Overloading << operator in C++ when using templates in linked list program

Question

I\'m trying to implement a linked list. But I\'m receiving an error when I try overloading the << operator. This is my program:

#include
#i         


        

Answer 1

Since they take a class template argument, your friends need to be function templates, or specializations of function templates:

// function template friend
template <typename T2>
friend ostream& operator<<(ostream& out, const List<T2>& li);

Note that one would usually overload this operator to take a reference, not a pointer, as in the example above.

The drawback is that this is probably not restrictive enough: ostream& operator<<(ostream& out, const List<Widget>& li) would be a friend of List<int>. Since this is not usually the desired behaviour, you can provide a template specialization, which would restrict friendship to the same T as that with which the class is instantiated:

// function template, declared outside of the class:
template <typename T>
ostream& operator<<(ostream& out, const List<T2>& li);

// function template specialization, declared inside the class:
friend ostream& operator<< <>(ostream& out, const List<T>& li);