error: 'AVL' was not declared in this scope

这个错误通常是由于代码中使用了未声明的标识符引起的。在本例中，可能是因为你没有正确地声明 `AVL` 类，或者没有包含头文件。

以下是一个示例 `AVL` 类的声明和定义，以及使用该类实现 AVL 树的代码：

#include <iostream>

using namespace std;

struct Node {
    int data;
    int height;
    Node *left;
    Node *right;

    Node(int val) {
        data = val;
        height = 1;
        left = nullptr;
        right = nullptr;
    }
};

class AVL {
private:
    Node *root;

    int height(Node *node) {
        if (node == nullptr) {
            return 0;
        }

        return node->height;
    }

    int balanceFactor(Node *node) {
        if (node == nullptr) {
            return 0;
        }

        return height(node->left) - height(node->right);
    }

    Node *rotateRight(Node *node) {
        Node *leftChild = node->left;
        Node *rightSubtree = leftChild->right;

        // Perform rotation
        leftChild->right = node;
        node->left = rightSubtree;

        // Update heights
        node->height = max(height(node->left), height(node->right)) + 1;
        leftChild->height = max(height(leftChild->left), height(leftChild->right)) + 1;

        return leftChild;
    }

    Node *rotateLeft(Node *node) {
        Node *rightChild = node->right;
        Node *leftSubtree = rightChild->left;

        // Perform rotation
        rightChild->left = node;
        node->right = leftSubtree;

        // Update heights
        node->height = max(height(node->left), height(node->right)) + 1;
        rightChild->height = max(height(rightChild->left), height(rightChild->right)) + 1;

        return rightChild;
    }

    Node *insert(Node *node, int val) {
        // Perform standard BST insertion
        if (node == nullptr) {
            return new Node(val);
        }

        if (val < node->data) {
            node->left = insert(node->left, val);
        } else if (val > node->data) {
            node->right = insert(node->right, val);
        } else {
            // Duplicate keys not allowed
            return node;
        }

        // Update the height of the current node
        node->height = max(height(node->left), height(node->right)) + 1;

        // Check the balance factor of the current node
        int balance = balanceFactor(node);

        // Left Left Case
        if (balance > 1 && val < node->left->data) {
            return rotateRight(node);
        }

        // Right Right Case
        if (balance < -1 && val > node->right->data) {
            return rotateLeft(node);
        }

        // Left Right Case
        if (balance > 1 && val > node->left->data) {
            node->left = rotateLeft(node->left);
            return rotateRight(node);
        }

        // Right Left Case
        if (balance < -1 && val < node->right->data) {
            node->right = rotateRight(node->right);
            return rotateLeft(node);
        }

        // Return the unchanged node pointer
        return node;
    }

    void inorder(Node *node) {
        if (node == nullptr) {
            return;
        }

        inorder(node->left);
        cout << node->data << " ";
        inorder(node->right);
    }

public:
    AVL() {
        root = nullptr;
    }

    void insert(int val) {
        root = insert(root, val);
    }

    void inorder() {
        inorder(root);
        cout << endl;
    }
};

int main() {
    AVL tree;

    tree.insert(10);
    tree.insert(20);
    tree.insert(30);
    tree.insert(40);
    tree.insert(50);

    tree.inorder();

    return 0;
}

在 `main` 函数中，我们首先创建了一个 `AVL` 对象 `tree`，然后通过调用 `insert` 方法插入一些节点，最后通过调用 `inorder` 方法遍历 AVL 树并输出结果。

如果你仍然遇到类似的错误，请检查是否正确包含了头文件，并确保在代码中正确地声明了 `AVL` 类。