front_insert_iterator Class

Describes an iterator adaptor that satisfies the requirements of an output iterator. It inserts, rather than overwrites, elements into the front of a sequence. Thus it provides semantics that are different from the overwrite semantics provided by the iterators of the C++ sequence containers. The front_insert_iterator class is templatized on the type of container.

Syntax

template <class Container>
class front_insert_iterator;

Parameters

Container
The type of container into the front of which elements are to be inserted by a front_insert_iterator.

Remarks

The container must satisfy the requirements for a front insertion sequence where is it possible to insert elements at the beginning of the sequence in amortized constant time. The C++ Standard Library sequence containers defined by the deque Class and list Class provide the needed push_front member function and satisfy these requirements. By contrast, sequence containers defined by the vector Class don't satisfy these requirements and can't be adapted to use with front_insert_iterators. A front_insert_iterator must always be initialized with its container.

Constructors

Constructor	Description
front_insert_iterator	Creates an iterator that can insert elements at the front of a specified container object.

Typedefs

Type name	Description
container_type	A type that represents the container into which a front insertion is to be made.
reference	A type that provides a reference to an element in a sequence controlled by the associated container.

Operators

Operator	Description
operator*	Dereferencing operator used to implement the output iterator expression * i = x for a front insertion.
operator++	Increments the front_insert_iterator to the next location into which a value may be stored.
operator=	Assignment operator used to implement the output iterator expression * i = x for a front insertion.

Requirements

Header: <iterator>

Namespace: std

front_insert_iterator::container_type

A type that represents the container into which a front insertion is to be made.

typedef Container container_type;

Remarks

The type is a synonym for the template parameter Container.

Example

// front_insert_iterator_container_type.cpp
// compile with: /EHsc
#include <iterator>
#include <list>
#include <iostream>

int main( )
{
   using namespace std;

   list<int> L1;
   front_insert_iterator<list<int> >::container_type L2 = L1;
   front_inserter ( L2 ) = 20;
   front_inserter ( L2 ) = 10;
   front_inserter ( L2 ) = 40;

   list <int>::iterator vIter;
   cout << "The list L2 is: ( ";
   for ( vIter = L2.begin ( ) ; vIter != L2.end ( ); vIter++)
      cout << *vIter << " ";
   cout << ")." << endl;
}
/* Output:
The list L2 is: ( 40 10 20 ).
*/

front_insert_iterator::front_insert_iterator

Creates an iterator that can insert elements at the front of a specified container object.

explicit front_insert_iterator(Container& _Cont);

Parameters

_Cont
The container object into which the front_insert_iterator is to insert elements.

Return Value

A front_insert_iterator for the parameter container object.

Example

// front_insert_iterator_front_insert_iterator.cpp
// compile with: /EHsc
#include <iterator>
#include <list>
#include <iostream>

int main( )
{
   using namespace std;
   int i;
   list <int>::iterator L_Iter;

   list<int> L;
   for (i = -1 ; i < 9 ; ++i )
   {
      L.push_back ( 2 * i );
   }

   cout << "The list L is:\n ( ";
   for ( L_Iter = L.begin( ) ; L_Iter != L.end( ); L_Iter++)
      cout << *L_Iter << " ";
   cout << ")." << endl;

   // Using the member function to insert an element
   front_inserter ( L ) = 20;

   // Alternatively, one may use the template function
   front_insert_iterator< list < int> > Iter(L);
*Iter = 30;

   cout << "After the front insertions, the list L is:\n ( ";
   for ( L_Iter = L.begin( ) ; L_Iter != L.end( ); L_Iter++)
      cout << *L_Iter << " ";
   cout << ")." << endl;
}
/* Output:
The list L is:
( -2 0 2 4 6 8 10 12 14 16 ).
After the front insertions, the list L is:
( 30 20 -2 0 2 4 6 8 10 12 14 16 ).
*/

front_insert_iterator::operator*

Dereferences the insert iterator returning the element it addresses.

front_insert_iterator<Container>& operator*();

Return Value

The member function returns the value of the element addressed.

Remarks

Used to implement the output iterator expression *Iter = value. If Iter is an iterator that addresses an element in a sequence, then *Iter = value replaces that element with value and doesn't change the total number of elements in the sequence.

Example

// front_insert_iterator_deref.cpp
// compile with: /EHsc
#include <iterator>
#include <list>
#include <iostream>

int main( )
{
   using namespace std;
   int i;
   list <int>::iterator L_Iter;

   list<int> L;
   for ( i = -1 ; i < 9 ; ++i )
   {
      L.push_back ( 2 * i );
   }

   cout << "The list L is:\n ( ";
   for ( L_Iter = L.begin( ) ; L_Iter != L.end( ); L_Iter++)
      cout << *L_Iter << " ";
   cout << ")." << endl;

   front_insert_iterator< list < int> > Iter(L);
*Iter = 20;

   // Alternatively, you may use
   front_inserter ( L ) = 30;

   cout << "After the front insertions, the list L is:\n ( ";
   for ( L_Iter = L.begin( ) ; L_Iter != L.end( ); L_Iter++)
      cout << *L_Iter << " ";
   cout << ")." << endl;
}
/* Output:
The list L is:
( -2 0 2 4 6 8 10 12 14 16 ).
After the front insertions, the list L is:
( 30 20 -2 0 2 4 6 8 10 12 14 16 ).
*/

front_insert_iterator::operator++

Increments the back_insert_iterator to the next location into which a value may be stored.

front_insert_iterator<Container>& operator++();

front_insert_iterator<Container> operator++(int);

Return Value

A front_insert_iterator addressing the next location into which a value may be stored.

Remarks

Both preincrementation and postincrementation operators return the same result.

Example

// front_insert_iterator_op_incre.cpp
// compile with: /EHsc
#include <iterator>
#include <list>
#include <iostream>

int main( )
{
   using namespace std;

   list<int> L1;
   front_insert_iterator<list<int> > iter ( L1 );
*iter = 10;
   iter++;
*iter = 20;
   iter++;
*iter = 30;
   iter++;

   list <int>::iterator vIter;
   cout << "The list L1 is: ( ";
   for ( vIter = L1.begin ( ) ; vIter != L1.end ( ); vIter++ )
      cout << *vIter << " ";
   cout << ")." << endl;
}
/* Output:
The list L1 is: ( 30 20 10 ).
*/

front_insert_iterator::operator=

Appends (pushes) a value onto the front of the container.

front_insert_iterator<Container>& operator=(typename Container::const_reference val);

front_insert_iterator<Container>& operator=(typename Container::value_type&& val);

Parameters

val
The value to be assigned to the container.

Return Value

A reference to the last element inserted at the front of the container.

Remarks

The first member operator evaluates container.push_front( val), then returns *this.

The second member operator evaluates

container->push_front((typename Container::value_type&&) val),

then returns *this.

Example

// front_insert_iterator_op_assign.cpp
// compile with: /EHsc
#include <iterator>
#include <list>
#include <iostream>

int main( )
{
   using namespace std;

   list<int> L1;
   front_insert_iterator<list<int> > iter ( L1 );
*iter = 10;
   iter++;
*iter = 20;
   iter++;
*iter = 30;
   iter++;

   list <int>::iterator vIter;
   cout << "The list L1 is: ( ";
   for ( vIter = L1.begin ( ) ; vIter != L1.end ( ); vIter++ )
      cout << *vIter << " ";
   cout << ")." << endl;
}
/* Output:
The list L1 is: ( 30 20 10 ).
*/

front_insert_iterator::reference

A type that provides a reference to an element in a sequence controlled by the associated container.

typedef typename Container::reference reference;

Example

// front_insert_iterator_reference.cpp
// compile with: /EHsc
#include <iterator>
#include <list>
#include <iostream>

int main( )
{
   using namespace std;

   list<int> L;
   front_insert_iterator<list<int> > fiivIter( L );
*fiivIter = 10;
*fiivIter = 20;
*fiivIter = 30;

   list<int>::iterator LIter;
   cout << "The list L is: ( ";
   for ( LIter = L.begin ( ) ; LIter != L.end ( ); LIter++)
      cout << *LIter << " ";
   cout << ")." << endl;

   front_insert_iterator<list<int> >::reference
        RefFirst = *(L.begin ( ));
   cout << "The first element in the list L is: "
        << RefFirst << "." << endl;
}
/* Output:
The list L is: ( 30 20 10 ).
The first element in the list L is: 30.
*/

See also

<iterator>
Thread Safety in the C++ Standard Library
C++ Standard Library Reference