std::set<boost::weak_ptr> and find()

This is a discussion on std::set<boost::weak_ptr> and find() within the C++ Programming forums, part of the General Programming Boards category; I'm curious: weak_ptr doesn't provide the user with operator== and yet I'm able to successfully use the find member function ...

  1. #1
    (?<!re)tired Mario F.'s Avatar
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    std::set<boost::weak_ptr> and find()

    I'm curious:

    weak_ptr doesn't provide the user with operator== and yet I'm able to successfully use the find member function of std::set.

    I've done some debugging to try and see how this is happening, and apparently weak_ptr is not even being converted to a shared_ptr in order to do the equality check. I went as far down as stl_tree.h where the actual equality check is being done. At that point a pointer to weak_ptr has been created.

    What exactly determines that an std::set find() member function works with weak pointers, but not a std::vector one?
    Last edited by Mario F.; 07-31-2006 at 05:07 PM.
    The programmer’s wife tells him: “Run to the store and pick up a loaf of bread. If they have eggs, get a dozen.”
    The programmer comes home with 12 loaves of bread.


    Originally Posted by brewbuck:
    Reimplementing a large system in another language to get a 25% performance boost is nonsense. It would be cheaper to just get a computer which is 25% faster.

  2. #2
    Registered User whiteflags's Avatar
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    > What exactly determines that an std::set find() member function works with weak pointers, bit not a std::vector one?
    sets, as well as other associative STL containers, store their keys in sorted order to allow fast traversal, so if you searched for something in the set you can actually use one of the other comparison operators to find something. This is why weak pointers don't work with vector well, because vector is strictly random access and not always sorted.

  3. #3
    Code Goddess Prelude's Avatar
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    >What exactly determines that an std::set find() member function works with weak pointers, but not a std::vector one?
    std::vector doesn't have a find member function, to start with. You would have to use the find function from <algorithm>. By default, std::set uses the equivalent of operator< for it's relational tests, an operator which boost::weak_ptr implements.
    My best code is written with the delete key.

  4. #4
    (?<!re)tired Mario F.'s Avatar
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    I actually wanted to mean the find algorithm for std::vector. My bad.

    Thanks both. Didn't think the find() member function wouldn't use ==, despite being a vector.
    The programmer’s wife tells him: “Run to the store and pick up a loaf of bread. If they have eggs, get a dozen.”
    The programmer comes home with 12 loaves of bread.


    Originally Posted by brewbuck:
    Reimplementing a large system in another language to get a 25% performance boost is nonsense. It would be cheaper to just get a computer which is 25% faster.

  5. #5
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    With the vector you can use the lagorithms binary_search, lower_bound, upper_bound and/or equal_range to utilize operator< instead of operator==.

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