«

Hi all.

Can I do this to duplicate the contents of struct a in struct b

struct myStruct a, b, *aptr, *bptr;
aptr = a;
bptr = b;
Do something to initialize contents of structure a
memcpy(bptr, aptr, sizeof(myStruct));

Or is not not a good idea to assume the memory used
by my structure is contiguous ?
should I instead do it field by field.

Kev.

Sourcerer wrote:

> Hi all.
>
> Can I do this to duplicate the contents of struct a in struct b
>
> struct myStruct a, b, *aptr, *bptr;
> aptr = a;

Illegal. Did you mean &a?

> bptr = b;

Ditto &b.

> Do something to initialize contents of structure a
> memcpy(bptr, aptr, sizeof(myStruct));
>
> Or is not not a good idea to assume the memory used
> by my structure is contiguous ?

It may have gaps, but sizeof will cover them all. But why not

memcpy( bptr, aptr, sizeof (*bptr) );

if you're set on using memcpy?

> should I instead do it field by field.

What's wrong with

a = b;

?

--
Chris "electric hedgehog" Dollin
C FAQs at: http://www.faqs.org/faqs/by-newsgrou...mp.lang.c.html
C welcome: http://www.angelfire.com/ms3/bchambl...me_to_clc.html

Sourcerer <> writes:

> Hi all.
>
> Can I do this to duplicate the contents of struct a in struct b
>
> struct myStruct a, b, *aptr, *bptr;
> aptr = a;
> bptr = b;
> Do something to initialize contents of structure a
> memcpy(bptr, aptr, sizeof(myStruct));

memcpy (&b, &a, sizeof b); or simply b = a;

On Tue, 15 Jun 2004 14:13:22 +0100, Chris Dollin <>
wrote:

>Sourcerer wrote:
>
>> Hi all.
>>
>> Can I do this to duplicate the contents of struct a in struct b
>>
>> struct myStruct a, b, *aptr, *bptr;
>> aptr = a;
>
>Illegal. Did you mean &a?
>
>> bptr = b;
>
>Ditto &b.
>
>> Do something to initialize contents of structure a
>> memcpy(bptr, aptr, sizeof(myStruct));
>>
>> Or is not not a good idea to assume the memory used
>> by my structure is contiguous ?
>
>It may have gaps, but sizeof will cover them all. But why not
>
> memcpy( bptr, aptr, sizeof (*bptr) );
>
>if you're set on using memcpy?
>
>> should I instead do it field by field.
>
>What's wrong with
>
> a = b;
>
>?

Wow ! that was fast.

I hadn't realized a = b would do it.

so, I could have

struct mystruct
{
char str[50];
}

struct mystruct a, b;
sprintf(a.str, "Testing 123");
b = a;

and then b.str would also contain "Testing 123" ?

Sourcerer wrote:
> Hi all.
>
> Can I do this to duplicate the contents of struct a in struct b
>
> struct myStruct a, b, *aptr, *bptr;
> aptr = a;
aptr = &a;

> bptr = b;
bptr = &b;

> Do something to initialize contents of structure a
> memcpy(bptr, aptr, sizeof(myStruct));
Try these:
b = a;
memcpy(&b, &a, sizeof(struct myStruct));


>
> Or is not not a good idea to assume the memory used
> by my structure is contiguous ?
> should I instead do it field by field.
I believe that the total space occupied by a structure
is contiguous; however, the compiller is allowed to
add padding between the fields.

When copying one structure to another, memcpy can be
used. But you should have a policy when it comes
to pointer fields:
1. Copy only the pointer and have multiple pointers
to one object.
2. Create a new copy of the target object, thus having
two copies of the target object.
3. Reference counting: multiple pointers to one
object, but the number of references must be zero
before the target object is deleted.

I would copy field by field when copying to or from
a buffer (unsigned char array). Padding bytes may
be copied from one structure to another, but a buffer
may not have padding between the fields.

>
> Kev.
>



--
Thomas Matthews

C++ newsgroup welcome message:
http://www.slack.net/~shiva/welcome.txt
C++ Faq: http://www.parashift.com/c++-faq-lite
C Faq: http://www.eskimo.com/~scs/c-faq/top.html
alt.comp.lang.learn.c-c++ faq:
http://www.raos.demon.uk/acllc-c++/faq.html
Other sites:
http://www.josuttis.com -- C++ STL Library book
http://www.sgi.com/tech/stl -- Standard Template Library

Sourcerer wrote:

> Hi all.
>
> Can I do this to duplicate the contents of struct a in struct b
>
> struct myStruct a, b, *aptr, *bptr;
> aptr = a;
> bptr = b;
> Do something to initialize contents of structure a
> memcpy(bptr, aptr, sizeof(myStruct));
>
> Or is not not a good idea to assume the memory used
> by my structure is contiguous ?
> should I instead do it field by field.
>
> Kev.

Juste use:
b = a;

- Dario

Sourcerer wrote:
>
> Can I do this to duplicate the contents of struct a in struct b
>
> struct myStruct a, b, *aptr, *bptr;
> aptr = a;
> bptr = b;
> Do something to initialize contents of structure a
> memcpy(bptr, aptr, sizeof(myStruct));
>
> Or is not not a good idea to assume the memory used
> by my structure is contiguous ?
> should I instead do it field by field.

That better be:

aptr = &a;
bptr = &b;

but you can duplicate the contents by:

b = a;

What you cannot do is check equality etc. by:

if (b == a) ...; /* not allowed */

structs are not arrays.

--
A: Because it fouls the order in which people normally read text.
Q: Why is top-posting such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

Sourcerer wrote:
> Hi all.
>
> Can I do this to duplicate the contents of struct a in struct b
>
> struct myStruct a, b, *aptr, *bptr;
> aptr = a;
> bptr = b;
> Do something to initialize contents of structure a
> memcpy(bptr, aptr, sizeof(myStruct));

Why not
struct myStruct a = { /* initialize a */ }, b;
b = a;
and forget all your side issues.

"Martin Ambuhl" <> wrote in message
news:...
> Sourcerer wrote:
> > Hi all.
> >
> > Can I do this to duplicate the contents of struct a in struct b
> >
> > struct myStruct a, b, *aptr, *bptr;
> > aptr = a;
> > bptr = b;
> > Do something to initialize contents of structure a
> > memcpy(bptr, aptr, sizeof(myStruct));
>
> Why not
> struct myStruct a = { /* initialize a */ }, b;
> b = a;
> and forget all your side issues.

Does the assignment above invoke undefined behaviour if not all members of a
are initialised?

What about the "equivalent" memcpy()?

Alex

Alex Fraser wrote:

> "Martin Ambuhl" <> wrote in message
> news:...
>
>>Sourcerer wrote:
>>
>>>Hi all.
>>>
>>>Can I do this to duplicate the contents of struct a in struct b
>>>
>>>struct myStruct a, b, *aptr, *bptr;
>>>aptr = a;
>>>bptr = b;
>>>Do something to initialize contents of structure a
>>>memcpy(bptr, aptr, sizeof(myStruct));
>>
>>Why not
>> struct myStruct a = { /* initialize a */ }, b;
>> b = a;
>>and forget all your side issues.
>
>
> Does the assignment above invoke undefined behaviour if not all members of a
> are initialised?

Just how, given the presence of an initializer, do you propose to
partially initialized the structure?

> What about the "equivalent" memcpy()?

Who cares about function calls when a simple assignment will do?