How to declare a c++ function that returns std::list or takes std::list as a parameter type?

Question

I've been trying to make a function in c++ that takes a list, performs some operation on it, and returns it. but the function won't accept std::list as a valid return type or as a parameter type.

#include <iostream>
#include <list>

std::list list_function(int n, std::list progress) {

}

int main() {
    /* std::list<int> test_list = {1, 2, 3, 4, 4854};

    test_list.push_back(10); // some unrelated testing

    for (int x : test_list) {
        std::cout << x << '\n';
    } */
}

I can't find any answers online, what do I need to do?

Answer 1

std::list is a template library, so you need a template argument to use that as a type.

For example:

#include <list>

std::list<int> list_function(int n, std::list<int> progress) {
    return std::list<int>();
}

int main(void) {
    std::list<int> a = list_function(10, std::list<int>());
    return 0;
}

If you want to create a generic function, you can also make your function templated like this:

#include <list>

template<typename T>
std::list<T> list_function(int n, std::list<T> progress) {
    return std::list<T>();
}

int main(void) {
    std::list<int> a = list_function(10, std::list<int>());
    std::list<double> b = list_function(10, std::list<double>());
    return 0;
}