C++ Iterator - CoolMind

Introduction
Common Iterator Wrapper
- Inserters
- IO related
Common functions

Introduction

Iterator is an essential part in STL. We can use iterator to:

traverse the elements of sequence or associative containers.
work with STL’s algorithm

There are five kinds of iterators:

Input Iterator
- Forward Iterator: only moves ahead
- Bidirectional Iterator: can move forward or backward
- Random Access Iterator: can randomly any valid position
Output Iterator
- All above three iterators can be an output iterator.

Common Iterator Wrapper

Inserters

front_inserter
back_inserter
inserter

In most STL’s algorithms, users are responsible for allocating space for the output. But most time we donot know the space we need in advance. For example, copy

vector<int> source{1,2,3};
vector<int> target;
copy(source.begin(), source.end(), target.begin());

The above program may crash. We can fix it by initializing the target with a size of source.size(). It is easy that we forget this kind of initialization. back_inserter comes to help here.

copy(source.begin(), source.end(), back_inserter(target));

back_inserter turns the moving the forward operation into a push_back operation.

vector<int> source{1,2,3};
deque<int> target;
set<int> s;

// the same with: `front_inserter` and `inserter`

copy(source.begin(), source.end(), front_inserter(target));
copy(source.begin(), source.end(), inserter(s, s.end()));

istream_iterator
- used to consume input stream
ostream_iterator
- used to print values

istream_iterator

vector<int> v;
istringstream s("1 2 3");

istream_iterator<int> ii(s);
copy(ii, istream_iterator<int>(), back_inserter(v));

It automatically coverts the input stream to the specified data type and put them into the container.

ostream_iterator

If we want to print all the elements in a container, how can we implement it? We can use a range for clause.

vector<int> v{1,2,3};
for (int vv: v) {
    cout << vv << ", "
}
cout << endl;

//^1, 2, 3, $

We can also use a ostream_iterator.

copy(v.begin(), v.end(), ostream_iterator<int>(cout, ", "));
cout << endl;

//^1, 2, 3, $

Common functions

advance(iter, distance): no iterator returned
distance(start_iter, end_iter): returns the distance
next(iter, distance=1): returns updated iterator
prev(iter, distance=1): returns updated iterator
begin(container): returns the starting iterator
end(container): returns the last iterator