C++ Class Templates

In this tutorial, we will learn about class templates in C++ with the help of examples.

Templates are powerful features of C++ which allows us to write generic programs. There are two ways we can implement templates:

Similar to function templates, we can use class templates to create a single class to work with different data types.

Class templates come in handy as they can make our code shorter and more manageable.


Class Template Declaration

A class template starts with the keyword template followed by template parameter(s) inside <> which is followed by the class declaration.

template <class T>
class className {
  private:
    T var;
    ... .. ...
  public:
    T functionName(T arg);
    ... .. ...
};

In the above declaration, T is the template argument which is a placeholder for the data type used, and class is a keyword.

Inside the class body, a member variable var and a member function functionName() are both of type T.


Creating a Class Template Object

Once we've declared and defined a class template, we can create its objects in other classes or functions (such as the main() function) with the following syntax

className<dataType> classObject;

For example,

className<int> classObject;
className<float> classObject;
className<string> classObject;

Example 1: C++ Class Templates

// C++ program to demonstrate the use of class templates

#include <iostream>
using namespace std;

// Class template
template <class T>
class Number {
   private:
    // Variable of type T
    T num;

   public:
    Number(T n) : num(n) {}   // constructor

    T getNum() {
        return num;
    }
};

int main() {

    // create object with int type
    Number<int> numberInt(7);

    // create object with double type
    Number<double> numberDouble(7.7);

    cout << "int Number = " << numberInt.getNum() << endl;
    cout << "double Number = " << numberDouble.getNum() << endl;

    return 0;
}

Output

int Number = 7
double Number = 7.7

In this program. we have created a class template Number with the code

template <class T>
class Number {
   private:
    T num;

   public:
    Number(T n) : num(n) {}
    T getNum() { return num; }
};

Notice that the variable num, the constructor argument n, and the function getNum() are of type T, or have a return type T. That means that they can be of any type.

In main(), we have implemented the class template by creating its objects

Number<int> numberInt(7);
Number<double> numberDouble(7.7);

Notice the codes Number<int> and Number<double> in the code above.

This creates a class definition each for int and float, which are then used accordingly.

It is compulsory to specify the type when declaring objects of class templates. Otherwise, the compiler will produce an error.

//Error
Number numberInt(7);
Number numberDouble(7.7);

Defining a Class Member Outside the Class Template

Suppose we need to define a function outside of the class template. We can do this with the following code:

template <class T>
class ClassName {
    ... .. ...
    // Function prototype
    returnType functionName();
};

// Function definition
template <class T>
returnType ClassName<T>::functionName() {
    // code
}

Notice that the code template <class T> is repeated while defining the function outside of the class. This is necessary and is part of the syntax.

If we look at the code in Example 1, we have a function getNum() that is defined inside the class template Number.

We can define getNum() outside of Number with the following code:

template <class T>
class Number {
    ... .. ...
    // Function prototype
    T getnum();
};

// Function definition
template <class T>
T Number<T>::getNum() {
    return num;
}

Example 2: Simple Calculator Using Class Templates

This program uses a class template to perform addition, subtraction, multiplication and division of two variables num1 and num2.

The variables can be of any type, though we have only used int and float types in this example.

#include <iostream>
using namespace std;

template <class T>
class Calculator {
   private:
    T num1, num2;

   public:
    Calculator(T n1, T n2) {
        num1 = n1;
        num2 = n2;
    }

    void displayResult() {
        cout << "Numbers: " << num1 << " and " << num2 << "." << endl;
        cout << num1 << " + " << num2 << " = " << add() << endl;
        cout << num1 << " - " << num2 << " = " << subtract() << endl;
        cout << num1 << " * " << num2 << " = " << multiply() << endl;
        cout << num1 << " / " << num2 << " = " << divide() << endl;
    }

    T add() { return num1 + num2; }
    T subtract() { return num1 - num2; }
    T multiply() { return num1 * num2; }
    T divide() { return num1 / num2; }
};

int main() {
    Calculator<int> intCalc(2, 1);
    Calculator<float> floatCalc(2.4, 1.2);

    cout << "Int results:" << endl;
    intCalc.displayResult();

    cout << endl
         << "Float results:" << endl;
    floatCalc.displayResult();

    return 0;
}

Output

Int results:
Numbers: 2 and 1.
2 + 1 = 3
2 - 1 = 1
2 * 1 = 2
2 / 1 = 2

Float results:
Numbers: 2.4 and 1.2.
2.4 + 1.2 = 3.6
2.4 - 1.2 = 1.2
2.4 * 1.2 = 2.88
2.4 / 1.2 = 2

In the above program, we have declared a class template Calculator.

The class contains two private members of type T: num1 & num2, and a constructor to initialize the members.

We also have add(), subtract(), multiply(), and divide() functions that have the return type T. We also have a void function displayResult() that prints out the results of the other functions.

In main(), we have created two objects of Calculator: one for int data type and another for float data type.

Calculator<int> intCalc(2, 1);
Calculator<float> floatCalc(2.4, 1.2);

This prompts the compiler to create two class definitions for the respective data types during compilation.


C++ Class Templates With Multiple Parameters

In C++, we can use multiple template parameters and even use default arguments for those parameters. For example,

template <class T, class U, class V = int>
class ClassName {
  private:
    T member1;
    U member2;
    V member3;
    ... .. ...
  public:
    ... .. ...
};

Example 3: C++ Templates With Multiple Parameters

#include <iostream>
using namespace std;

// Class template with multiple and default parameters
template <class T, class U, class V = char>
class ClassTemplate {
   private:
    T var1;
    U var2;
    V var3;

   public:
    ClassTemplate(T v1, U v2, V v3) : var1(v1), var2(v2), var3(v3) {}  // constructor

    void printVar() {
        cout << "var1 = " << var1 << endl;
        cout << "var2 = " << var2 << endl;
        cout << "var3 = " << var3 << endl;
    }
};

int main() {
    // create object with int, double and char types
    ClassTemplate<int, double> obj1(7, 7.7, 'c');
    cout << "obj1 values: " << endl;
    obj1.printVar();

    // create object with int, double and bool types
    ClassTemplate<double, char, bool> obj2(8.8, 'a', false);
    cout << "\nobj2 values: " << endl;
    obj2.printVar();

    return 0;
}

Output

obj1 values: 
var1 = 7
var2 = 7.7
var3 = c

obj2 values: 
var1 = 8.8
var2 = a
var3 = 0

In this program, we have created a class template, named ClassTemplate, with three parameters, with one of them being a default parameter.

template <class T, class U, class V = char>
class ClassTemplate {
  // code  
};

Notice the code class V = char. This means that V is a default parameter whose default type is char.

Inside ClassTemplate, we declare 3 variables var1, var2 and var3, each corresponding to one of the template parameters.

class ClassTemplate {
   private:
    T var1;
    U var2;
    V var3;
    ... .. ...
    ... .. ...
};

In main(), we create two objects of ClassTemplate with the code

// create object with int, double and char types
ClassTemplate<int, double> obj1(7, 7.7, 'c');

// create object with double, char and bool types
ClassTemplate<double, char, bool> obj2(8, 8.8, false);

Here,

Object T U V
obj1 int double char
obj2 double char bool

For obj1, T = int, U = double and V = char.

For obj2, T = double, U = char and V = bool.

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