[boost] proxies and const

From: Eric Niebler (eric_at_[hidden])
Date: 2007-07-23 20:09:36

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?

FWIW, it came up in the context of BOOST_FOREACH's handling of proxies, 
so it's not (far) off-topic.

-- 
Eric Niebler
Boost Consulting
www.boost-consulting.com
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