Template non-type arguments in C++

Prerequisite: Templates in C++

Generally, a C++ template, with a single argument looks like this:

template<typename template_name>

But it has been seen that a template can have multiple arguments. The syntax for the same would be:

template<class T1, class T2, class T3, ………, class Tn>

where, n is the number of arguments.

It is also possible to use non-type arguments (basic/derived data types) i.e., in addition to the type argument T, it can also use other arguments such as strings, function names, constant expressions, and built-in data types.

Example 1:

template <class T, int size>
class Array {
private:

    // Automatic array initialization
    T Arr[size]
    .....
    .....
};

Explanation:

In the above example, the template supplies the size of the array as an argument. This implies that the size of the array is known to the compiler at the compile time itself. The arguments must be specified whenever a template class is created.

Example 2:

// Array of 10 integers
Array<int, 10> a1

// Array of 5 double type numbers
Array<double, 5> a2

// String of size 9 
Array<char, 10>  a3 

where size is given as an argument to the template class.

Following are the arguments that are allowed:

• Constant Expressions
• Addresses of function or objects with external linkage
• Addresses of static class members.

Below is the program to illustrate the Non-type templates:

C++

// C++ program to implement bubble sort
// by using Non-type as function parameters
#include <iostream>
using namespace std;

// Function to swap two numbers
template <class T>
void swap_(T* x, T* y)
{
    T temp = *x;
    *x = *y;
    *y = temp;
}

// Function to implement the Bubble Sort
template <class T, int size>
void bubble_sort(T arr[])
{
    for (int i = 0; i < size - 1; i++) {

        // Last i elements are already
        // in place
        for (int j = 0; j < size - i - 1; j++) {
            if (arr[j] > arr[j + 1]) {

                // Swap operation
                swap_(&arr[j], &arr[j + 1]);
            }
        }
    }
}

// Function to print an array
template <class T, int size>
void printArray(T arr[])
{
    int i;
    for (i = 0; i < size - 1; i++) {
        cout << arr[i] << ", ";
    }

    cout << arr[size - 1] << endl;
}

// Driver Code
int main()
{
    // Given array arr[]
    float arr[] = { 1.1, 1.2, 0.3, 4.55, 1.56, 0.6 };
    const int size_arr = sizeof(arr) / sizeof(arr[0]);

    // Size of the array passed as
    // an argument to the function
    bubble_sort<float, size_arr>(arr);

    cout << "Sorted Array is: ";
    printArray<float, size_arr>(arr);

    return 0;
}

Output:

Sorted Array is: 0.3, 0.6, 1.1, 1.2, 1.56, 4.55