Re: What is the best way to iterate over a map? (or any paired container)

"Earl Purple" <>
16 Aug 2006 06:37:44 -0400
Diego Martins wrote:

I appreciate your quick answers, but all of them were boost related and
I can't use boost in my software due to project policies.
Anyway, I want to learn how to build an iterator wrapper.

No, I showed you how to do it yourself, whilst pointing out at the same
time that you didn't need to because it was already in boost. But if
you can't use boost then either:

- write your own with the steps above
- go to the boost online site, copy the source and add it to your
project standalone although I don't know if that's legal unless you at
least put the copyright details at the top and your company might not
approve of that.

according to example above, can you explain the reason of "Oper op"

Oper is the transformer that takes pair<A,B> to produce B. I gave you
one example, there is a better example that has been given here.

Note you'll need iterator_traits to extract the type of *Iter from
Iter. I think that std::iterator_traits may do that already (typename
std::iterator_traits<Iter>::reference_type possibly). Note you want a
reference so if you dereference Foo* you get Foo& not Foo. If you
dereference const Foo * you get const Foo &). I'm not wholly familiar.

// now write function transform_iterator() to produce one

I can't figure out why we need an aux function
what is wrong with:
SecondIteratorWrapper wrap(map.begin());

If yes, no need for for_each_second() and the similars

something like:
std::map<string,int> map;


What do you think about that?

What does SecondWrap produce? How is it going to be incremented?
SecondWrap would have to be a function returning the transform_iterator
type we are producing.

sounds good, but I am still not getting the whole picture. Can you
gimme an example of your transform_iterator with this transformer,

The transformer is the "Op" parameter for the constructor of the
transform_iterator. The advantage of using this rather than
for_each_second or even for_each_transformed (allowing for a generic
transformer) is that you can use the transform_iterator on any
algorithm, not only for_each.

The example would be:

std::for_each( make_transform_iterator( myMap.begin(), convert2nd ),
make_transform_iterator( myMap.end(), convert2nd ), func );

Note that you wouldn't need to use transform_iterator with std::copy.
You just use std::transform. So

std::transform( myMap.begin(), myMap.end(), std::ostream_iterator< T >(
os, "\n" ), convert2nd );

will do the trick, no need to use std:copy and transform_iterator.

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