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shelley-greer

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Lecture 5
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  1. Lecture 5 OOP Course

  2. 5. Templates

  3. Motivation • A function for finding minimum value • For floats we will need another function • Can we parameterize this function with a type? • Towards Generic Programming • Like objects are generalized into classes, we would like to generalize classes and functions intotemplates • int min(int a, int b){ • return (a<b ? a : b) • } • float min(float a, float b){ • return (a<b ? a : b) • }

  4. Motivation void swap(int& a, int& b){ int temp = a; a = b; b = temp; } void swap(short& a, short& b){ short temp = a; a = b; b = temp; } void swap(string& a, string& b){ string temp = a; a = b; b = temp; } void swap(Elephant& a, Elephant& b){ Elephant temp = a; a = b; b = temp; } void swap(Parrot& a, Parrot& b){ Parrot temp = a; a = b; b = temp; } void swap(char& a, char& b){ char temp = a; a = b; b = temp; } void swap(boolean& a, boolean& b){ boolean temp = a; a = b; b = temp; } void swap(Eagle& a, Eagle& b){ Eagle temp = a; a = b; b = temp; } void swap(Snake& a, Snake& b){ Snake temp = a; a = b; b = temp; } void swap(Cat& a, Cat& b){ Cat temp = a; a = b; b = temp; } void swap(double& a, double& b){ double temp = a; a = b; b = temp; } void swap(Crocodile& a, Crocodile& b){ Crocodile temp = a; a = b; b = temp; } void swap(float& a, float& b){ float temp = a; a = b; b = temp; }

  5. Templates • C++ provides a super macro mechanism that is identified by the keyword template • Templates can be thought of as MACROs for functions and classes, that need not be explicitly called by the programmer • Templates are controlled by the compiler (and not by the pre-processor) • Templates arguments are either class-types or const-expressions of a fixed type • Arguments that are passed to temple functions are checked for type-mismatch

  6. Function Template • Preserving the semantics of a function • Automatically generated instance of a function, varying by type • The programmer parameterizes all or a subset of the types in the interface • The function implementation remains invariant over a set of function instances, each handling a unique data type template <class Type> Type min(Type a, Type b) { return ( a>b ? a : b); }

  7. Function Temples Cont. Declaration • Template <calssT> • T min(T a, T b){ • return (a<b ? a : b) • } operator< should be defined for calss T Usage • int main() • { int a=3, b=4; • char c= ‘a’, d = ‘b’; • cout << min(a,b); // o.k • cout << min(c,d); // o.k • cout << min(a,c); // Error!! • return 0; Defining additional function int min(int, int) Will prevent the compilation error

  8. Function Temples Cont. • For each call to a function template, the compiler will try to generate an actual function with the appropriate prototype • Overloading for function of the same name is done in this order: • Look for an exact match among non-template functions • Look for an exact match that can be generated from function template • Look for the best match non-template functions, using • Trivial conversion • Promotions (e.g. short to int) • Standard conversions (e.g. int to double) • User-defined conversions (e.g. double to complex)

  9. Class Templates • Class templates are used to define generic classes • An actual class is created only when an actual object defined, using the class template with actual parameters templete<class E, int size> class Buffer; Buffer<char*, 1024> buf_chrp; Buffer<int, 1024> buf_int;

  10. Example: class Bag • add • getOne • isEmpty • isFull • currentSize • capacity

  11. Bags With and Without Templates

  12. Bags With and Without Templates

  13. Bags With and Without Templates

  14. Bags With and Without Templates • Differences between .template, .cpp files: • Occurrences of BagOfInt :: are replaced by Bag<Item> :: • Member functions, constructors, destructor are preceded by template<class Item> • Constructors are Bag; destructor is ~Bag • Assumption:Item is a class • See use of NULL in add, getOne

  15. Bags With and Without Templates

  16. Bags With and Without Templates

  17. Bags With and Without Templates

  18. Bags With and Without Templates

  19. Bags With and Without Templates

  20. Bags With and Without Templates

  21. Bags With and Without Templates

  22. Instantiating Bag Objects Bag<int> bag(40); // locally allocated bag for 40 ptrs to int Bag<int> *bagPtr = new Bag<int>(60); // bagPtr holds the address of a dynamically // allocated bag that stores up to 60 pointers to int Bag< Bag<int> > x(25); // locally allocated. bag x can hold 25 pointers to // bags, each of which can hold pointers to int Bag< Bag<int > > *y = new Bag< Bag<int > > (30); // y holds the address of a dynamically allocated bag // that stores up to 30 pointers to bags of pointers // to int

  23. Instantiating Bag Objects • When Bag<Bag<int>> *y = new Bag<Bag<int>>(30); is first encountered: • First, the class Bag<int> is instantiated • Then the class Bag<Bag<int>> is instantiated • Then the object y is instantiated • When Bag<int> and Bag<Bag<int>> are encountered subsequently, the classes already exist

  24. Example: Ordered list

  25. Operations on OrderedList Assuming we already have the class List<Item>, orderedList<Item> will use List<Item> to store the elements in a sorted order

  26. OrderedList.h #include “List.h” template <class Item> class OrderedList { public: OrderedList(unsigned int capacity = MAX_LIST); // constructor for an empty ordered list that // can hold up to capacity items; default // size is defined in List.h ~OrderedList( ); // destructor bool isEmpty( ) const; // true if list is empty, false otherwise

  27. OrderedList.h int getLength( ) const; // returns the current size of the list Item remove (unsignedint pos); // remove the value at location pos and return it // precondition: pos must be a legal list position Item retrieve (unsignedint pos) const; // return value at pos without modifying the list. // precondition: pos must be a legal list position // cont’d..

  28. OrderedList.h void insert (Item item); // insert item at appropriate pos’n in list int find (Item item) const; // return pos’n of first occurrence of // item, or -1 if item isn’t found private: List<Item> m_container; // to hold the list of Items }; // end of header file

  29. OrderedList.template #include <stdlib.h> template <class Item> OrderedList<Item> :: OrderedList (unsignedint capacity ) : m_container(capacity) { } template <class Item> OrderedList<Item> :: ~OrderedList ( ) { } // cont’d..

  30. OrderedList.template template <class Item> bool OrderedList<Item> :: isEmpty( ) const { return m_container.isEmpty( ); } template <class Item> int OrderedList<Item> :: getLength( ) const { return m_container.getLength( ); } // cont’d..

  31. OrderedList.template template <class Item> Item OrderedList<Item> :: remove (unsignedint pos) { return m_container.remove(pos); } template <class Item> Item OrderedList<Item> :: retrieve (unsignedint pos) const { return m_container.retrieve(pos); } // cont’d..

  32. OrderedList.template template <class Item> void OrderedList<Item> :: insert (Item item) { for ( int k = 1; k <= getLength( ); k++ ) if ( item < retrieve(k) ) break; m_container.insert( k, item ); }// cont’d..

  33. OrderedList.template template <class Item> int OrderedList<Item> :: find (Item item) const { for ( int k=1; k <= getLength( ); k++ ) if ( item == retrieve(k) ) return k; return –1; } // end of OrderedList implementation Next lecture: we will see a better way to implement OrederedList<Item>, using Inheritance