C++ Output Iterator

• ✅ Output Iterator is an iterator used to modify the value in the container.
• ✅ Dereferencing an output iterator allows us to alter the value of the container.
• ✅ It does not allow us to read the value from the container.
• ✅ It is a one-way and write-only iterator.
• ✅ It can be incremented, but cannot be decremented.
• ✅ Operators that can be used for an output iterator are increment operator(++), decrement operator(--) and assignment operator(=) .
• ✅ There are two main subclasses of an Output Iterator are: insert iterator ostream iterator
• ✅ insert iterator
• ✅ ostream iterator

Insert Iterator

• ✅ An insert iterator is an iterator used to insert the element in a specified position.
• ✅ An assignment operator on the insert_iterator inserts the new element at the current position.

Syntax

Example
Copy
template<class Container, class Iterator>

insert_iterator<container> inserter(Container &x,Iterator it);
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Parameters

x : It is the container on which the new element is to be inserted.

it : It is an iterator objeccpp tutorialing to the position which is to be modified.

Let's see a simple example:

#include <iostream>     // std::cout

#include <iterator>     // std::front_inserter

#include <vector>         // std::list

#include <algorithm>    // std::copy

using namespace std;

int main () {

  vector<int> v1,v2;

  for (int i=1; i<=5; i++)

  { 

  v1.push_back(i); 

  v2.push_back(i+2);

  }

 vector<int>::iterator it = v1.begin();

  advance (it,3);

 copy (v2.begin(),v2.end(),inserter(v1,it));

  cout<<"Elements of v1 are :";

  for (  it = v1.begin(); it!= v1.end(); ++it )

  cout << ' ' << *it;

  cout << '\n';

  return 0;

}

Output:

Elements of v1 are : 1 2 3 3 4 5 6 7 4 5

In the above example, insert_iterator is applied on the copy algorithm to insert the elements of the vector v2 into the vector v1 at a specified position pointed by it.

Ostream iterator

• ✅ An ostream iterators are the output iterators used to write to the output stream such as cout successively.
• ✅ An ostream iterator is created using a basic_ostream object.
• ✅ When an assigenment operator is used on the ostream iterator, it inserts a new element into the output stream.

Syntax

Example
Copy
template<class T, class charT=char, class traits=char_traits<charT>>

class ostream_iterator;
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Member functions of Ostream Iterator class

Ostream_iterator<T, charT, traits>& operator=(const T& value);

Ostream_iterator<T, charT, traits>& operator*();

Ostream_iterator<T, charT, traits>& operator++();

Ostream_iterator<T, charT, traits>& operator++(int);

Parameters

• ✅ T : It is the type of elements to be inserted into the container.
• ✅ charT : The type of elements that ostream can handle, for example, char ostream.
• ✅ traits : These are the character traits that the stream handles for the elements.

Let's see a simple example:

#include <iostream>

#include<iterator>

#include<vector>

#include<algorithm>

using namespace std;

int main()

{

   vector<int> v;

   for(int i=1;i<=5;i++)

   {

       v.push_back(i*10);

   }

 ostream_iterator<int> out(cout,",");

 copy(v.begin(),v.end(),out);

    return 0;

}

Output:

10,20,30,40,50

In the above example, out is an object of the ostream_iterator used to add the delimiter ',' between the vector elements.

Let's see another simple example of ostream iterator:

#include <iostream>

#include<iterator>

#include<vector>

#include<algorithm>

using namespace std;

int main()

{

   ostream_iterator<int> out(cout,",");

   *out = 5;

   out++;

   *out = 10;

   out++;

   *out = 15;

   return 0;

}

Output:

5,10,15,

Features Of Output Iterator

• ✅ Equality/Inequality Operator : Output iterators cannot be compared either by using equality or inequality operator. Suppose X and Y are the two iterators:
Example
Copy
X==Y;  invalid

X!=Y;   invalid
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• ✅ Dereferencing : An output iterator can be dereferenced as an lvalue.
• ✅ Incrementable : An output iterator can be incremented by using operator++ function.
Example
Copy
X++;

++X;
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Limitations Of Output Iterator

• ✅ Assigning but no accessing : We can assign an output iterator as an lvalue, but we cannot access them as an rvalue.

Suppose 'A' is an output iterator type and 'x' is a integer variable:

*A = x;                   // valid

  x = *A;                 // invalid

• ✅ It cannot be decremented : We can increment the output iterator by using operator++ function, but we cannot decrement the output iterator.

Suppose 'A' is an output iterator type:

A++;            // not valid

++A;            // not valid

• ✅ Multi-pass algorithm : An output iterator cannot be used as a multi-pass algorithm. Since an output iterator is unidirectional and can move only forward. Therefore, it cannot be used to move through the container multiple times
• ✅ Relational Operators : An output iterator cannot be compared by using any of the relational operators.

Suppose 'A' and 'B' are the two iterators:

A = =B;        // not valid

A = =B;        // not valid

• ✅ Arithmetic Operators : An output iterator cannot be used with the arithmetic operators. Therefore, we can say that the output iterator only moves forward in a sequential manner.

Suppose 'A' is an output iterator :

A + 2;               // invalid

A + 5;               // invalid