iterators in c++ stl

An iterator is an object that can navigate over elements of STL containers.(eg of containers:vectors) All iterator represents a certain position in a container. 

We can utilize iterators to traverse through the elements of container .They can be imagined as something like a pointer pointing to some location and we can get to content of the particular location utilizing them.

Basic Operations of iterators :-

*begin() :- This function is used to return the beginning position of the container.

*end() :- This function is used to return the after end position of the container.

// C++ code to demonstrate the working of
// iterator, begin() and end()

#include<iostream>
#include<iterator> // for iterators
#include<vector> // for vectors

using namespace std;

int main()
{
    vector<int> a = { 1, 2, 3, 4, 5 };
    
    // Declaring iterator to a vector
    vector<int>::iterator p;
    
    // Displaying vector elements using begin() and end()
    cout << "The vector elements are : ";
    for (p = a.begin(); p < a.end(); p++)
        cout << *p << " ";
    
    return 0;    
}

Output:

The vector elements are : 1 2 3 4 5 

advance() :- This function is used to increment the iterator position till the specified number mentioned in its arguments.

// C++ code to demonstrate the working of
// advance()

#include<iostream>
#include<iterator> // for iterators
#include<vector> // for vectors

using namespace std;

int main()
{
    vector<int> a = { 1, 2, 3, 4, 5 };
    vector<int>::iterator p = a.begin(); // Declaring iterator to a vector
    advance(p, 3);      // Using advance() to increment iterator position
                        // points to 4
    
    // Displaying iterator position
    cout << "The position of iterator after advancing is : ";
    cout << *p << " ";
    
    return 0;
    
}

Output:

The position of iterator after advancing is:4

*next() :- This function returns the new iterator that the iterator would point after advancing the positions mentioned in its arguments.

*prev() :- This function returns the new iterator that the iterator would point after decrementing the positions mentioned in its arguments.

// C++ code to demonstrate the working of
// next() and prev() 

#include<iostream>
#include<iterator> // for iterators
#include<vector> // for vectors

using namespace std;

int main()
{
    vector<int> a = { 1, 2, 3, 4, 5 };
    
    // Declaring iterators to a vector
    vector<int>::iterator p = a.begin();
    vector<int>::iterator ftr = a.end();
   
   
    // Using next() to return new iterator
    // points to 4
    auto it = next(p, 3);
    
    // Using prev() to return new iterator
    // points to 3
    auto it1 = prev(ftr, 3);
    
    // Displaying iterator position
    cout << "The position of new iterator using next() is : ";
    cout << *it << " ";
    cout << endl;
    
    // Displaying iterator position
    cout << "The position of new iterator using prev()  is : ";
    cout << *it1 << " ";
    cout << endl;
    
    return 0; 
}

Output:

The position of new iterator using next() is : 4 
The position of new iterator using prev()  is : 3 

*inserter() :- This function is used to insert the elements at any position in the container. It accepts 2 arguments, the container and iterator to position where the elements have to be inserted.

// C++ code to demonstrate the working of
// inserter()

#include<iostream>
#include<iterator> // for iterators
#include<vector> // for vectors

using namespace std;

int main()
{
    vector<int> ar = { 1, 2, 3, 4, 5 };
    vector<int> ar1 = {10, 20, 30}; 
    
    // Declaring iterator to a vector
    vector<int>::iterator ptr = ar.begin();
   
    // Using advance to set position
    advance(ptr, 3);
    
    // copying 1 vector elements in other using inserter()
    // inserts ar1 after 3rd position in ar
    copy(ar1.begin(), ar1.end(), inserter(ar,ptr));
    
    // Displaying new vector elements
    cout << "The new vector after inserting elements is : ";
    for (int &x : ar) 
        cout << x << " ";
    
    return 0;    
}

Output:

The new vector after inserting elements is : 1 2 3 10 20 30 4 5