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

From: David Abrahams (dave_at_[hidden])
Date: 2004-12-05 18:58:52

Tobias Schwinger wrote:
> 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 

It most certainly does apply.  There may be some cases in which it 
doesn't, but the usual case is that it does.

> - modification of the proxy object actually means 
> changing the stored data internally referenced by it.

Normally the reason for a proxy is that there isn't any "stored data 
internally referenced by it."  If there's a persistent lvalue, why not 
just return a real reference instead of a proxy?

> 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) ?

Just hand-code your own operator->.

> 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 ?

It is assumed that if you passed non-const R for the reference type, you 
are trying to make a writable iterator, and R will have a (usually 
non-const) assignment operator that takes a value_type argument.  If we 
returned a const object, it would be impossible to write

    *p = x;

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