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

From: Tobias Schwinger (tschwinger_at_[hidden])
Date: 2004-12-05 10:04:37

David Abrahams wrote:
> 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.

I see - thanks for your detailed reply.

When using proxy classes as references the assumption that modifications 
of a temporary object are lost at the end of this very object's lifetime 
does not apply - modification of the proxy object actually means 
changing the stored data internally referenced by it.
In this case it would be OK if not preferable to have operator-> return 
a pointer to non-const...

Is there any way to achieve this (besides hand-coding the whole iterator) ?

Or any hope that there will be a parameter to explicitly disable this 
extra-safety ?

And why does operator* give me a mutable (temporary) object then, by the 
way ?

--
Tobias