Re: [boost] [iterator] Proposal to add is_const_iterator trait

From: David Abrahams (dave_at_[hidden])
Date: 2007-05-12 18:15:41

on Sat May 12 2007, Andrey Semashev <andysem-AT-mail.ru> wrote:

> Hello David,

<please avoid overquoting>

> I apologize for returning to this, but as I'm reading the docs and
> looking at my use cases I have before me I don't see how would I elide
> is_const_iterator when using iterator_adaptor. Suppose this:
>
>   template< typename ValueT >
>   class MyList
>   {
>   public:
>     typedef ValueT value_type;
>
>     // etc. all other common typedefs except iterators
>
>   private:
>     struct MyNode
>     {
>       ValueT value;
>       int m_SomeOtherData;
>     };
>     typedef std::list< MyNode > underlying_container;
>
>     template< typename >
>     class MyIterator;
>
>   public:
>     typedef MyIterator<
>       typename underlying_container::iterator
>     > iterator;
>
>     typedef MyIterator<
>       typename underlying_container::const_iterator
>     > const_iterator;
>
>   private:
>     template< typename ItT >
>     class MyIterator :
>       public iterator_adaptor<
>         MyIterator< ItT >,
>         ItT,
>         value_type,  // I have to substitute value type

No, just leave it out.  The default works.

>         use_default, // Category fits well
>         ???  // What should I write for reference type?
>              // I've provided value type, so value_type&
>              // is not valid when ItT is const_iterator

Just leave it out.  The default works.

>       >
>     {
>       typedef typename iterator_adaptor<
>         ...
>       >::reference reference;
>
>       reference dereference()
>       {
>         return this->base()->value;
>       }
>     };
>   };
>
> So, once again I have to detect ItT constness and form up the correct
> reference type for the iterator_adaptor. Am I missing something?

...unless I'm missing something.

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