Cursors, Vector - Object Oriented Programming - Lecture Slides, Slides of Object Oriented Programming

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Object-Oriented Programming

(OOP)

Lecture No. 40

Recap

• Generic algorithm requires three

operations (++, *, !=)

• Implementation of these operations in

Vector class

• Problems
  • No support for multiple traversals
  • Supports only a single traversal strategy
  • Inconsistent behavior
  • Operator !=

Vector

template< class T > class Vector { private: T* ptr; int size; public: Vector( int = 10 ); Vector( const Vector< T >& ); ~Vector(); int getSize() const;

…Vector

const Vector< T >& operator =( const Vector< T >& ); T& operator ; T* first(); T* beyond(); T* next( T* );

};

…Vector

template< class T >

T* Vector< T >::next( T* current )

{

if ( current < (ptr + size) ) return ( current + 1 ); // else return current;

}

Example – Cursor

int main() {

Vector< int > iv( 3 ); iv[0] = 10; iv[1] = 20; iv[2] = 30; int* first = iv.first(); int* beyond = iv.beyond(); int* found = find(first,beyond,20); return 0;

}

…Cursors

• This technique works fine for a contiguous

sequence such as Vector

• However it does now work with containers

that use complicated data structures

• There we have to rely on the container

traversal operations

Example – Works Fine

a b c d e f g …

Cursor

Example – Problem

int main() {

Set< int > is( 3 ); is.add( 10 ); is.add( 20 ); is.add( 30 ); ET* first = iv.first(); ET* beyond = iv.beyond(); ET* found = find(first, beyond, 20); return 0;

}

…Example – Problem

template< typename P, typename T > P find( P start, P beyond, const T& x ) { while ( start != beyond && *start != x ) ++start; // Error

return start; }

…Works Fine

int main() {

Set< int > is( 3 ); is.add( 10 ); is.add( 20 ); is.add( 30 ); int* found = find( is, 20 ); return 0;

}

Cursors – Conclusion

• Now we can have more than one traversal

pending on the aggregate object

a b c d e f g …

Cursor 1 Cursor 2

Iterators

• Iterator is an object that traverses a

container without exposing its internal

representation

• Iterators are for containers exactly like

pointers are for ordinary data structures

Generic Iterators

• A generic iterator works with any kind of

container

• To do so a generic iterator requires its

container to provide three operations

• T* first()
• T* beyond()
• T* next( T* )