How to derive abstract template classes, with template-types as function parameters (C++11)

I've been assigned to write a class "binaryExpressionTree" which is derived from the abstract template class "binaryTreeType." binaryExpressionTree is of type String. As part of the assignment, I have to override these 3 virtual functions from binaryTreeType:

virtual bool search(const elemType& searchItem) const = 0;


    virtual void insert(const elemType& insertItem) = 0;


    virtual void deleteNode(const elemType& deleteItem) = 0;

Where "elemType" is the placeholder for the template class type. The problem is, since binaryExpressionTree is a derived class of type String, it doesn't know what "elemType" means and I would need to change searchItem, insertItem and deleteItem to string& objects. But once I do, the compiler no longer recognizes that I am overriding virtual functions (as I've changed their parameters), and declares binaryExpressionTree to be an abstract class. How do I work around this, so that I can override the functions and make binaryExpressionTree non-abstract?