Sorting an STL Vector of pointers

This is a discussion on Sorting an STL Vector of pointers within the C++ Programming forums, part of the General Programming Boards category; Hi all, I have a stl::vector containing pointers to objects. I need to sort this vector based on a member ...

  1. #1
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    Sorting an STL Vector of pointers

    Hi all,

    I have a stl::vector containing pointers to objects. I need to sort this vector based on a member variable (accessible by a getter function). Can someone give me a clue as to how to go about this?

    I've tried a few things - but nothing has really worked, I think mainly because every bit of information I can find on this doesn't talk about pointers...

    For instance, one of the methods I tried was:

    Code:
    class Ccharacter {
       int m_spawnOrder;
    
       int getSpawnOrder();
    
    public:
        Ccharacter(int spawnOrder);
        bool operator<(const Ccharacter& a) const;
    
    };
    
    Ccharacter::Ccharacter(int spawnOrder) {
        m_spawnOrder = spawnOrder
    }
    
    int Ccharacter::getSpawnOrder() {
        return m_spawnOrder;
    }
    
    bool Ccharacter::operator<(const Ccharacter& a) const {
      
       return this->getSpawnOrder() < a.getSpawnOrder();
    }
    I have a vector like such that I need to sort:

    Code:
    vector <Ccharacter *> listOfCharacters;
    The operator< is not getting called at all in this scenario.

    Cheers,

    B
    Last edited by Beamu; 02-07-2009 at 02:50 AM. Reason: Fixed an obvious syntax error

  2. #2
    Cat without Hat CornedBee's Avatar
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    Of course not. You're comparing Ccharacter*, which has its own < operator.

    You need to tell sort not to use the operator at all, but instead your own comparison predicate.
    Code:
    struct compareCharacters {
      bool operator ()(Ccharacter *lhs, Ccharacter *rhs) { return *lhs < *rhs; }
    };
    
    std::sort(listOfCharacters.begin(), listOfCharacters.end(), compareCharacters());
    There are more generic ways of solving this problem, but this will do for now.
    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."
    - Flon's Law

  3. #3
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    Exceptional thank you! Although - i'm not 100% sure why it worked - would someone be so kind as to give me a quick explanation as to why I need the the struct to do the test? I thought sort would use the < if no function was passed to it...

    But again thanks

  4. #4
    Cat without Hat CornedBee's Avatar
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    It does, but that's the problem. For Ccharacter *lhs, Ccharacter *rhs, it does lhs < rhs, but you want *lhs < *rhs. Thus the struct (called a "functor" or "function object", because it overloads operator ()) passed as the third argument.
    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."
    - Flon's Law

  5. #5
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    OH! I see - that makes perfect sense! Thanks very much for the help.

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