Re: [Boost-users] Problem with sub_range

From: Thorsten Ottosen (thorsten.ottosen_at_[hidden])
Date: 2006-11-17 11:31:20

abir basak wrote:
> Hi,
>    I am not sure if it is the proper newsgroup to get answers
>     related to boost usage.
>    I am facing a problem with boost::sub_range for a const container
>    which is not allowing me to access the indexed operator.
>    Below a short program to demonstrate the problem. Necessary headers
>    are included. Boost version 1.33.1
>    typedef vector<int> VI;
>    VI v(10);
>    int data[] = {0,1,2,3,4,5,6,7,8,9};
>    copy(data,data+10,v.begin());///inserted a few int to std::vector<int>

Hm... this is not legal.

May I suggest

     v = boost::assign::list_of(0)(1)(2)(3)(4)(5)(6) ... ;

?

>    ///prints 0 1 2 3 4 5 6 7 8 9
>    copy(v.begin(),v.end(),ostream_iterator<int>(cout," ")); cout<<endl;
>    const VI& cv = v; /// a const reference to the container.
>    VI::iterator it = v.begin(); ///got an random access iterator
>    it[1] = 100; /// index operator allows to change it
>    copy(v.begin(),v.end(),ostream_iterator<int>(cout," ")); cout<<endl;
>    ///prints  0 100 2 3 4 ...
>    VI::const_iterator it1 = v.begin(); ///got a const_iterator
>    cout<<it1[1]<<endl; /// returns const_reference, assignment is not 
> allowed, access is allowed. prints 100
> boost::sub_range<VI> r(v.begin()+2,v.begin()+8); ///got a sub_range
>    ///prints 2 3 4 5 6 7
>    copy(r.begin(),r.end(),ostream_iterator<int>(cout," "));
>    cout<<endl; r[2] = 400; cout<<r[2]<<endl;/// prints 400
>    ///prints 2 3 400 5 6 7
>    copy(r.begin(),r.end(),ostream_iterator<int>(cout," "));
>    ///so far everything is fine. Here problem starts
>    /// a const sub_range ?
>    boost::sub_range<const VI> r1(cv.begin()+2,cv.begin()+8);
>    ///prints 2 3 400 5 6 7
>    copy(r1.begin(),r1.end(),ostream_iterator<int>(cout," "));cout<<endl;
>    ///next statement is don't compile
>    cout<<r1[2]<<endl; ///=> not allowed
>    gives error under Visual Studio 7.1
> cannot convert from 'const std::allocator<_Ty>::value_type' to 
> 'boost::iterator_range<IteratorT>::value_type &'
>   Why it tried to convert it to value_type& instead of const value_type& ?
> cout<<typeid(r1.begin()).name()<<endl; says r1.begin() is a 
> const_iteartor. So the statement cout<<r1[2]<<endl; should return a 
> const_reference.
>   Am I mis interpreting the const version of sub_range ? If so, what is 
> a const counterpart of sub_range, which behaves just like a pair of 
> const_iterator ?

This is an error in the range lib that should be fixed in the upcoming 
version of boost.

-Thorsten