Re: [Boost-users] Problem with sub_range

From: abir basak (abirbasak_at_[hidden])
Date: 2006-11-20 01:41:17

Thorsten Ottosen wrote:
> 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.
It is "not legal" or "not preferred" ? I think it is legal, as I have a 
statement like VI v(10); rather than VI v; copy doesn't grow memory, or 
change size. But here I am just copying! Of course the other (and better 
  ) way is,
VI v;
int data[] = {0,1,2,3,4,5,6,7,8,9};
v.assign(data,data+10);
> 
> 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.
Is the correction is in the CVS head ? Or can you suggest what should be 
the correction,
if it is a small correction?
Thanks for answering.
abir
> -Thorsten