Head Tail List implementation in C++17

head_tail_list.cpp

#include <iostream>
#include <memory>
#include <optional>

class not_found_error : public std::logic_error {
    using std::logic_error::logic_error;
};

// Head - Tail List
// Inspired by Prolog lists
template<typename T>
class List {
public:
    List() = default;

    List(std::initializer_list<T> l) {
        insert(l.begin(), l.end());
    }
    
    void insert(const T* it, const T* end_it) {
        if (it == end_it) return;
        m_head = *it;
        m_tail = std::make_unique<List<T>>();
        (*m_tail)->insert(std::next(it), end_it);
    }
    
    int getElementIndex(const T& element, size_t starting_index=0) const {
        if (element == m_head) {
            return starting_index;
        }
        else if (!m_tail) { 
            throw not_found_error("value not found in list");
            return -1;
        }
        else
            return (*m_tail)->getElementIndex(element, starting_index+1);
    }
    
    T head() const { return m_head; }
    List<T>& tail() const { return *(m_tail.value()); }

private:    
    T m_head;
    std::optional<std::unique_ptr<List<T>>> m_tail = std::nullopt;
};

int main() {
    List<int> list{1, 2, 3}; // could also be represented as [1, 2, 3] or [1, [2, [3]]
    // get first element
    std::cout << list.head() << std::endl;
    // get first element of tail (that is second element)
    std::cout << list.tail().head() << std::endl; 

    std::cout << "element 1, index: " << list.getElementIndex(1) << std::endl;
    std::cout << "element 4, index: " << list.getElementIndex(4) << std::endl; // throws error - there is no 4 in [1, 2, 3]

    return 0;
}