front_insert_iterator

Overview

The STL algorithms usually overwrite elements of the range passed to them as parameters. Instead, sometimes it's useful to prepend new elements to the range. front_insert_iterator designed exactly to do it.

Details

A front_insert_iterator is a special output iterator which can be  generated  for containers that support push_front()  method as in the standard containers  deque and list.

Syntax

The syntax is as below. Template parameter Container represents the container that is updated. 

template < class Container > 
class  front_insert_iterator;

Members

 It defines following types.

member type	definition
conainer_type	Container
iterator_category	output_iterator_tag
value_type	void
difference_type	void
pointer	void
reference	void

Operation

An front_insert_iterator supports three operators *, ++ and =. 

The assignment operator, used both during dereferencing or directly, causes the container to expand by one element. For example, both *i = x and i=x adds a new element.

Dereferencing operator does nothing.

Both pre and post Increments the front_insert_iterator does nothing.

This can be clearly seen in the example below. 

list<int> v{3,4};
front_insert_iterator<list<int> > frontiter ( v );

//v:{2,3,4}
*frontiter = 2;

//v:{1,2,3,4}
frontiter = 1;

Functionality

Constructors

Name	Description
front_insert_iterator(Container& c)	Initializes the underlying pointer to the container to addressof(c). Example: list<int> v{1, 2, 3, 4, 5}; front_insert_iterator<list<int>> frontiter (v);

Overloaded operators

Name	Example
reference operator*()	No Operation.
front_insert_iterator& operator++() front_insert_iterator operator++(int)	No Operation.
pointer operator=()	Prepends new element. Example: list<int> v{3,4}; front_insert_iterator<list<int> > frontiter ( v ); //v:{2,3,4} *frontiter = 2; //v:{1,2,3,4} frontiter = 1;

front_inserter

An utility function to create a front_insert_iterator from a container that supports push_front() method such as  deque, list.

Syntax

The syntax is as below. Template parameter Container represents the container for which 

front_inserter_iterator is created. Container represents must support push_front() function as in  deque and list.

template < class Container > 
front_insert_iterator<Container >  front_inserter( Container& c );

The following example shows its usage.

list<int> v{1,2,3,4}, v2;
//same as 
//for_each(rbegin(v),rend(v),[&v2](int i){push_front(i);});
//v2:{1,2,3,4}
copy (v.rbegin(),v.rend(),front_inserter(v2));