C++ iterator

[aamir121a]

I was meaning to understand in C++ what are iterator , I have gone through whats on the web , however still did not get it. could someone please provide explanation , with some example ,
I have always found , seeing an example helps me better than just plain words.

[itsmeandnobodyelse]

An iterator object is associated to a container so that it points to one element of that container or to an invalid pseudo element which signals the end of the iteration. The iterator provides a function 'next' - mostly via the ++ operator - which makes the iterator object pointing to the next element.

// the container
std::vector<int> cont;
cont.push_back(1);
cont.push_back(2);
// the iterator (pointing to nowhere and not yet associated to the container)
std::vector<int>::iterator iter;

for (iter = cont.begin();  // now iter is associated to the container and points to first element
      iter !=cont.end();     // and runs until the iter is end iterator of the container 
      ++iter)                     // points to next element after each cycle
{
     int i = *iter;    // get value of element from iterator by dereferencing (it is 1 or 2)
     std::cout << i << std::endl;
}

[aamir121a]

thank you , except for the obvious , as looping through the container , what are there practical uses.

[Lindley]

thank you , except for the obvious , as looping through the container , what are there practical uses.

Well, looping through a container seems to be a very practical use to me, especially for a container such as a std::list or std::map that can't be simply indexed like a vector can.

Due to their ability to make any kind of container "look the same", iterators are also used in almost every STL algorithm such as std::transform() as a generic way to specify data without caring where it's stored.

[aamir121a]

thanks again much appreciated

[JohnW@Wessex]

Custom iterators can also be useful when you want to iterate from something other than 'first to last'. Many standard containers have a reverse iterator, where incrementing will step backwards through the container. This means that only one algorithm need be written which assumes the availability of a ++ operator in the iterator. It's up to the iterator to define what ++ actually does.