facing problem in understanding this....

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  1. #1
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    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"

  2. #2
    pwns nooblars
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    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.

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    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

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    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.....

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    pwns nooblars
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    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.

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    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....

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    Registered User MacNilly's Avatar
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    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.

  8. #8
    Its hard... But im here swgh's Avatar
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    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

  9. #9
    Cat without Hat CornedBee's Avatar
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    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."
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  10. #10
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    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.

  11. #11
    and the hat of wrongness Salem's Avatar
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    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.
    If at first you don't succeed, try writing your phone number on the exam paper.
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