Chapter 3 Templates Saurav Karmakar Spring 2007

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

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, swapping, sorting & searching.

3.2 Function Templates typedef: keyword for defining new type from an existing one.Ex: typedef double Object; void swap( Object &lhs, Object &rhs){ Object temp = lhs; lhs = rhs; rhs = temp;}

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

Function template example: Swap routine 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 void swap(Object & lhs, Object & rhs) { Object tmp = lhs; lhs = rhs; rhs = tmp; }// figure 3.2

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

3.3 A Sorting Function Template template void insertionSort(vector &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; }

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

3.4 Class Templates A class can be a template. Example: vector is a class template C++ Class Templates are used where we have multiple copies of code for different data types with the same logic. If a set of functions or classes have the same functionality for different data types, they becomes good candidates for being written as Templates. 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)

A class template example: template 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

Typical template interface template class ClassName{ public: //public members private: //private member };

Typical member implementation template ReturnType ClassName ::memberName(p arameterList) /*const*/ { // member body }

template class MemoryCell { public: explicit MemoryCell(const Object & initVal = Object()); const Object & read() const; const write(const Object & x); private: Object storedValue; };// figure 3.10 Interface of the template

Summary Discussion of template facilities Template for generic algorithms