Problems with templates

[iw0XoG1T]

This will compile link and run:

link_list.cpp:

#include <iostream>
#include <stdexcept>
#include <Link.h>

using namespace std;

void keep_open()
{
	char ch;
	cout << "enter any character: ";
	cin >> ch;
}

int main(int argc, char* argv[])
{
	Link<string>* odd = new Link<string>("999");
	odd->insert(new Link<string>("997"));
	cout << odd << endl;
	keep_open();
	return 0;
}

Link.h

using namespace std;

template<class T>
class Link
{
public:
	Link<T>(T v, Link<T>* p = 0, Link* s = 0)
		:private_curr(v), prev(p), succ(s){}
	
	Link<T>* insert(Link<T>* li)
	{
		li->succ = this;
		li->prev = prev;
		prev = li;
		return this;
	}

	T curr() {return private_curr;}

private:
	T private_curr;
	Link<T>* prev;
	Link<T>* succ;
};

ostream& operator<<(ostream& os, Link<string>* li);

#endif 

but if I change the header to this:

using namespace std;

template<class T>
class Link
{
public:
	Link<T>(T v, Link<T>* p = 0, Link* s = 0)
		:private_curr(v), prev(p), succ(s){}
	
	Link<T>* insert(Link<T>* li);

	T curr() {return private_curr;}

private:
	T private_curr;
	Link<T>* prev;
	Link<T>* succ;
};

ostream& operator<<(ostream& os, Link<string>* li);

#endif 

and add this soure file (link.cpp):

#include <Link.h>

using namespace std;

ostream& operator<<(ostream& os, Link<string>* li) {return os << li->curr();}

template<class T>
Link<T>* Link<T>::insert(Link<T>* li)
{
	li->succ = this;
	li->prev = prev;
	prev = li;
	return this;
}

It will compile but not link and this is the message I get:

g++ link.o link_list.o -lstdc++ -o link_test
link_list.o:link_list.cpp:(.text+0x3d4): undefined reference to `Link<std::string>::insert(Link<std::string>*)'

Why can't I define the member function in a separate source file?

[firedraco]

Because it's a template. You must have all the templated functions available in the header file on linking.

[iw0XoG1T]

Does this rule apply to all functions or only class member functions, because I seem to be having the same problem with templated functions?
Also if you have to define functions in the header doesn't that make templated functions awkward to use?

[jsmith]

It applies to template functions as well.

As for making it awkward to use, no, I would disagree. Make the header file cluttered? Yes.
In that case, you could put the implementations in a file named, say, link.inl, and #include it at
the end of link.h. This keeps the header file neater and satisfies the requirement at the same time.

[iw0XoG1T]

I just ran a test and you were right it is not awkward--I had a misconception about how I could use headers (I am glad I made the statement because I learned something).

As for adding an include at the end that is really clever--it would never have occurred to me--thanks for the tip.