Thread: need help assigning one structure to another using pointers

  1. #1
    Registered User
    Join Date
    May 2010

    Post need help assigning one structure to another using pointers

    i have two pointer to structures named *currMB and *PrevMB and i want to assign PrevMB into currMB.
    will the C++ code below work.

    struct Macroblock *currMB;
    struct Macroblock *PrevMB;

    *currMB= & PrevMB;

    and if i want to average these two structures and put the output in currMB.

    then can i write

    currMB = (currMB+PrevMB)/2;

  2. #2
    spurious conceit MK27's Avatar
    Join Date
    Jul 2008
    segmentation fault
    Quote Originally Posted by naeem561 View Post
    *currMB= & PrevMB;
    This will probably be a compiler error, but if not you would be assigning the address of PrevMB to the content of currMB, which is not what you want.

    currMB = PrevMB;
    Both pointers will point to the same struct. If you want to copy the contents:
    memcpy(currMB,PrevMB,sizeof(struct Macroblock));
    then can i write

    currMB = (currMB+PrevMB)/2;
    For sure not.
    C programming resources:
    GNU C Function and Macro Index -- glibc reference manual
    The C Book -- nice online learner guide
    Current ISO draft standard
    CCAN -- new CPAN like open source library repository
    3 (different) GNU debugger tutorials: #1 -- #2 -- #3
    cpwiki -- our wiki on sourceforge

  3. #3
    and the Hat of Guessing tabstop's Avatar
    Join Date
    Nov 2007
    *currMB is a struct, while &PrevMB is a pointer to a pointer to a struct, so they're not comparable at all.

    (currMB+PrevMB)/2 is the average of the addresses stored in those variables (since that's what currMB and PrevMB are storing, addresses).

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