how to call a function template that takes different class templates as parameters list

Question

I need to make a template function that takes 2 std::array s with different sizes, but I don't know how to call it from the main function:

// write a function to combine two sorted arrays and keep the resulting array sorted

#include<iostream>
#include<array>
using namespace std;

template <class T, size_t size>
template <class T, size_t size_1>
template <class T, size_t size_2>
array<T, size> combine(array<T, size_1> a, array<T, size_2> b)
{
    int i, j, k;
    i = j = k = 0;
    array<T, size> c;
    while (i < size_1 && k < size_2) {
        if (a[i] < b[k]) {
            c[j] = a[i];
            j++;
            i++;
        } else {
            c[j] = b[k];
            j++;
            k++;
        }
    }
    if (i < size_1) {
        for (int q = j; q < size_1; q++)
            c[j] = a[q];
    } else {
        for (int e = k; e < size_2; q++)
            c[j] = b[e];
    }
    return c;
}

int main()
{
    std::array<int, 5> a = { 2, 5, 15, 18, 40 };
    std::array<int, 6> b = { 1, 4, 8, 10, 12, 20 };
    std::array<int, 11> c;
    c = combine<int>(a, b);
    for (int i = 0; i < c.size(); i++) {
        cout << c[i];
    }

    return 0;
}

Answer 1

What you are trying to do is pass in two array s with different sizes, and return an array whose size is the sum of the two sizes.

You can declare your function this way:

template <typename T, std::size_t X, std::size_t Y>
std::array<T, X+Y> combine (std::array<T, X> a, std::array<T, Y> b)
{
    //...
}

This way, template function argument deduction will work. So, you can avoid using explicit template parameters to call the function.

    std::array<int, 5> a = { 2, 5, 15, 18, 40 };
    std::array<int, 6> b = { 1, 4, 8, 10, 12, 20 };
    auto c = combine(a, b);