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

12 May 2007

      Hello David,

Thursday, May 10, 2007, 2:49:56 PM, you wrote:
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on Wed May 09 2007, Andrey Semashev <andysem-AT-mail.ru> wrote:
...
...
Hello David,
Wednesday, May 9, 2007, 8:27:53 PM, you wrote:
...
on Tue May 08 2007, Andrey Semashev <andysem-AT-mail.ru> wrote:
...
...
Hello,
Quite often I find myself writing an is_const_iterator trait which
looks like this:
[snip]
...
...
Maybe it is worth adding to Boost.Iterators library? Or maybe there
already is something like that?
...
What's it useful for?
Mostly I used it to implement my iterators for some adopted STL-like
containers. It shortens the iterator_facade template parameters list:
template< NodeItT >
  struct MyIterator :
      public iterator_facade<
          MyIterator< NodeItT >,
          typename mpl::if_<
              is_const_iterator< NodeItT >,
              const value_type,
              value_type
          >::type,
          typename iterator_category< NodeItT >::type
      >
  {
  };
...
Is there some good reason you're not using iterator_adaptor here?
This looks like a classic use case for it.  That would eliminate the
need for is_const_iterator in this case.
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
        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
      >
    {
      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?

-- 
Best regards,
 Andrey                            mailto:andysem@mail.ru