list.cpp

template <typename Data>
class LinkedList
{
private:
    // the node holds only the data we want to store
    // and the link to the next node in the list
    struct Node
    {
        using node_ptr = std::unique_ptr<Node>;
        Data data{};
        node_ptr next = nullptr;
    };
    using node_ptr = std::unique_ptr<Node>;

    // we always need to know where the link starts, thus we need a head node
    node_ptr head = nullptr;

    // we also need to keep track of how many numbers are in the list
    std::size_t size = 0;

public:
    ...
};