Advanced Programming

Introduction to C++ Templates

Case-Study

• What we have: A function that retrieves the minimum float in an array of floats

• What we want: A function that retrieves the minimum element in a traversable list of arbitrary type according to arbitrary comparison criterion.

Case-Study (Cont.)

int findMin(float arr[], int n){

int min = 0;

for(int i = 1; i < n; i++){

if(arr[i] < arr[min]) {min = i;}}

return min;}

Supporting Arbitrary Typetemplate<class T>int findMin(T arr[], int n){

int min = 0;

for(int i = 1; i < n; i++){

if(arr[i] < arr[min]) {min = i;}}

return min;}

Supporting Arbitrary Criteriontemplate<class T, class Comparison>int findMin(T arr[], int n, Comparison c){

int min = 0;

for(int i = 1; i < n; i++){

if(c(arr[i], arr[min])) {min = i;}}

return min;}

Supporting Arbitrary Criterion (Cont.)

Comparison can be:- A function

- An object implementing () operator

Supporting Arbitrary Criterion (Cont.)

Example using a function:bool lessAlpha(const Student &s1, const Student

&s2) {return s1.mName < s2.mName;}Student array[10];int firstInList = findMin(array, 10, lessAlpha);

Supporting Arbitrary Criterion (Cont.)

Example using a function:template<class T>bool lessThan(const T &s1, const T &s2) {return

s1 < s2;}int[10];int minIdx = findMin(array, 10, lessThan<int>);

Supporting Arbitrary Condition (Cont.)

Example using an object:class CourseComp{

public: CourseComp(int courseKey) : mKey(courseKey) {}bool operator()(const Student &s1, const Student &s2) {return s1.getGrade(mKey) < s2.getGrade(mKey);}private: int mKey;

}

int array[10];Int lowestIn450 = findMin(array, 10, CourseComp(450));

Supporting Arbitrary ListLet’s first rewrite the function as follows

template<class T, class Comparison>T* findMin(T* begin, T* end, Comparison c){

T* min = begin;

for(T* it = begin; it != end; ++it){

if(c(*it, *min)) {min = it;}}

return min;}

How can it be invoked ???

Supporting Arbitrary List (Cont.)

int array[50];int* minPtr = findMin(array, array+50,

lessThan<int>);min = *minPtr;

Supporting Arbitrary List (Cont.)We can further rewrite the function as follows

template<class T, class Comparison>T findMin(T begin, T end, Comparison c){

T min = begin;

for(T it = begin; it != end; ++it){

if(c(*it, *min)) {min = it;}}

return min;}

And it should still work !

Supporting Arbitrary List (Cont.)

• Suppose we have a linked liststruct Node{

int mData;Node *mNext;

}Node *list;

How can we use findMin to find the minimum element in the linked list ??? (No code changes)

Supporting Arbitrary List (Cont.)struct ListIterator{

Listiterator(Node *node) : mCurrent(node) {}

ListIterator &operator++() {

mCurrent = mCurrent->mNext;return *this;

}

bool operator!=(const ListIterator &it) const {return mCurrent != it->mCurrent;}

const int & operator *() {return mCurrent->mData;}

Node *mCurrent;}

Supporting Arbitrary List (Cont.)

ListIterator b(list);ListIterator e(null);ListIterator minIt = findMin(b, e, lessThan<int>);Int min = *minIt;