not able to understand this tiny tiny method

This is a discussion on not able to understand this tiny tiny method within the C++ Programming forums, part of the General Programming Boards category; I just saw a tiny method called concrete which is an one line function but I am not able to ...

  1. #1
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    not able to understand this tiny tiny method

    I just saw a tiny method called concrete which is an one line function but I am not able to understand what it does but I am Interested.

    http://cep.xor.aps.anl.gov/software/...ce.html#l00130

    Look at Line 00130

    Code:
    static inline Node *concrete(QMapData::Node *node) {
      return reinterpret_cast<Node *>(reinterpret_cast<char *>(node) - payload());
    }
    payload() will return an Integer but why reinterpret_cast<char *>(node) - payload() ??
    does reinterpret_cast returns an integer ??
    and why cast an Integer to a pointer to an object ??
    Last edited by noobcpp; 10-20-2008 at 11:00 AM.

  2. #2
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    It does this so that it moves the pointer over by byte units, then casts that pointer to a node.

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    Sorry didn't understand correctly.
    I can understand why it cats char* instead of void* is Qt's business logic and that doesn't makes much headech.
    Oh! cause Its 1 byte.

    but why it does reinterpret_cast<char *>(node) - payload()
    is not understood yet.

    does reinterpret_cast returns an integer ??
    if it returns a pointer (I belive it does)
    It could just do node--

    Is it because QMapData::Node contains another data *forward[1]; ??

    But If so return value of payload() is not same of the size of Node so shouldn't it crash due to orphan pointer access ??
    Last edited by noobcpp; 10-20-2008 at 11:29 AM.

  4. #4
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    Lets say we have 32-bit sized memory objects

    Code:
    [0123][4567][89AB]
    What if I wanted to place my pointer to one of thse objects right

    Code:
    [0123][4567][89AB]
        ^
    There. How can I possibly do that? What if I called the pointer a char instead. That way I can move over an arbitary number of bytes instead of moving in 4-byte increments. Get it now?

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    Thanks understood now.

  6. #6
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    Yeah no problem. Its actually a reasonably common practice to move pointers like that for alignment purposes.

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