iterator design help

[Rex_chaos]

Hi there,
I am writing an iterator for a container. Just like a typical
iterator,

template <typename ADT>
class MyIter
{
...
MyContainer<ADT >& refc;
public:
MyIter(MyContai ner<ADT>& cont) :refc(a) {...}

MyIter& begin() {...; return *this;}
MyIter& end() {...; return *this;}
MyIter& operator++() {...; return *this;}
...
};

While MyIter is applied to a template algorithm like std::transform,
the copy constructor will be actived because of the reference to cont.
It lowers effeciency a lot. Maybe I should use a pointer as instead
for speed purpose

....
MyContainer<ADT >* refc;
public:
MyIter(MyContai ner<ADT>& cont) :refc(a) {...}
....

However, it is another problem. Suppose the container passed to the
iterator has been released, refc will then become an invalid pointer
!!! I can't release the pointer (refc) in the destructor because I
don't know if any object own the container.

[Victor Bazarov]

Re: iterator design help

"Rex_chaos" <rex_chaos@21cn .com> wrote...[color=blue]
> I am writing an iterator for a container. Just like a typical
> iterator,
>
> template <typename ADT>
> class MyIter
> {
> ...
> MyContainer<ADT >& refc;
> public:
> MyIter(MyContai ner<ADT>& cont) :refc(a) {...}
>
> MyIter& begin() {...; return *this;}
> MyIter& end() {...; return *this;}[/color]

I think you're confused. "begin" and "end" should be members of
the container, not of the iterator.
[color=blue]
> MyIter& operator++() {...; return *this;}
> ...
> };
>
> While MyIter is applied to a template algorithm like std::transform,
> the copy constructor will be actived because of the reference to cont.[/color]

I am not sure I understand the sentence above.
[color=blue]
> It lowers effeciency a lot. Maybe I should use a pointer as instead
> for speed purpose
>
> ...
> MyContainer<ADT >* refc;
> public:
> MyIter(MyContai ner<ADT>& cont) :refc(a) {...}
> ...
>
> However, it is another problem. Suppose the container passed to the
> iterator has been released, refc will then become an invalid pointer
> !!! I can't release the pointer (refc) in the destructor because I
> don't know if any object own the container.[/color]

That's the problem with all iterators. They become invalid when the
container they were iterating is destroyed. Simply document that
behaviour and think no more of it.

Victor

[Rex_chaos]

Re: iterator design help

> > template <typename ADT>[color=blue][color=green]
> > class MyIter
> > {
> > ...
> > MyContainer<ADT >& refc;
> > public:
> > MyIter(MyContai ner<ADT>& cont) :refc(a) {...}
> >
> > MyIter& begin() {...; return *this;}
> > MyIter& end() {...; return *this;}[/color]
>
> I think you're confused. "begin" and "end" should be members of
> the container, not of the iterator.[/color]
Thanks for your reply. Yes, I should put "begin" and "end" inside the
container. However, it's something confusing me. Should I return a
reference or just a value of the iterator? i.e Which one is better?

MyIter& begin() {...; return *this;} // defined in MyContainer
MyIter& operator++() {...; return *this;} // defined in MyIter

or

MyIter begin() {...; return *this;} // defined in MyContainer
MyIter operator++() {...; return *this;} // defined in MyIter

Should I have "const" here like

MyIter& begin() const {...}

[Julián Albo]

Re: iterator design help

Rex_chaos escribió:
[color=blue]
> MyIter begin() {...; return *this;} // defined in MyContainer
> MyIter operator++() {...; return *this;} // defined in MyIter
>
> Should I have "const" here like
>
> MyIter& begin() const {...}[/color]

You probably need:

MyIter begin ()

And

MyConstIter begin () const

Regards.

[Victor Bazarov]

Re: iterator design help

"Rex_chaos" <rex_chaos@21cn .com> wrote in message
news:f7a7417.03 10210956.27b4e8 84@posting.goog le.com...[color=blue][color=green][color=darkred]
> > > template <typename ADT>
> > > class MyIter
> > > {
> > > ...
> > > MyContainer<ADT >& refc;
> > > public:
> > > MyIter(MyContai ner<ADT>& cont) :refc(a) {...}
> > >
> > > MyIter& begin() {...; return *this;}
> > > MyIter& end() {...; return *this;}[/color]
> >
> > I think you're confused. "begin" and "end" should be members of
> > the container, not of the iterator.[/color]
> Thanks for your reply. Yes, I should put "begin" and "end" inside the
> container. However, it's something confusing me. Should I return a
> reference or just a value of the iterator? i.e Which one is better?[/color]

How can you return a reference? A reference to what? Are you
going to keep _the_only_ iterator in a container? What if I need
two independent iterators? Just think about it for a minute.
[color=blue]
> MyIter& begin() {...; return *this;} // defined in MyContainer
> MyIter& operator++() {...; return *this;} // defined in MyIter
>
> or
>
> MyIter begin() {...; return *this;} // defined in MyContainer
> MyIter operator++() {...; return *this;} // defined in MyIter
>
> Should I have "const" here like
>
> MyIter& begin() const {...}[/color]

Just like Julian suggested, you should have a special MyConstIter
complementing MyIter.

Get the Josuttis' book on the Standard library and read about
iterators. It will be time well spent.

Victor