How to define a static private method outside the class in c++ - class

Question

#include <iostream>
class BST {
private:
struct Bst_node
{
int data;
struct Bst_node* left;
struct Bst_node* right;
}*root;
// static Bst_node* get_inorder_predecessor(Bst_node* ptr);
static BST::Bst_node* get_inorder_predecessor(Bst_node* ptr)
{
ptr = ptr->left;
if (ptr == NULL) return NULL;
while (ptr->right != NULL)
{
ptr = ptr->right;
}
return ptr;
}
public:
void create()
{
root = new Bst_node;
root->data = 8;
root->right = NULL;
root->left = new Bst_node;
root->left->data = 10;
root->left->left = NULL;
root->left->right = NULL;
}
void print()
{
Bst_node * ptr = get_inorder_predecessor(this->root);
if(ptr == NULL) std::cout<<"NULL\n";
else std::cout<<ptr->data<<std::endl;
}
};
int main()
{
BST obj;
obj.create();
obj.print();
return 0;
}
The above code is working fine but when i define 'get_inorder_predecessor()' function outside the class it gives error see the below code.
#include <iostream>
class BST {
private:
struct Bst_node
{
int data;
struct Bst_node* left;
struct Bst_node* right;
}*root;
static Bst_node* get_inorder_predecessor(Bst_node* ptr);
public:
void create()
{`
root = new Bst_node;
root->data = 8;
root->right = NULL;
root->left = new Bst_node;
root->left->data = 10;
root->left->left = NULL;
root->left->right = NULL;
}
void print()
{
Bst_node * ptr = get_inorder_predecessor(this->root);
if(ptr == NULL) std::cout<<"NULL\n";
else std::cout<<ptr->data<<std::endl;
}
};
static BST::Bst_node* get_inorder_predecessor(Bst_node* ptr)
{
ptr = ptr->left;
if (ptr == NULL) return NULL;
while (ptr->right != NULL)
{
ptr = ptr->right;
}
return ptr;
}
int main()
{
BST obj;
obj.create();
obj.print();
return 0;
}
See the ERRORS:
'''binary.cpp:33:13: error: 'struct BST::Bst_node' is private within this context
static BST::Bst_node* get_inorder_predecessor(Bst_node* ptr)
^~~~~~~~
binary.cpp:4:12: note: declared private here
struct Bst_node
^~~~~~~~
binary.cpp:33:47: error: 'Bst_node' was not declared in this scope
static BST::Bst_node* get_inorder_predecessor(Bst_node* ptr)
^~~~~~~~
binary.cpp:33:57: error: 'ptr' was not declared in this scope
static BST::Bst_node* get_inorder_predecessor(Bst_node* ptr)
^~~
'''