Re: [boost] proxies and const

From: Thorsten Ottosen (thorsten.ottosen_at_[hidden])
Date: 2007-07-27 08:10:01

Eric Niebler skrev:
> General design question here. This came up on the user's list, and it 
> made me question something I believed to be true. When writing a proxy 
> for, e.g., a std container, I see two options for handling const.
> 
> 0) Const-ness depends on the const-ness of the object being proxied:
> 
> struct vector_proxy {
>    std::vector<int> & vec;
>    typedef std::vector<int>::iterator iterator;
>    typedef std::vector<int>::iterator const_iterator;
>    iterator begin() const { return vec.begin(); }
>    ...
> };
> 
> 1) Const-ness depends on the proxy object itself:
> 
> struct vector_proxy {
>    std::vector<int> & vec;
>    typedef std::vector<int>::iterator iterator;
>    typedef std::vector<int>::const_iterator const_iterator;
>    iterator begin() { return vec.begin(); }
>    const_iterator begin() const { return vec.begin(); }
>    ...
> };
> 
> I think a loooong time ago, I preferred (1) but these days (0) is more 
> natural for me, but I can't say why, exactly. Just feels like that's how 
> it should be. Thoughts?
> 

There's a relevant piece in Boost.Range:

http://www.boost.org/libs/range/doc/utility_class.html

boost::iterator_range falls into case 1, whereas boost::sub_range falls 
into case 2.

I think case one has been the preferred way in C++0x3 because the 
language lacks rvalue references. Without rvalue references we often
say

template< class Range >
void SomeAlgo( const iterator_range<Range>& );

Wiht rvalue references, I think option 2 will gain popularity.

-Thorsten