swap function of STL container

This is a discussion on swap function of STL container within the C++ Programming forums, part of the General Programming Boards category; Hello everyone, All swap functions in STL container class is exception safe and nothrow, right? I have searched for swap ...

  1. #1
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    swap function of STL container

    Hello everyone,


    All swap functions in STL container class is exception safe and nothrow, right? I have searched for swap for vector and deque, but seems no explicit document about the exception safe level.


    thanks in advance,
    George

  2. #2
    C++ Witch laserlight's Avatar
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    The 2003 edition of C++ Standard states:
    Quote Originally Posted by Section 23.1
    no swap() function throws an exception unless that exception is thrown by the copy constructor or assignment operator of the container’s Compare object
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    Hi laserlight,


    I am confused about what means "copy constructor or assignment operator of the containerís Compare object"?

    1. Compare operator will utilize copy constructor?

    2. Compare operator will utilize assignment operator?

    Quote Originally Posted by laserlight View Post
    The 2003 edition of C++ Standard states:

    regards,
    George

  4. #4
    C++ Witch laserlight's Avatar
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    The associative containers have a Compare object since they internally order their elements. This Compare object has a copy constructor and an assignment operator. Since vector and deque do not have Compare objects, their swap functions are guaranteed not to throw.
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    Thanks laserlight,


    I think you quoted C++ Standard implies that Compare object will be used for some containers when we do swap.

    My question, why compare object is needed if we want to swap content of two container? Compare I think it means something like greater than, less than, etc.

    Quote Originally Posted by laserlight View Post
    The associative containers have a Compare object since they internally order their elements. This Compare object has a copy constructor and an assignment operator. Since vector and deque do not have Compare objects, their swap functions are guaranteed not to throw.

    regards,
    George

  6. #6
    C++ Witch laserlight's Avatar
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    My question, why compare object is needed if we want to swap content of two container? Compare I think it means something like greater than, less than, etc.
    You have to swap the Compare objects too.
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    Hi laserlight,


    I only know comparasion operator, and I do not know what is Compare object. What is Compare object (I think compare object is some special member of a class in your context)?

    I searched Google and wikipedia, but no result for "Compare object C++". It is appreciated if you could share some links about it or give your description please?

    Quote Originally Posted by laserlight View Post
    You have to swap the Compare objects too.

    regards,
    George

  8. #8
    C++ Witch laserlight's Avatar
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    I only know comparasion operator, and I do not know what is Compare object. What is Compare object (I think compare object is some special member of a class in your context)?
    Take for example std::map:
    Code:
    template <class Key, class T, class Compare = less<Key>,
              class Allocator = allocator<pair<const Key, T> > >
    class map {
    public:
        // ...
        typedef Compare key_compare;
        // ...
    protected:
        class value_compare : public binary_function<value_type,value_type,bool> {
            friend class map;
        protected:
            Compare comp;
            value_compare(Compare c) : comp(c) {}
        public:
            bool operator()(const value_type& x, const value_type& y) const {
                return comp(x.first, y.first);
            }
        }
    public:
        explicit map(const Compare& comp = Compare(), const Allocator& = Allocator());
        // ...
    };
    The Compare object is just some function object that is used to compare the keys of the map such that they can be ordered.
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    Hi laserlight,


    I do not understand why compare function object may throw exception. It contains no data members normally.

    Here is how STL implements less in MSVC 2008. Any comments? If possible, could you show sample about how and when exception is thrown during construction or assignment for compare object please?

    Code:
    		// TEMPLATE STRUCT less
    template<class _Ty>
    	struct less
    		: public binary_function<_Ty, _Ty, bool>
    	{	// functor for operator<
    	bool operator()(const _Ty& _Left, const _Ty& _Right) const
    		{	// apply operator< to operands
    		return (_Left < _Right);
    		}
    	};

    Quote Originally Posted by laserlight View Post
    Take for example std::map:
    Code:
    template <class Key, class T, class Compare = less<Key>,
              class Allocator = allocator<pair<const Key, T> > >
    class map {
    public:
        // ...
        typedef Compare key_compare;
        // ...
    protected:
        class value_compare : public binary_function<value_type,value_type,bool> {
            friend class map;
        protected:
            Compare comp;
            value_compare(Compare c) : comp(c) {}
        public:
            bool operator()(const value_type& x, const value_type& y) const {
                return comp(x.first, y.first);
            }
        }
    public:
        explicit map(const Compare& comp = Compare(), const Allocator& = Allocator());
        // ...
    };
    The Compare object is just some function object that is used to compare the keys of the map such that they can be ordered.

    regards,
    George

  10. #10
    C++ Witch laserlight's Avatar
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    I do not understand why compare function object may throw exception. It contains no data members normally.
    The keyword is "normally".

    Here is how STL implements less in MSVC 2008. Any comments? If possible, could you show sample about how and when exception is thrown during construction or assignment for compare object please?
    I cannot think of a good example right now, but a rather contrived example is one where the Compare object queries a database during construction to find out how it should compare.
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    The user of the map can supply his or her own compare object. The standard does not require that compare object not to throw in it's copy constructor or assignment operator, and the copy constructor or assignment operator may be used during the swap, so there is no way for the standard to guarantee that the swap won't throw.

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    Thanks laserlight and Daved,


    Question answered.

    [QUOTE=Daved;735079]The user of the map can supply his or her own compare object. The standard does not require that compare object not to throw in it's copy constructor or assignment operator, and the copy constructor or assignment operator may be used during the swap, so there is no way for the standard to guarantee that the swap won't throw.[/QUOT]


    regards,
    George

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