facing problem in understanding this....

enggabhinandan · 10-25-2006, 12:01 AM

Hi,
I'm facing one more problem, i'm not able to understand the below way of casting, whats actually happening here....

Code:

#define X(t,m) (size_t) ( (char *) &((t*)0)->m - (char *)(t*)0)

whats the meaning of " &((t*)0)" .....is it something like "address of zero cast to pointer of t"

Wraithan · 10-25-2006, 12:05 AM

Go to your teacher and request either some tutoring or to be removed from the class.

Or really just read your book or pay attention in class.

In anycase stop posting every little question you get.

nadroj · 10-25-2006, 12:07 AM

it looks like: the address of member 'm', from (t) pointer 0.. heh, its really confusing.

break it up into parts to visualize what it is:

Code:

&(          )->m           <---- 'address of' member variable 'm', from this object:
    (t*) 0                  <---- zero casted to a 't' pointer.. which is NULL

or thats how i see it

enggabhinandan · 10-25-2006, 12:11 AM

Thanks Nadroj....thank u very much
and Wraithan if u dont want to suggest then please keep quiet, the forums are present to help the people who are studying on there own. i'm studying on my own and if i find any problem in understanding then ofcourse i will seek the help in this forum, if u r not interested in answering then why cant u be quiet.....

Wraithan · 10-25-2006, 12:25 AM

Most of your questions so far have either been homework/exam questions. And the others with a little googling you would have found the answers.

Refer to the link in my sig for a more indepth reasoning.

enggabhinandan · 10-25-2006, 12:33 AM

i had taken few tests online to check my level of understanding and those question were the one which i never went through and could not understand at all.....for u it may look like very silly but for me a small thing information makes lot of difference....

MacNilly · 10-25-2006, 12:44 AM

Quote:

Originally Posted by enggabhinandan

Hi,
I'm facing one more problem, i'm not able to understand the below way of casting, whats actually happening here....

Code:

#define X(t,m) (size_t) ( (char *) &((t*)0)->m - (char *)(t*)0)

whats the meaning of " &((t*)0)" .....is it something like "address of zero cast to pointer of t"

That code is god awful. There is a struct or class involved because of the -> operator. the (char *) are casts to bytes and well X is just really really vague name for a macro.

swgh · 10-25-2006, 02:46 AM

On a side note I hate using #define. I only use it when using header gaurds in header files

But #define is used much more in C than C++. I usually use a constant variable instead

**CornedBee** · 10-25-2006, 02:50 AM

Looks like an implementation of the offsetof macro to me. These things are not to be looked at, IMO.

kevin_cat · 10-25-2006, 03:34 AM

Quote:

Originally Posted by CornedBee

Looks like an implementation of the offsetof macro to me. These things are not to be looked at, IMO.

I think so, too!

Code:

&((t*)0)->m

is the address of member m. And

Code:

 (t*)0

is the address of struct t.

**Salem** · 10-25-2006, 05:30 AM

Need to find the solution for the Macro
Not showing effort gets you black marks
Repeatedly asking the same question isn't good either.

10-25-2006, 12:01 AM	#1
enggabhinandan Registered User Join Date: Mar 2006 Location: Bangalore, INDIA Posts: 43	facing problem in understanding this.... Hi, I'm facing one more problem, i'm not able to understand the below way of casting, whats actually happening here.... Code: #define X(t,m) (size_t) ( (char ) &((t)0)->m - (char )(t)0) whats the meaning of " &((t)0)" .....is it something like "address of zero cast to pointer of t"*
enggabhinandan is offline

10-25-2006, 12:05 AM	#2
Wraithan pwns nooblars Join Date: Oct 2005 Location: Portland, Or Posts: 1,094	Go to your teacher and request either some tutoring or to be removed from the class. Or really just read your book or pay attention in class. In anycase stop posting every little question you get. __________________ Enlighten Yourself I'm back!
Wraithan is offline

10-25-2006, 12:07 AM	#3
nadroj Registered User Join Date: Oct 2006 Location: Canada Posts: 1,219	it looks like: the address of member 'm', from (t) pointer 0.. heh, its really confusing. break it up into parts to visualize what it is: Code: &( )->m <---- 'address of' member variable 'm', from this object: (t*) 0 <---- zero casted to a 't' pointer.. which is NULL or thats how i see it
nadroj is offline

10-25-2006, 12:11 AM	#4
enggabhinandan Registered User Join Date: Mar 2006 Location: Bangalore, INDIA Posts: 43	Thanks Nadroj....thank u very much and Wraithan if u dont want to suggest then please keep quiet, the forums are present to help the people who are studying on there own. i'm studying on my own and if i find any problem in understanding then ofcourse i will seek the help in this forum, if u r not interested in answering then why cant u be quiet.....
enggabhinandan is offline

10-25-2006, 12:25 AM	#5
Wraithan pwns nooblars Join Date: Oct 2005 Location: Portland, Or Posts: 1,094	Most of your questions so far have either been homework/exam questions. And the others with a little googling you would have found the answers. Refer to the link in my sig for a more indepth reasoning. __________________ Enlighten Yourself I'm back!
Wraithan is offline

10-25-2006, 12:33 AM	#6
enggabhinandan Registered User Join Date: Mar 2006 Location: Bangalore, INDIA Posts: 43	i had taken few tests online to check my level of understanding and those question were the one which i never went through and could not understand at all.....for u it may look like very silly but for me a small thing information makes lot of difference....
enggabhinandan is offline

10-25-2006, 02:46 AM	#8
swgh Its hard... But im here Join Date: Apr 2005 Location: England Posts: 1,467	On a side note I hate using #define. I only use it when using header gaurds in header files But #define is used much more in C than C++. I usually use a constant variable instead
swgh is offline

10-25-2006, 02:50 AM	#9
CornedBee Cat without Hat Join Date: Apr 2003 Posts: 8,492	Looks like an implementation of the offsetof macro to me. These things are not to be looked at, IMO. __________________ All the buzzt! CornedBee "There is not now, nor has there ever been, nor will there ever be, any programming language in which it is the least bit difficult to write bad code." - Flon's Law
CornedBee is offline

10-25-2006, 05:30 AM	#11
Salem and the hat of Jobseeking Join Date: Aug 2001 Location: The edge of the known universe Posts: 21,630	Need to find the solution for the Macro Not showing effort gets you black marks Repeatedly asking the same question isn't good either. __________________ If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
Salem is offline

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