C++ Templates Gordon College CPS212

Overview Templates Definition: a pattern for creating classes or functions as instances of the template at compile time Easily create a large range of related functions or classes Terms: Function template - the blueprint of the related functions Template instance - a specific function made from a function template (instantiating the template) Type of programming: Generic programming Template metaprogramming

Class Templates Class templates Allow type-specific versions of generic classes Format: template class ClassName{ definition } Need not use "T", any identifier will work To create an object of the class, type ClassName myObject; Use Example: Listor doubleListor;

Class Templates Template class functions Declared normally, but preceded by template Generic data in class listed as type T Binary scope resolution operator used :: Template class function definition: template MyClass ::MyClass(int size) { myArray = new T[size]; } Constructor definition - creates an array of type T

Class Template Example (see template_class_demo) template class store { public : store ( const T & item = T ()); T getValue () const ; void setValue ( const T & item); friend ostream& operator & obj) { ostr << "Value = " << obj.value; return ostr; } private : T value; }; Class Declaration

Class Template Example (see template_class_demo) // use an initialization list to assign value template store ::store ( const T & item): value(item) {} // return the current value template T store ::getValue () const { return value; } // assign the argument as the new data member value template void store ::setValue ( const T & item) { value = item; } Class Implementation

Class Template Example (see template_class_demo) int main () { // declare three different types of store objects store intStore(5); store realStore(2.718); store strStore("Template"); cout << "The values stored by the objects are:" << endl ; cout << intStore << endl ; cout << realStore << endl ; cout << strStore << endl ; cout << endl ; cout << "The concatenation:" << endl ; // access current value strStore and concatenate " Class" // update the value in strStore with the new string strStore.setValue( strStore.getValue() + " Class" ); cout << strStore << endl ; return 0; } Class Usage The values stored by the objects are: Value = 5 Value = Value = Template The concatenation: Value = Template Class Output

Class Templates and Non-type Parameters Can use non-type parameters (T) in templates Can have default argument Treated as const Example: template Listor mostRecentSalesFigures; Declares object of type: Listor This may appear in the class definition: T stackHolder[ elements ]; //array to hold stack Creates array at compile time, rather than dynamic allocation at execution time

Function Templates Defines a pattern for any number of functions whose definitions depend on the template parameters. Can overload a function template with a non-template function or with other function templates. Used throughout the STL sort, copy, for_each, etc.

Function Template Example template void selectionSort ( T arr[], int n) { int smallIndex; // index of smallest element in the sublist int pass, j; T temp; // pass has the range 0 to n-2 for (pass = 0; pass < n-1; pass++). Function Declaration

Function Template Example int itest[20]={4,7,8,9,2,12,3,2,100,34,54,67,54,4,5,87,89,4,5,23}; string stest[10]={"this", "is", "a", "test", "I", "hope", "it", "works", "It", "worked"}; for ( int i = 0;i<20;i++) cout<<" "<<itest[i]; cout<<endl; for ( int i = 0;i<10;i++) cout<<" "<<stest[i]; cout<<endl; cout << "SORTED" <<endl; selectionSort(itest, 20); for ( int i = 0;i<20;i++) cout<<" "<<itest[i]; cout<<endl; selectionSort(stest, 10); for ( int i = 0;i<10;i++) cout<<" "<<stest[i]; cout<<endl; Function Usage this is a test I hope it works It worked SORTED I It a hope is it test this worked works output

Operator Templates Create operator template in the same manner as function templates Rules for deducing argument types apply Example: Template Bigint operator+(const Bingint& x, const T& y); Bigint a; a = a + 42;//argument type deduction a = operator+ (a,42);//explicit argument type

Operator Template Example template class Array { private: T* pType; int itsSize; public: Array(int itsSize = DefaultSize); Array(const Array &rhs); ~Array(){delete [] pType;} Array& operator=(const Array&); T& operator[](int offset){ return pType[offset];} const T& operator[](int offset) const { return pType[offset];} int GetSize() const {return itsSize;} }; Operator Declaration

Operator Template Example Array theArray; Array theZoo; Animal *pAnimal; for(int i=0; i<theArray.GetSize(); i++) { theArray[i] = i*2; pAnimal = new Animal(i*3); theZoo[i] = *pAnimal; delete pAnimal; } for(int j=0; j<theArray.GetSize(); j++) { cout << "theArray[" << j << "]: "; cout << theArray[j] << ”\t\t"; cout << "theZoo[" << j << "]: "; cout << theZoo[j] << endl; } Operator usage: see template_operator_zoo