Re: [Boost-users] FW: custom iterator_adaptor::pointer

From: David Abrahams (dave_at_[hidden])
Date: 2005-10-18 07:22:15

"Sigg  Christian" <sigg_at_[hidden]> writes:

> Dear boosters,
>
> I'm trying to figure out why there is no way to define the 
> iterator_adaptor's pointer type.

Because 

 a. Generic algorithms never need operator-> in the first place, so
    supplying an operator-> usually has little value in itself.  It's
    just a requirement that must be met to create a legal iterator

 b. For any reference type you specify, the library will already
    supply a pointer type that's sufficient to meet the standard's
    requirements.

 c. You can always override the library's choices in your derived class.

> I'm trying to write an iterator that concurrently accesses 
> two containers, but the interface should look like it's only 
> one container. Therefore, it is necessary to write my own 
> value, reference and pointer type. 

It's usually not necessary to write your own pointer type.

> However, the iterator_adaptors pointer type is always value*.

That's not true, actually.  The library supplies an
operator_arrow_proxy that's used whenever your reference type is not a
true C++ reference.

> Is it save to just overload the pointer typedef and reimplement 
> operator->()?

You can certainly do that.  Whether it's safe or not may be another
matter.

> I'm not too much of a c++ expert, so if anyone could comment 
> on that, I would greatly appreciate it.

Be sure that the category you specify for your iterator is compatible
with the associated types you supply.  For example, a forward,
bidirectional, or random access iterator must always have a reference
type of T& or T const&, where T is the value_type.

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com