c++ Templates

What is a template? Templates are type-generic versions of functions and/or classes Template functions and template classes can be used with any data type Typing is determined by the client Type enforcement is maintained for the type that is initiated by the client More than one type can be created for more than one object created from the same template class vector ivector; vector svector;

Template versus Typedef typedef int item; item maximal(item a, item b) { if (a > b) return a; else return b; }

Template versus Typedef template Item maximal(Item a, Item b) { if (a > b) return a; else return b; }

How To Make a Template Class: Class Definition Add template line before class definition in header file

Template versus Typedef class bag { public: typedef int value_type;... }

Template versus Typedef Template class bag { public: typedef int value_type;... }

How To Make a Template Class: Functions Use class name as usual within class definition in header file Outside of class definition, tack onto the name of your class. e.g. use bag instead of bag when referring to the bag class name. Use Item instead of other data type for objects of your class. Ask yourself is this really a string (or whatever) or is it really an Item? return values parameter lists

Examples return values Member function (ex. pp. 296) prototype (in class def): Item grab(); implementation (after class def): template Item bag ::grab() { code here} Non Member function prototype (after class def): bag operator +(parameter list here); implementation (after class def): template bag operator+(parameter list here) { code here}

Examples parameter lists Member function prototype (in class def) void insert(const Item& entry); implementation (after class def, bag needed) void bag ::insert(const Item& entry) { code here} Non Member function prototype (after class def) bag operator +(const bag & b1, const bag & b2); implementation (after class def, no bag needed) bag operator +(const bag & b1, const bag & b2); { code here}

How To Make a Template Class: Implementation Make the implementation visible Typing is determined by the client compilation is performed when class becomes an object No using namespace std in implementation std:: required instead for all std namespace references Separate.cpp implementation file is not allowed. either put the entire implementation in header after class definition or remove implementation from project and add the #include class.cpp line after the class definition in the header file

How To Make a Template Class: Client #include.h file using namespace std is okay in client Declare an object of your template class with the. bag letters; bag scores; bag verbs: //include string class

Templates are Type Safe #include void TestVector() { vector nums; nums.add(7); vector words; words.add(“apples”); nums.add(“banana”); //compile error char c = words.getAt(0); //compile error vector s = nums; //compile error }

typedef Rewind typedef is an alias to a data type that can be defined in a class Can be used to aid in maintenance (easy to change data type throughout class Can be used to make another name for data types like std::size_t typedef std::size_t size_type Can also be used to identify items in a bag (was that a regular int or a bag item int?) typedef int value_type Even in a templated bag typedef Item value_type

Typedef Considerations with Template Implementations Example from text pp. 296 Before template bag::size_type bag::count(const Item& target) const After template bag ::size_type bag ::count(const Item& target) const Doesn't work typename bag ::size_type bag ::count(const Item& target) const compiler has no idea that bag ::size_type is a data type. It needs a new keyword typename

The Standard Template Library is a library of generic container classes which are both efficient and functional Templated- can handle many types In namespace std Google for “C++ STL” for documentation C++ STL developed in early 90's at Hewlett Packard Laboratories Alex Stepanov and Meng Lee became part of C++ standard in 1994 implementations available by late 90's

STL STL is part of a movement toward libraries of reusable code function libraries (reusable algorithms) class libraries (reusable ADTs) STL separates data and algorithms

STL Components Containers  templates for classes which hold a collection of elements Algorithms  templates for functions which operate on a range of elements from a container  range is specified by iterators Iterators  give access to the elements in a container  allow for movement from one element to another

STL Container Classes Sequences - elements arranged in a linear order and accessed by relative or absolute position vector - dynamic array. fast inserts only at the end; random access list - linked list. fast inserts anywhere; no random access deque - doubly ended queue. Fast inserts at the beginning and the end. Associative Containers - elements are key values for access by content not position Associative containers organize their data using keys which allow elements stored in the container to be accessed randomly Examples: set- a collection of unique items multi-set- a collection of non-unique items map – a two dimensional array (table)

STL Algorithms are function templates designed to operate on a sequence of elements rather than methods the sequence is designated by two iterators most container classes have the following two methods begin( ) - returns an iterator positioned at the container's first element end( ) - returns an iterator positioned past the container's last element (past-the-end)  some examples of STL algorithms find (iter1, iter2, value) //returns an iterator max_element (iter1, iter2) //returns an iterator sort (iter1, iter2) //sorts for data type

STL Iterators The following iterator operations are supported for all of the library containers *iter - Returns a reference to the element referenced by iterator iter. iter++ - Increment iter (move to the next element). iter-- - Decrement iter (move to the previous element). iter1 == iter2 - Compare two iterators for equality or inequality. Two iterators are equal if they both reference the same element in the same container or if they are positioned "one past the end" of the container. An iterator positioned "one past the end" is sometimes called the off-the-end iterator

Iterator Example #include using namespace std; // This program demonstrates the use of an iterator int main() { cout << "Iterator example" << endl; vector vals; vector :: iterator p; for(int i = 0; i < 10; ++i) vals.push_back(i*2+1); // output using an iterator for(p = vals.begin( ); p < vals.end( ); ++p) cout << *p << endl; }

list class another STL container class used for storing a linear collection of like items comparison to a vector? linked list vs array is the underlying data structure no indexing (iterators are bidirectional) inserts and deletes anywhere are done in a constant amount of time

a list data structure

Basic list class methods list( ); // construct an empty list list (const list & aList); // copy constructor ~list( ); // destructor list operator= (const list & aList); // assignment operator bool empty( ); int size( );

More List Methods L.push_back(value) // append value to L L.push_front(value) // insert value at front of L L.insert(pos, value) // insert value into L at // position indicated by iterator pos L.front( ) // return L's first element L.back( ) // return L's last element L.begin( ) // return an iterator positioned at start L.end( ) // return the"past the end" iterator L.sort( ) // sort L's elements

#include using namespace std; int main ( ) { list L; L.push_back (9); L.push_back (7); L.push_back (5); L.push_back (3); list ::iterator p; for (p = L.begin ( ); p != L.end ( ); p++) cout << *p << endl; for (p = L.begin ( ); p != L.end ( ); p++) (*p)++; for (p = L.begin ( ); p != L.end ( ); p++) cout << *p << endl; return 0; }

Another List Example int i; list int_list; list ::iterator iter; for (i=1; i<=10; i++) int_list.push_back(i); for (iter = int_list.begin(); iter != int_list.end(); iter++) cout << *iter << “ “ ;

Iterators in Loops General pattern for (i = c.begin(); i != c.end(); i++) { statements to access item}

STL Set and Multiset #include <set data stored in sorted order set holds sorted unique values as keys multiset holds sorted non-unique values functions include insert erase swap clear size