C++ Access private member of subclass from superclass

C++ Access private member of subclass from superclass

The title might be a bit misleading. I have the following problem: I have
a tree consisting of leaves and internal nodes. The user should be able to
store any information in the leaves and the tree has some methods which
get a set of user-defined values and need to access the corresponding
leaves in constant time (not amortized).
I came up with the following idea but it does not work because
unfortunately I cannot access private members of a subclass: The user
creates the tree and also for each leaf an instance of UserElement which
contains the user_defined value for the corresponding leaf. Once a method
like doSomethingWithTheTree(list>) is called and the tree is built, the
tree creates the corresponding leaves and saves it in the private field
leaf. Whenever the user wants to call a method with some of the leaves
corresponding to its user_defined values, he/she just has to call the
method by giving the corresponding UserElements and the tree can retrieve
the corresponding leaves in constant time.
class Tree {
public:
template <typename T>
class UserElement {
private:
T user_value;
tree_node* leaf; // this has to be private for
// everyone outside the class `Tree`
public:
T getInf() {
return user_value;
}
void setInf(T i) {
user_value = i;
}
};
void doSomethingWithTheTree(list<UserElement<T>> elements) {
...
// I want to be able to access elem.leaf for all elements
}
}