Re: [Boost-users] boost::iterator_adaptor

From: David Abrahams (dave_at_[hidden])
Date: 2006-05-12 18:26:51

"Delfin Rojas" <drojas_at_[hidden]> writes:

>> Grisha Spivak wrote:
>>
>> <snip>
>>
>>     it[2] = -22;
>>     it[3] = it[5];
>>     std::cout << arr[2*2] << ' ' << arr[3*2] << ' ' << 
>> arr[5*2] << '\n';
>> 
>> It prints "-22 33 55" instead of "-22 55 55" (which I 
>> expected) - so "it[3] = it[5];" assignment didn't modifies array.
>> Is it intended behaviour? Or maybe I'm doing something wrong?
>> 
>> 
>
> If you change it[3] = it[5]; with it[3] = 55; it will work. The problem is
> your iterator_facade::operator[] returns a
> boost::detail::operator_brackets_proxy which, I'm not sure why, does not
> seem to implement operator=(operator_brackets_proxy const &). There may be a
> good reason for this but perhaps the author(s) of the library can answer
> that.

We didn't think of it ;-)

http://www.boost.org/libs/iterator/doc/iterator_facade.html#operator

explains why operator[] returns a proxy. You can write:

         int x = it[5];
         it[3] = x;

to work around this problem.

Jeremy and Thomas: do you think we should change the
Mutable_RandomAccessIterator concept to require that

  it[n] = e work 

for any e convertible to the iterator's value_type?

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com