C++ templates extend the function template concepts to classes too, and enable us to write object-oriented generic code. In the previous sections, you learned the use of function templates and overloading. In this section, you will learn writing template classes that open up more interesting generic programming concepts.

A class template lets you parameterize the data type on the class level via a template type expression.

Let's understand a class template with the following example:

//myalgorithm.h
#include <iostream>
#include <algorithm>
#include <array>
#include <iterator>
using namespace std;

template <typename T, int size>
class MyAlgorithm {

public:
        MyAlgorithm() { } 
        ~MyAlgorithm() { }

        
        void sort( array<T, size> &data ) {
             for ( int i=0; i<size; ++i ) {
                 for ( int j=0; j<size; ++j ) {
                     if ( data[i] < data[j] )
                         swap ( data[i], data[j] );
                 }
             }
        }

        void sort ( T data[size] );

};

template <typename T, int size>
inline void MyAlgorithm<T, size>::sort ( T data[size] ) {
       for ( int i=0; i<size; ++i ) {
           for ( int j=0; j<size; ++j ) {
               if ( data[i] < data[j] )
                  swap ( data[i], data[j] );
           }
       }
}

Let's use myalgorithm.h in the following main.cpp program as follows:

#include "myalgorithm.h"

int main() {

    MyAlgorithm<int, 10> algorithm1;
 
    array<int, 10> a = { 10, 5, 15, 20, 25, 18, 1, 100, 90, 18 };
 
    cout << "\nArray values before sorting ..." << endl;
    copy ( a.begin(), a.end(), ostream_iterator<int>(cout, "\t") );
    cout << endl;

    algorithm1.sort ( a );

    cout << "\nArray values after sorting ..." << endl;
    copy ( a.begin(), a.end(), ostream_iterator<int>(cout, "\t") );
    cout << endl;

    MyAlgorithm<int, 10> algorithm2;
    double d[] = { 100.0, 20.5, 200.5, 300.8, 186.78, 1.1 };

    cout << "\nArray values before sorting ..." << endl;
    copy ( d.begin(), d.end(), ostream_iterator<double>(cout, "\t") );
    cout << endl;

    algorithm2.sort ( d );

    cout << "\nArray values after sorting ..." << endl;
    copy ( d.begin(), d.end(), ostream_iterator<double>(cout, "\t") );
    cout << endl;

    return 0;  

}

Let's quickly compile the program using the following command:

g++ main.cpp -std=c++17

./a.out

The output is as follows:

Array values before sorting ...
10  5   15   20   25   18   1   100   90   18

Array values after sorting ...
1   5   10   15   18   18   20   25   90   100

Array values before sorting ...
100   20.5   200.5   300.8   186.78   1.1

Array values after sorting ...
1.1     20.5   100   186.78  200.5  300.8