number of elements in pointer

This is a discussion on number of elements in pointer within the C Programming forums, part of the General Programming Boards category; Hey guys, here's a newbie question: suppose I have a ptr to an int, and I allocate memory to it ...

  1. #1
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    number of elements in pointer

    Hey guys, here's a newbie question:

    suppose I have a ptr to an int, and I allocate memory to it dynamically.

    after all the allocations are done, how can I get the number of ints (elements) in my array?

    i feel like I can do this with some fancy sizeof stuff, instead of maintaining a counter..


    Thanks for the help!!

  2. #2
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    You can't.

    You have to maintain a counter.

    If you want someone to keep the counter for you, use a vector.

  3. #3
    Registered User whiteflags's Avatar
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    Well there is sizeof foo / sizeof foo[0]; but that expression only works for arrays that are in scope. So don't use it after you pass foo to a function, the type is different.

  4. #4
    Registered User jeffcobb's Avatar
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    Well usually you can do something like:
    Code:
    int *ptrArray = NULL;
    int nNumberOfInts = 20;
    unsigned totalSize = nNumberOfInts * sizeof(int);
    ptrArray = (int*) malloc(totalSize);
    // ptrArray now points to a block of memory that is
    // number of elements * the size of an int
    // so
    printf("Total size of block of %d integers should be: %d\n", nNumberOfInts, totalSize);
    printf( "Or, sizeof(ptrArray[0] * nNumberOfInts = %d\n", sizeof(ptrArray[0]) * nNumberOfInts);
    Output:
    Code:
    jeff@jeff-gate:~/dev/arraysize$ ./arraysize_test 
    arraysize_test 1.0
    Testing: arraysize
    Total size of block of 20 integers should be: 80
    Or, sizeof(ptrArray[0] * nNumberOfInts = 80
    jeff@jeff-gate:~/dev/arraysize$
    Guess it works
    Last edited by jeffcobb; 05-11-2010 at 11:51 PM.
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  5. #5
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    And it won't work for dynamic arrays.

    sizeof() is a compile-time operator.

  6. #6
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    I see. So I can do that with arrays (the sizeof stuff), but not pointers.

    Thanks for the help guys! you are all really fast!
    Last edited by dayalsoap; 05-11-2010 at 11:42 PM.

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