Generic Class

◀ Friend Class and Friend Function▶ Traps and Tips
Amazon The main purpose of a generic class is to provide genericity so that the same member function does not need to be rewritten to accept a different type of argument. Different programming languages support genericity in different ways. In C++, a class template is used to provide genericity for a class. The word “template” in C++ in fact is linked to genericity.

The Standard Template Library we will be covering in Chapter 7 is a case in point. Most of you should be well familiar with a template class; if you don’t, consult the Internet or a book for its syntax.


The following is a sample program using a template to represent a two-dimensional array of various types. In main() I create one array of int and one array of char and manipulate them the same way to show you how the template class matrix is written to provide genericity.
#include <iostream>
using namespace std;

/* you can replace <class Type> with <typename Type> */
template <class Type>
class matrix{
private:
Type **array; int rows; int cols; public: matrix(int r, int c); matrix(matrix & m); /* copy constructor */ Type* & operator[](int row) { return array[row]; } int getRows() const { return rows; } int getCols() const { return cols; } void showArray(); }; template <class Type> matrix<Type>::matrix(int r, int c) { rows = r; cols = c; array = new Type*[r]; for(int i=0; i<r; i++)
array[i] = new Type[c]; } template <class Type> matrix<Type>::matrix(matrix & m) { int i, j; delete [] array; rows = m.getRows(); cols = m.getCols(); array = new Type*[rows]; for(i=0; i<rows; i++) array[i] = new Type[cols]; for(i=0; i<rows; i++) for(j=0; j<cols; j++) array[i][j] = m[i][j]; } template <class Type> void matrix<Type>::showArray() {
int i, j; for(i=0; i<rows; i++) { for(j=0; j<cols; j++) cout << array[i][j] << '\t'; cout << endl; } } int main(){ int i, j; matrix<int> b(10,5); /* 10-by-5 array */ matrix<char> d(10,5); for(i=0; i<10; i++) for(j=0; j<5; j++) b[i][j] = i+j; /* assigning values to every single cell */ cout << "Here are the contents of b:\n";
b.showArray(); /* display the array */ matrix<int> a(b); /* copy b to a */ cout << "\nHere are the contents of a:\n"; a.showArray(); for(i=0; i<10; i++) for(j=0; j<5; j++) d[i][j] = 'a'+i+j; /* assigning values to every single cell */ cout << "Here are the contents of d:\n"; d.showArray(); /* display the array */ matrix<char> c(d); /* copy d to c */ cout << "\nHere are the contents of c:\n"; c.showArray();
return 0; }
Go ahead and add additional functions to this class template so that it can handle row and column additions and deletions.

Pay attention to the syntax of a class template and discern the differences between a class template and a normal class.
The primary use of a class template is, obviously, to accommodate a number of different data types. In most situations, however, a class is designed specifically to work with certain data types. Therefore, don’t get intimidated by a class template; it is not as important as you may think.
◀ Friend Class and Friend Function▶ Traps and Tips

fShare
Questions? Let me know!