19.2 Template non-type parameters

Non-type parameters

#include <iostream>

template <typename T, int size> // size is an integral non-type parameter
class StaticArray
{
private:
    // The non-type parameter controls the size of the array
    T m_array[size] {};

public:
    T* getArray();

    T& operator[](int index)
    {
        return m_array[index];
    }
};

// Showing how a function for a class with a non-type parameter is defined outside of the class
template <typename T, int size>
T* StaticArray<T, size>::getArray()
{
    return m_array;
}

int main()
{
    // declare an integer array with room for 12 integers
    StaticArray<int, 12> intArray;

    // Fill it up in order, then print it backwards
    for (int count { 0 }; count < 12; ++count)
        intArray[count] = count;

    for (int count { 11 }; count >= 0; --count)
        std::cout << intArray[count] << ' ';
    std::cout << '\n';

    // declare a double buffer with room for 4 doubles
    StaticArray<double, 4> doubleArray;

    for (int count { 0 }; count < 4; ++count)
        doubleArray[count] = 4.4 + 0.1 * count;

    for (int count { 0 }; count < 4; ++count)
        std::cout << doubleArray[count] << ' ';

    return 0;
}

template <int size>
class Foo
{
};

int main()
{
    int x{ 4 }; // x is non-constexpr
    Foo<x> f; // error: the template non-type argument must be constexpr

    return 0;
}