C++ Forward Iterators

Iterators :

Iterators serve as a link between algorithms and STL containers, allowing the data inside the container to be modified. They let you to iterate through the container, access and assign values, and apply various operators to them in order to get the required outcome.

Forward Iterators :

Both of the input and output iterators are served by forward iterators. As a result, these iterators are called input-output operators. The values may be accessed (input iterator’s functionality) as well as assigned (functionality of output iterators). In one-way iterators, as their name implies, only forward movement is allowed. Additionally, Bidirectional and Random access iterators are valid forward iterators. In C++, the Forward Iterator can be used to read and write to a container, hence it can be claimed that the forward iterator is essentially a mix of the Forward and Random Access iterators. Forward iterators are commonly used to read the contents of a container from the start to end.

The forward iterator uses just the increment operator (++) to traverse all of the items of a container. In C++, a forward iterator is sometimes referred to as a multi-pass iterator.

Operations on Forward Iterators :

The following is a list of operations that are commonly performed on forward iterators :

• The forward iterator is usually constructible by default.

Expression :

A i;

• In most cases, the forward iterator can be copied.

Expression :

A i(j);

• In most cases, the forward iterator can be copied.

Expression :

j = i;

• In general, an equality or inequality operator can be used to compare the forward iterator.

Expression :

x++;
++x

• In general, the forward iterator can be increased.

Expression :

x++;
++x

• In most cases, a forward iterator may be dereferenced as a lvalue.

Expression :

*x=t;

• In most cases, a forward iterator can be dereferenced to a rvalue.

Expression :

*x;

In the demonstration above, 'A' represents a forward iterator type, i and j represent forward iterator type objects, and t represents an object that has been directed by the iterator type object.

Features of the Forward Iterators :

The following are some of the characteristics of forward iterators:

Equality/Inequality operator :

The forward iterator may be compared simply by utilizing an equality or inequality operator.

Dereferencing :

The forward iterator may be dereferenced for both these values, such as a lvalue and a rvalue, and coders can allocate the value to the output iterator and give a value to it.

Incrementable :

The forward iterator can be incremented, but it can't be decremented.

Example of Forward Iterator in C++ :

#include <iostream>
#include <vector>
using namespace std;
int main()
{	// Declaring a vector vect1
	vector<int> vect1 = { 10, 20, 30, 40, 50 };


	// Declaring the iterator itr1
	vector<int>::iterator itr1;


	for (itr1 = vect1.begin(); itr1 != vect1.end(); ++itr1) {
		// Assigning the values to the locations that are pointed by the iterator
		*itr1 = 10;
	}


	for (itr1 = vect1.begin(); itr1 != vect1.end(); ++itr1) {
		// Accessing the values at the locations that are pointed by the iterator
		cout << (*itr1) << " ";
	}


	return 0;
}

Output :

10 10 10 10 10

Explanation :

In the above example, we declared a vector list called vect1 and an iterator called itr1. Then we assigned the values to the locations that were pointed by the iterator pointer. After that, we accessed the values at the locations that were pointed by the iterator pointer. As we can see, the iterator may be accessed as well as assigned a value, indicating that it is at least a forward iterator which can be any element in the hierarchy.

Drawbacks of Forward Iterator :

The following are the limits that are applicable to the Forward Iterator in general:

• Relational operator :
  Users can utilize an equality with the forward iterator in most cases, but the constraint is that the other iterators would not be applied to the forward iterator in C++.

• Arithmetic operator:
  The forward iterator does not support this sort of operator.

• Decrementable :
  Because the forward iterator only advances forward, it is not feasible to increment it.

• Random Access :
  The forward iterator can only iterate across the items in a container, therefore random access to an element is not feasible.

• Unidirectional :
  These iterators are unidirectional, as previously stated. They can only move ahead, that is, they can only be increased. You can't take anything away from them.

• Offset Dereference Operator ([]) :
  The forward iterators do not support the offset dereference operator ([]). As a result, the offset operator cannot be used to dereference a forward iterator.