pointer arithmetic.. why doesn't this work?

This is a discussion on pointer arithmetic.. why doesn't this work? within the C++ Programming forums, part of the General Programming Boards category; hey all, I'm trying to compare a node in an STL linked list to the next node in the list. ...

  1. #1
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    pointer arithmetic.. why doesn't this work?

    hey all, I'm trying to compare a node in an STL linked list to the next node in the list.

    I have this line to compare:

    Code:
    if(*pos == *(pos + 1)) // compare to the next node
    where pos is a list<int>::iterator.

    this seems like it should work to me, but my compiler (MetroWerks CodeWarrior IDE 4.1, MacOS9.1) gives me "Illegal operand" as an error.

    I probably just don't understand pointer arithmetic like I thought: what can I do? Thanks!

  2. #2
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    Well for one, + operator isnt defined for linked list iterators. Its only defined for iterators where the data structure its iterating over is stored contiguously in memory (an array). For example, if you have a list node N, whose to say the next node in the list is at mem location N+1? In most cases it wont be.

    Although in a way it would kinda make sense for the + operator to just move the iterator down the list, that behavior isnt what somone would normally associate with "pointer arithmetic", which is what the iterators are supposed to model.
    C Code. C Code Run. Run Code Run... Please!

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  3. #3
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    to get around this youll probably have to create a temporary iterator that points to the element just before the current element:

    prev = list.begin();
    curr = prev;
    ++curr;

    if (*prev == *curr)

    then just increment both if you want to check down the entire list
    C Code. C Code Run. Run Code Run... Please!

    "Love is like a blackhole, you fall into it... then you get ripped apart"

  4. #4
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    Hey, thanks a lot. I wasn't aware that the + operator didn't work for list iterators!

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