const iterator
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Thread: const iterator

  1. #1
    Join Date
    Aug 2001
    Posts
    81

    const iterator

    Hi,

    I have a problem with a constant iterator. This is an example of the situation

    class B
    {
    public:
    B(){}

    std::list<int> m_List;
    std::list<int>::iterator Iter;


    const int Func ( ) const
    {
    std::list<int>::iterator IterFunc;
    IterFunc = m_List.begin ( );
    return 2;
    }
    };


    int main ( )
    {
    B ObjB;
    ObjB.Func ( );

    return 0;
    }

    I have this compilation error

    error C2679: binary '=' : no operator found which takes a right-hand operand of type 'std::list<_Ty>::_Const_iterator<_Secure_validation>' (or there is no acceptable conversion)

    If I remove the const at the end of Func it solves the problem but I need this function to be const. How I can solve this problem

    Thank you

  2. #2
    Join Date
    Oct 2008
    Posts
    1,168

    Re: const iterator

    Code:
    const int Func ( ) const
    {
    std::list<int>::const_iterator IterFunc;
    IterFunc = m_List.begin ( );
    return 2;
    }

  3. #3
    Join Date
    Aug 2001
    Posts
    81

    Re: const iterator

    Perfect, Thank you for your answer

  4. #4
    Join Date
    Jun 2009
    Location
    France
    Posts
    2,358

    Re: const iterator

    Quote Originally Posted by Geof View Post
    Hi,

    I have a problem with a constant iterator. This is an example of the situation

    class B
    {
    public:
    B(){}

    std::list<int> m_List;
    std::list<int>::iterator Iter;


    const int Func ( ) const
    {
    std::list<int>::iterator IterFunc;
    IterFunc = m_List.begin ( );
    return 2;
    }
    };


    int main ( )
    {
    B ObjB;
    ObjB.Func ( );

    return 0;
    }

    I have this compilation error

    error C2679: binary '=' : no operator found which takes a right-hand operand of type 'std::list<_Ty>::_Const_iterator<_Secure_validation>' (or there is no acceptable conversion)

    If I remove the const at the end of Func it solves the problem but I need this function to be const. How I can solve this problem

    Thank you
    Func is const, that means inside Func, all member variables are const, including m_list. When you call begin on m_list, you are calling the const version of begin, which returns a const iterator.

    You cannot assign a const_iterator to an iterator, because that would break const correctness,

    If you are not planning to use iterfunc to modify you list, then declare it const_iterator. If you ARE planning to use it to modify your list, then don't call Func const, since it isn't.

    You should know that constness IS part of function signatures. Writing:
    Code:
    myList.begin()
    could call two different functions depending on your list constness:

    Code:
    iterator begin(); //non const version, returning a non const iterator
    const_iterator begin() const; //const version returning a const_iterator
    Your compiler will always try to use non-const functions if your object is not const, and const functions if your object is const.

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