Template Class

Example

//template_class.h
#include <iostream>

template< typename T>
class Node {
public: 
	Node(T pay, Node<T> *nx) : payload(pay), next(nx) { }
	
	void print() const {
		const Node<T> *cur = this;
		while(cur != NULL) {
			std::cout << cur->payload << ' ';
			cur = cur ->next;
		}
		std::cout << std::endl;
	}
	
private:
	T payload;
	Node<T> *next;
};

Template classes and header files

If we separate this template class into a .h file and .cpp file we get ERRORS:

//template_class2.h
#ifndef NODE_TEMPLATE_H__
#define NODE_TEMPLATE_H__
template< typename T >
public:
class Node {
	Node(T pay, Node<T> *nx) : payload(pay), next(nx) {}
	void print() const;
private:
	T payload;
	Node<T> *nx;
};

//template_class2.cpp
#include "template_class2.h"
#include <iostream>

template< typename T> 
void Node<T>::print() const {
 	const Node<T> *cur = this;
	while(cur != NULL) {
		std::cout << cur ->payload << ' ';
		cur = cur->next;
	}
	std::cout << std::endl;
}

When we try to use it we get errors in certain areas:

Node<float> f3 = {95.1f, NULL}; // instantiates Node<float>
Node<float> f2 = {34.4f, f3};
d2.print(); // instantiate Node<float>::print

Node<int> i2 = {32, NULL}; // instantiates Node<int>
Node<int> i1 = {3, i2};
i1.print(); // instantiates Node<int>::print

• When instantiating, compiler needs the relevant template classes and functions to be fully defined already because the compiler acts lazily when initiating templates.
  • Lazily instantiating Node<float> and Node<int> is fine because both are defined in the header.
  • Instantiating Node<float>::print() and Node<int>::print() won't work because they are in separate source file that is not #included (by convention we never #include# cpp files)
• In contrast to typical functions or class use, where definition can be in separate .cpp files. i.e. template classes have to be typically defined in the header files.
• Solutions:
  • Move Node<T>::print() definition back into header.
  • Put member function definitions in a separate file and include it, but don't give it a cpp extension. i.e. a different extension such as inc or .inl and use #include "template_class2.inc" along with #inlcude "template_class2.h" (or use #inlcude "template_class2.inc" at the end of the header file).