c++ - g++ is giving "unresolved overloaded function type" with template parameters -

reduced basic form, i'm trying binary search tree:

template <class item, class key> class bst { public:     item val;     key key;     bst *left;     bst *right; private: };  template <class item, class key, class process, class param> void preorder_processing1(bst<item, key> *root, process f, param g) {     if (root == null) return;     f(root->val, g);     preorder_processing1(root->left, f, g);     preorder_processing1(root->right, f, g); }  template <class item, class key> void print_path(const bst<item, key>* root, const key target) {     if (root->key != target)     {         cout << root->key << " " << root->val << endl;         if (target < root->key)             print_path(root->right, target);         else             print_path(root->left, target);     }     else         cout << root->key << " " << root->val << endl; }  template <class item, class key> void print_if_leaf(const bst<item, key>* test_node, const bst<item, key>* root) {     if (test_node->right == null && test_node->left == null)     {         print_path(root, test_node->key);         cout << endl;     } }  template <class item, class key> void print_paths_bst(const bst<item, key>* root) {     preorder_processing1(root, print_if_leaf, root); } 

when call print_paths_bst, this: error: no matching function call ‘preorder_processing1(const bst<int, int>*&, <unresolved overloaded function type>, const bst<int, int>*&)’

i've tried forcing cast on preorder_processing1 call

preorder_processing1(root, print_if_leaf<item, key>, root); 

...but hasn't resolved problem.

does see what's wrong this?

template <class item, class key, class process, class param> void preorder_processing1(bst<item, key> *root, process f, param g) 

this needs take const bst<item, key> *

additionally, call f(root->val, g); have problems - root->val isn't bst<item, key> const *. mean f(root, g)