* Assign some value to person.sex here */

if ( person.sex == 'M' ) {
person.age = malloc( sizeof *person.age );
*person.age = 17;
}

Of course, you also have to take care of deallocating the memory before
the structure goes out of scope.

Obvioulsy, this looks stupid in this example (the int pointer probably
requires at least as much memory as an integer), but there are several
situations where such an approach makes a lot of sense.

Regards, Jens
--
\ Jens Thoms Toerring ___ [e-mail]
\______________ ____________ http://toerring.de

[Christopher Benson-Manica]

raghu <ra*********@gm ail.comwrote:

Okay but the problem is i have number of such type of members in a
structure. And instead of a byte I want to check the bit positions. If
i followed ur approach then i have to waste the memory of 4 bytes for
each member. But i don't want to happen like that.

One idea that buys you infinite flexibility at the cost of one void *,
one int, and 2^n-1 structure definitions (where n is the number of these
members that you don't want to automatically allocate memory for) -
making it a terribly silly idea, really - follows:

enum struct_types { FOO, BAR, FOO_AND_BAR };

struct my_struct {
int baz;
/* other fields */
int struct_type;
void *variable_struc t;
};

struct foo {
int foo;
};

struct bar {
int bar;
};

struct foo_and_bar {
int foo;
int bar;
};

struct my_struct *allocate_my_st ruct( int type ) {
/* Error checking of malloc() calls omitted */
struct my_struct *ret=malloc( sizeof *ret );
switch(type) {
case FOO:
ret->variable_struc t=malloc( sizeof(struct foo) );
break;
case BAR:
ret->variable_struc t=malloc( sizeof(struct bar) );
break;
case FOO_AND_BAR:
ret->variable_struc t=malloc( sizeof(struct foo_and_bar) );
break;
default:
ret->variable_struc t=NULL;
}
ret->struct_type=ty pe;
return ret;
}

--
C. Benson Manica | I *should* know what I'm talking about - if I
cbmanica(at)gma il.com | don't, I need to know. Flames welcome.

[mark_bluemel]

raghu wrote:

... the problem is i have number of such type of members in a
structure. And instead of a byte I want to check the bit positions. If
i followed ur approach then i have to waste the memory of 4 bytes for
each member. But i don't want to happen like that.

First point - don't top-post. It's greatly disliked in this group and
you'll lose such friends as you have.

Second point - if that is your problem, then you should have told us
about it first, rather than the trivial reduction of it. I think we
still need a clearer idea of what you are really trying to do before we
can give more than the most general guidance. Christopher's suggestion
seems fairly close though I'd also consider what could be achieved with
(for example) the use of unions...

[Ancient_Hacker]

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.

[Ancient_Hacker]

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. You can fake it somewhat by
defining TWO structs, one with and the other without the age field,and
malloc the right one each time.

[CBFalconer]

raghu wrote:

>
Okay but the problem is i have number of such type of members in a
structure. And instead of a byte I want to check the bit positions.
If i followed ur approach then i have to waste the memory of 4
bytes for each member. But i don't want to happen like that.

Don't top-post, or you will likely be ignored in this group. Your
answer belongs after, or possibly intermixed with, the _snipped_
material you quote. Also you should improve your spelling. The
first person pronoun is I, capitalized. 'your' is spelt as shown.
Using such abortions as i and ur only makes your post hard to read
and is offensive.

--
Chuck F (cbfalconer at maineline dot net)
Available for consulting/temporary embedded and systems.
<http://cbfalconer.home .att.net>