Understanding C++ variable number of input parameters to test_function(Test<T>...)

Question

I have a generic class Test<T> and I want to have a function test_function() that has a variable number of Test<T> object input parameters with .... In the function, I want to iterate over all the parameters. The generic type T can be different across the parameters. Something like this:

template <typename T> class Test {
private:
    T value = (T)0;
    int test = 1;
public:
    Test() = default;
    int get_test() {
        return test;
    }
}

template <typename T> void test_function(const Test<T> tests...) {
    for(auto test : tests) {
        cout << test.get_test() << endl;
    }
}

When compiling, I get the errors:

error C3520: "T": Parameter pack must be extended in this context
error C3520: "tests": Parameter pack must be extended in this context
error C3312: no callable 'begin' function found for type 'unknown-type'
error C3312: no callable 'end' function found for type 'unknown-type'

What am I doing wrong?

EDIT: Is it possible to have a counter in the expansion?

EDIT 2: I figured it out with the counter:

template <typename ...T> void test_function(const Test<T> ...tests) {
    int i=0;
    ((cout << tests.get_test() << ", counter = " << i++ << endl), ...);
}

Answer 1

There are multiple problems here.

First, the correct variadic template function declaration should be:

template <typename ...T> void test_function(const Test<T> ...tests)

But that won't solve all the problems. The first one is that all the parameters are const objects, therefore the class method must also be a const class member:

int get_test() const {

Finally:

for(auto test : tests) {

tests is not a container that can be range-iterated over, like a vector. It is a parameter pack and needs to be expanded like one:

((cout << tests.get_test()), ...);

Tested with gcc 11:

#include <iostream>

using namespace std;

template <typename T> class Test {
private:
    T value = (T)0;
    int test = 1;
public:
    Test() = default;
    int get_test() const {
        return test;
    }
};

template <typename ...T> void test_function(const Test<T> ...tests) {

    ((cout << tests.get_test()), ...);
}

void foo()
{
    test_function(Test<int>{}, Test<char>{});
}