How to handle recursion in member functions?

Question

E.g., I have an empty function to clear a linked list:

void empty(Node* head) {
        if (head->next) { empty(head->next); }
        delete head;
        head = nullptr;
    }

But then I created a class for the linked list, so now I don't need to pass the head argument:

void empty() {
        if (head->next) { empty(head->next); }
        delete head;
        head = nullptr;
    }

But empty(head->next) line is obviously wrong as empty doesn't take any arguments. The idea comes to my mind to create a function inside a function (with lambda), something like this:

void empty() {
        std::function<void(Node*)> emptyWrapper = [&] (Node* l_head) {
            if (l_head->next) { emptyWrapper(l_head->next); }
            delete l_head;
            l_head = nullptr;
        };
        emptyWrapper(head);
    }

But I'm wondering if there's any better way to do it. Lambdas became kind of idee fixe for me recently.

Answer 1

A general approach is to declare a public member function that in turn calls a private static recursive member function.

Pay attention that the name empty sounds confusing. It is better to name the function for example like clear.

Here you are

#include <functional>

//...

class List
{
public:
    //...
    void clear() 
    {
        clear( head );
    }

private:
    static void clear( Node * &head )
    {
        if ( head )
        {
            delete std::exchange( head, head->next );
            clear( head ); 
        }
    }
    //...
}

The same approach can be used without defining an auxiliary static function.

void clear()
{
    if ( head )
    {
        delete std::exchange( head, head->next );
        clear();
    }
}

Here is a demonstrative program.

#include <iostream>
#include <iomanip>
#include <functional>

template <typename T>
class List
{
private:
    struct Node
    {
        T data;
        Node *next;
    } *head = nullptr;

public:
    void push_front( const T &data )
    {
        head = new Node { data, head };
    }

    friend std::ostream & operator <<( std::ostream &os, const List &list )
    {
        for ( Node *current = list.head; current; current = current->next )
        {
            os << current->data << " -> ";
        }

        return os << "null";
    }

    bool empty() const { return head== nullptr; }

    void clear()
    {
        if ( head )
        {
            delete std::exchange( head, head->next );
            clear();
        }
    }
};

int main() 
{
    List<int> list;

    const int N = 10;

    for ( int i = N; i != 0; )
    {
        list.push_front( i-- );
    }

    std::cout << list << '\n';

    list.clear();

    std::cout << "list is empty " << std::boolalpha << list.empty() << '\n';

    return 0;
}

The program output is

1 -> 2 -> 3 -> 4 -> 5 -> 6 -> 7 -> 8 -> 9 -> 10 -> null
list is empty true