Templates in C++ with Examples

A template is a simple yet very powerful tool in C++. The simple idea is to pass the data type as a parameter so that we don’t need to write the same code for different data types. For example, a software company may need to sort() for different data types. Rather than writing and maintaining multiple codes, we can write one sort() and pass the datatype as a parameter. 

C++ adds two new keywords to support templates: ‘template’ and ‘type name’. The second keyword can always be replaced by the keyword ‘class’.

How Do Templates Work?

Templates are expanded at compiler time. This is like macros. The difference is, that the compiler does type-checking before template expansion. The idea is simple, source code contains only function/class, but compiled code may contain multiple copies of the same function/class. 

Function Templates

We write a generic function that can be used for different data types. Examples of function templates are sort(), max(), min(), printArray(). 

To know more about the topic refer to Generics in C++.

Example:

C++

// C++ Program to demonstrate
// Use of template
#include <iostream>
using namespace std;

// One function works for all data types.  This would work
// even for user defined types if operator '>' is overloaded
template <typename T> T myMax(T x, T y)
{
    return (x > y) ? x : y;
}

int main()
{
    // Call myMax for int
    cout << myMax<int>(3, 7) << endl;
    // call myMax for double
    cout << myMax<double>(3.0, 7.0) << endl;
    // call myMax for char
    cout << myMax<char>('g', 'e') << endl;

    return 0;
}

Output

7
7
g

Example: Implementing Bubble Sort using templates in C++

C++

// C++ Program to implement
// Bubble sort
// using template function
#include <iostream>
using namespace std;

// A template function to implement bubble sort.
// We can use this for any data type that supports
// comparison operator < and swap works for it.
template <class T> void bubbleSort(T a[], int n)
{
    for (int i = 0; i < n - 1; i++)
        for (int j = n - 1; i < j; j--)
            if (a[j] < a[j - 1])
                swap(a[j], a[j - 1]);
}

// Driver Code
int main()
{
    int a[5] = { 10, 50, 30, 40, 20 };
    int n = sizeof(a) / sizeof(a[0]);

    // calls template function
    bubbleSort<int>(a, n);

    cout << " Sorted array : ";
    for (int i = 0; i < n; i++)
        cout << a[i] << " ";
    cout << endl;

    return 0;
}

Output

 Sorted array : 10 20 30 40 50 

Class Templates 

Class templates like function templates, class templates are useful when a class defines something that is independent of the data type. Can be useful for classes like LinkedList, BinaryTree, Stack, Queue, Array, etc. 

Example:

C++

// C++ Program to implement
// template Array class
#include <iostream>
using namespace std;

template <typename T> class Array {
private:
    T* ptr;
    int size;

public:
    Array(T arr[], int s);
    void print();
};

template <typename T> Array<T>::Array(T arr[], int s)
{
    ptr = new T[s];
    size = s;
    for (int i = 0; i < size; i++)
        ptr[i] = arr[i];
}

template <typename T> void Array<T>::print()
{
    for (int i = 0; i < size; i++)
        cout << " " << *(ptr + i);
    cout << endl;
}

int main()
{
    int arr[5] = { 1, 2, 3, 4, 5 };
    Array<int> a(arr, 5);
    a.print();
    return 0;
}

Output

 1 2 3 4 5

Can there be more than one argument for templates?

Yes, like normal parameters, we can pass more than one data type as arguments to templates. The following example demonstrates the same.

Example:

C++

// C++ Program to implement
// Use of template
#include <iostream>
using namespace std;

template <class T, class U> class A {
    T x;
    U y;

public:
    A() { cout << "Constructor Called" << endl; }
};

int main()
{
    A<char, char> a;
    A<int, double> b;
    return 0;
}

Output

Constructor Called
Constructor Called

Can we specify a default value for template arguments?

Yes, like normal parameters, we can specify default arguments to templates. The following example demonstrates the same. 

Example:

C++

// C++ Program to implement
// Use of template
#include <iostream>
using namespace std;

template <class T, class U = char> class A {
public:
    T x;
    U y;
    A() { cout << "Constructor Called" << endl; }
};

int main()
{
    // This will call A<char, char>
    A<char> a;

    return 0;
}

Output

Constructor Called

What is the difference between function overloading and templates?

Both function overloading and templates are examples of polymorphism features of OOP. Function overloading is used when multiple functions do quite similar (not identical) operations, templates are used when multiple functions do identical operations.

What happens when there is a static member in a template class/function?

Each instance of a template contains its own static variable. See Templates and Static variables for more details.

What is template specialization?

Template specialization allows us to have different codes for a particular data type. See Template Specialization for more details.

Can we pass non-type parameters to templates?

We can pass non-type arguments to templates. Non-type parameters are mainly used for specifying max or min values or any other constant value for a particular instance of a template. The important thing to note about non-type parameters is, that they must be const. The compiler must know the value of non-type parameters at compile time. Because the compiler needs to create functions/classes for a specified non-type value at compile time. In the below program, if we replace 10000 or 25 with a variable, we get a compiler error. 

Example:

C++

// C++ program to demonstrate
// working of non-type parameters
// to templates in C++
#include <iostream>
using namespace std;

template <class T, int max> int arrMin(T arr[], int n)
{
    int m = max;
    for (int i = 0; i < n; i++)
        if (arr[i] < m)
            m = arr[i];

    return m;
}

int main()
{
    int arr1[] = { 10, 20, 15, 12 };
    int n1 = sizeof(arr1) / sizeof(arr1[0]);

    char arr2[] = { 1, 2, 3 };
    int n2 = sizeof(arr2) / sizeof(arr2[0]);

    // Second template parameter
    // to arrMin must be a
    // constant
    cout << arrMin<int, 10000>(arr1, n1) << endl;
    cout << arrMin<char, 256>(arr2, n2);

    return 0;
}

Output

10
1

Here is an example of a C++ program to show different data types using a constructor and template. We will perform a few actions 

• passing character value by creating an object in the main() function.
• passing integer value by creating an object in the main() function.
• passing float value by creating an object in the main() function.

Example:

C++

// C++ program to show different data types using a
// constructor and template.
#include <iostream>
using namespace std;

// defining a class template
template <class T> class info {
public:
    // constructor of type template
    info(T A)
    {
        cout << "\n"
             << "A = " << A
             << " size of data in bytes:" << sizeof(A);
    }
    // end of info()
}; // end of class

// Main Function
int main()
{
    // clrscr();

    // passing character value by creating an objects
    info<char> p('x');

    // passing integer value by creating an object
    info<int> q(22);

    // passing float value by creating an object
    info<float> r(2.25);

    return 0;
}

Output

A = x size of data in bytes:1
A = 22 size of data in bytes:4
A = 2.25 size of data in bytes:4

For template metaprogramming, refer to the following article – Template Metaprogramming.

Take a Quiz on Templates. Java also supports these features. Java calls it generics.