[Boost-users] Re: [iterator] iterator_facade operator-> pointee always const

From: David Abrahams (dave_at_[hidden])
Date: 2004-12-04 14:53:47

Tobias Schwinger wrote:
> Hi,
> 
> While using boost::iterator_facade wirh a class type as argument for 
> both the 'Reference' and the 'Value' template type parameter I noticed 
> that I can't access non-const members via operator->.

Surely you can!  You must mean that you can't modify members via 
operator-> ?

>  *i is not const 
> and neither 'Value' nor 'Reference' are.
> 
> A naiv look at the source shows me that 
> operator_arrow_proxy::operator->() always returns 'T const *'.
> 
> Is this a bug or an intentional restriction ?

It's intentional.  When your iterator's reference type is not 
value_type& or value_type const&, your reference type is the same as the 
value_type or you are using a proxy reference.  Either way, if your 
iterator is readable,  the reference is implicitly convertible to 
value_type.  If your iterator is writable, it can be assigned from the 
value_type.  No persistent object (lvalue) of type value_type is 
revealed by dereferencing the iterator; you can at best see a temporary. 
  If iterator_facde handed you a pointer to non-const value_type, it 
would  allow you to modify this temporary, which would then disappear. 
We don't do that for the same reasons the language won't bind a 
non-const reference to a temporary.

-- 
Dave Abrahams
Boost Consulting
http://www.boost-consulting.com