STL sort throw exception?

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  1. #1
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    STL sort throw exception?

    Hello everyone,


    In Bjarne's book, it is mentioned that sort of STL may throw exception, like sorting elements in a vector.

    In what situation will sort throw exception? I can not find a case.


    thanks in advance,
    George

  2. #2
    The larch
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    Good!

    A search in Mingw implementation of algorithms didn't turn up any instances of "throw".
    I might be wrong.

    Thank you, anon. You sure know how to recognize different types of trees from quite a long way away.
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  3. #3
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    It is possible that this wording is for precautions only - allowing sort to throw an exception allows implementations to CHOOSE whether they throw an exception under some circumstances or not, whilst stating that it can't limits the implementation to only such ways that can not throw an exception. Just like there are plenty of compilers that allow things that are undefined by the C or C++ standard - because that particular compiler or C runtime has a suitable solution for this undefined behaviour.

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    I haven't looked at that passage, but he could very well be referring to an exception being thrown while sort is being run, not necessarily that the sort function itself throws an exception.

    A simple example is if the custom sorting algorithm you pass to sort can throw an exception, or if the copy assignment operator for your class throws an exception.

  5. #5
    Algorithm Dissector iMalc's Avatar
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    Assignment operators are allowed to throw, and vector sorting definitely involves doing that, therefore it has to be able to throw. Otherwise you'd get a partially sorted vector as a result and no notification that it is so.

    However it probably doesn't actually throw new exceptions itself, but rather it would catch and rethrow any user-thrown exceptions. If your operators don't throw, then the sort shouldn't throw.
    I think this all answers the original question as well.
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    Hi anon,


    In his book, Appendix E (Standard-Library Exception Safety), (E.1 Introduction), he mentioned it (in the 1st sample).

    Quote Originally Posted by anon View Post
    Good!

    A search in Mingw implementation of algorithms didn't turn up any instances of "throw".

    regards,
    George

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    Hi Daved,


    1.

    What is the difference between *an exception being thrown while sort is being run* and *sort function itself throws an exception*? Could you show some pseudo code please?

    Quote Originally Posted by Daved View Post
    I haven't looked at that passage, but he could very well be referring to an exception being thrown while sort is being run, not necessarily that the sort function itself throws an exception.
    2.

    Quote Originally Posted by Daved View Post
    A simple example is if the custom sorting algorithm you pass to sort can throw an exception, or if the copy assignment operator for your class throws an exception.
    Agree.


    regards,
    George

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    Agree, iMalc! Great!


    Quote Originally Posted by iMalc View Post
    Assignment operators are allowed to throw, and vector sorting definitely involves doing that, therefore it has to be able to throw. Otherwise you'd get a partially sorted vector as a result and no notification that it is so.

    However it probably doesn't actually throw new exceptions itself, but rather it would catch and rethrow any user-thrown exceptions. If your operators don't throw, then the sort shouldn't throw.
    I think this all answers the original question as well.

    regards,
    George

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    Hi Mats,


    I do not agree it is for precautions, but I think he means assignment operator (e.g. when doing swap) may throw exception.

    Quote Originally Posted by matsp View Post
    It is possible that this wording is for precautions only - allowing sort to throw an exception allows implementations to CHOOSE whether they throw an exception under some circumstances or not, whilst stating that it can't limits the implementation to only such ways that can not throw an exception. Just like there are plenty of compilers that allow things that are undefined by the C or C++ standard - because that particular compiler or C runtime has a suitable solution for this undefined behaviour.

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    regards,
    George

  10. #10
    C++まいる!Cをこわせ! Elysia's Avatar
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    Quote Originally Posted by George2 View Post
    1.

    What is the difference between *an exception being thrown while sort is being run* and *sort function itself throws an exception*? Could you show some pseudo code please?
    Typically when you call sort you provide a callback function, no?
    What Dave is referring to is that your callback function can throw an exception.

    Also, 4 posts in a row. Yeah, like a real spammer
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  11. #11
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    Code:
    void bar(int i)
    {
        if (i == 0)
            throw std::exception();
    
        // do work
    }
    
    void foo_1(int i)
    {
        bar(i);
    }
    
    void foo_2(int i)
    {
        if (i == 0)
            throw std::exception();
        bar(i);
    }
    If you call foo_1 then *an exception will be thrown while foo_1 is being run* but if you call foo_2 then the *foo_2 function itself throws an exception*.

  12. #12
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    Thanks Daved,


    My question is answered.

    Quote Originally Posted by Daved View Post
    Code:
    void bar(int i)
    {
        if (i == 0)
            throw std::exception();
    
        // do work
    }
    
    void foo_1(int i)
    {
        bar(i);
    }
    
    void foo_2(int i)
    {
        if (i == 0)
            throw std::exception();
        bar(i);
    }
    If you call foo_1 then *an exception will be thrown while foo_1 is being run* but if you call foo_2 then the *foo_2 function itself throws an exception*.

    regards,
    George

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