Why compiler(g++) cannot find the first add function?

Question

template <typename Key,typename Value>
void add(SBTNode<Key,Value>*& t, const SBTNode<Key,Value>* x)
{
    if (t==SBTNode<Key,Value>::getPNIL()) { t = x; return;}
    if (x->key<t->key) add(t->left,x);
    else if (t->key<x->key) add(t->right,x);
    else { t->value+=x->value; return; }
    maintain(t, t->key<x->key);
}

template <typename Key,typename Value>
class SBTree
{
private:
    SBTNode<Key,Value>* root;
public:
    SBTree():root(SBTNode<Key,Value>::getPNIL()) {}
    void add(Key& key,Value& value) {  add(root,new SBTNode<Key,Value>(key,value));}
};

Here,void add(Key& key,Value& value) { add(root,new SBTNode<Key,Value>(key,value));} };, I want to call the first add function, however, the compiler seems to think that the only candidate function is the void add(Key& key,Value& value).

 error: no matching function for call to 'SBTree<row_col, int>::add(SBTNode<row_col, int>*&, SBTNode<row_col, int>*)'
  void add(Key& key,Value& value) {  add(root,new SBTNode<Key,Value>(key,value));}
                                     ^~~
note: candidate: 'void SBTree<Key, Value>::add(Key&, Value&) [with Key = row_col; Value = int]'
  void add(Key& key,Value& value) {  add(root,new SBTNode<Key,Value>(key,value));}

I tried to replace to add<Key,Value>(root,new SBTNode<Key,Value>(key,value));，the compiler just tell me <,> is wrong.

error: expected primary-expression before ',' token
  void add(Key& key,Value& value) {  add<Key,Value>(root,new SBTNode<Key,Value>(key,value));}
                                            ^
error: expected primary-expression before '>' token
  void add(Key& key,Value& value) {  add<Key,Value>(root,new SBTNode<Key,Value>(key,value));}

When I give the two functions different names, all is right, what's wrong with the function overloading in this case?

Answer 1

To call the global add function hidden by the member function, prefix the function call with :: ( global scope operator)

template <typename Key,typename Value>
class SBTree
{
//...
    void add(Key& key,Value& value) { 
       ::add(root,new SBTNode<Key,Value>(key,value));
    }
};