cannot convert 'LinkedList::filter(void (*)(Node*))::<lambda(Node*)>' to 'void (*)(Node*)'

Im trying to implement a simple LinkedList class, but this error shows up and I don’t understand why.

struct Node {
public:
    int val;
    Node* next;
    Node(int v) : val(v), next(nullptr) {}
};

struct LinkedList {
public:
    Node* head;
    Node* tail;
    LinkedList() : head(nullptr), tail(nullptr) {}

    void append(int value) {
        Node* new_node = new Node(value);
        if (head == nullptr) {
            head = new_node;
            tail = new_node;
        } else {
            tail->next = new_node;
            tail = new_node;
        }
    }

    void traverse(void (*callback)(Node* node)) {
        Node* cur = head;
        while (cur != nullptr) {
            callback(cur);
            cur = cur->next;
        }
    }

    LinkedList filter(bool (*filter_function)(Node* node)) {
        LinkedList new_list = LinkedList();
        traverse([&](Node* node) { if(filter_function(node)) new_list.append(node->val); });
        return new_list;
    }
};

error is in this line

traverse([&](Node* node) { if(filter_function(node)) new_list.append(node->val); });

cannot convert 'LinkedList::filter(void (*)(Node*))::<lambda(Node*)>' to 'void (*)(Node*)'

>Solution :

As mentioned in the comments, a capturing lambda is not the same as a function pointer. Instead, behind the scenes, it is a fully-fledged object (because it has state).

Fortunately, there’s an easy fix – you can use the magical powers of std::function to abstract away all the messy details. To do this, all you have to do is #include <functional> and change this:

void traverse(void (*callback)(Node* node)) {

to this:

void traverse (std::function <void (Node* node)> callback) {

and you are golden.

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