6.5 A Function Template Output Operator

We'd like to provide an output operator for our BinaryTree class template. For a nontemplate class, we write

ostream& operator<<( ostream&, const int_BinaryTree& ); 

For the class template, we could provide an explicit instance for each generated class definition:

ostream& operator<<( ostream&, const BinaryTree<int>& ); 

But that's Sisyphus work: It's tedious, and it never ends. A better solution is to define the output operator as a function template:

template <typename elemType> 


inline ostream& 


operator<<( ostream &os, const BinaryTree<elemType> &bt ) 


{ 


    os << "Tree: " << endl; 


    bt.print( os ); 


    return os; 


} 

When we write

BinaryTree< string > bts; 


cout << bts << endl; 

an instance of the output operator is generated to support a second parameter of BinaryTree<string>. When we write

BinaryTree< int > bti; 


cout << bti << endl; 

an instance of the output operator is generated to support a second parameter of BinaryTree<int>, and so on.

print() is a private member function of the BinaryTree class template (refer to the online code listing for its definition). For the output operator to access print(), it must be made a friend of BinaryTree:

template <typename elemType> 


class BinaryTree { 


   friend ostream& operator<<( ostream&, 


                               const BinaryTree<elemType>& ); 


   // ... 


};