Chapter 3 Templates

1. Chapter 3 Templates Bernard Chen Spring 2006

2. Objective In Chapter 3, we will discuss: • The concept of a template • Function templates • Class templates • vector and matrix classes • Fancy stuff

3. 3.1 What is a Template • A mechanism for writing routines that work for arbitrary types w/o knowing these types (type independent). • Most likely be seen in generic algorithm implementations, i.e. find max, sorting & searching.

4. 3.2 Function Templates • typedef: keyword for defining new type from an existing one.Ex: typedef double Object; Object a = 3.5; // double a = 3.5 Object b=3; // int b=3

5. 3.2 Function Templates • A Function template is a design or pattern for a function which allows processors to generate an actual function from this design. • A function template is not an actual function, instead it’s a design or a pattern, for what could become an actual function.

6. typedef double Object; void swap(Object & lhs, Object & rhs) { Object tmp = lhs; lhs = rhs; rhs = tmp; } // figure 3.1 Note: swap is part of the STL template <class Object> void swap(Object & lhs, Object & rhs) { Object tmp = lhs; lhs = rhs; rhs = tmp; }// figure 3.2 Function template example: Swap routine

7. Swap routine used in main function: int main(){ int x =5, y = 7; double a = 2, b = 4; swap (x,y); // swap(int,int) swap(x,y); //reuse previous instantiation swap(a,b); //swap(double, double) //swap(x, b); // illegal: no match return 0; } // figure 3.3

8. 3.3 A Sorting Function Template template <class Comparable> void insertionSort(vector<Comparable> &a) { for( int p = 1; p < a.size() ; p++) { comparable tmp = a[p]; int j; for ( j=p ; j>0 ; j-- ) { if (temp < a[j-1]) { a[j]=a[j-1]; a[j-1]=tmp;} } } }

9. 3.3 A Sorting Function Template template <class Comparable> void insertionSort(vector<Comparable> &a) { for(int p = 1; p < a.size(); p++) { Comparable tmp = a[p]; int j; for(j = p; j > 0 && tmp < a[j-1]; j--) a[j] = a [j –1]; a[j] = tmp; } }

10. Insertion Sort example • The given insertionSort routine work for double, int, float but not char*, (primitive string), because operator “=” and “<” are undefined.

11. 3.4 Class Templates • A class can be a template. • Example: vector is a class template • When possible, constant reference should be used instead of call by value because if object is a large object, making a copy could be inefficient. (or illegal if copy constructor is disable or not defined)

12. A class template example: template <class Object> class MemoryCell{ public: explicit MemoryCell(const Object & initVal = Object()) :storedValue(initVal){} const Object & read() const {return storedValue;} void write(const Object & x) {storedValue = x;} private: Object storedValue; }; // figure 3.8

13. Typical template interface template<class object> class ClassName{ public: //public members private: //private member };

14. Typical member implementation template <class object> ReturnType ClassName<object>::memberName(parameterList) /*const*/ { // member body }

15. template<class Object> class MemoryCell { public: explicit MemoryCell(const Object & initVal = Object()); const Object & read() const; const write(const Object & x); private: Object storedValue; };// figure 3.10 template<class Object> class MemoryCell { public: explicit MemoryCell(const Object & initVal = Object()); const Object & read() const; const write(const Object & x); private: Object storedValue; };// figure 3.10 Separating the interface and implementation of the template

16. Implementing the Vector Class Template • Provides: Indexing, resizing, copying, and index range checking capabilities • figure 3.14 vector.h • figure 3.15 vector.cpp

17. 3.5 Templates of Templates: A Matrix Class • Figure 3.16 A complete matrix class • Matrix class is represented as a vector of vectors. • operator [] • No need for destructors, copy operator and assignment operator, (already exist in vector) • Note: There must be at least a space between “>” “>” in the declaration; otherwise compilers will confuse “>>” with the shift operator . Ex: vector<vector<Object> > array; // white space needed

18. 3.6 Fancy Templates • Multiple template parameters template <class keyType, class valueType> Class Map{ . . . }; Ex: map<string,int> zipCodes; Note: class “map” is in STL

19. Summary • Discussion of template facilities • Template for generic algorithms