C++: Can't link template program with separate files for class definition/declaration

Nylex · 04-15-2007, 10:54 AM

I have a really simple problem (I think), but I can't work out for the life of me what it is. I have 3 files:

Code:

// link.h

#ifndef LINK_H
#define LINK_H

template<class T> class Link
{
 private:
  T data;
  Link<T> *nextLink;

 public:
  Link(T d);
  T get();
  void update(T newData);
};

#endif

Code:

// link.cpp

#include "link.h"

template<class T> Link<T>::Link(T d)
{
  data = d;
}

template<class T> T Link<T>::get()
{
  return data;
}

template<class T> void Link<T>::update(T newData)
{
  data = newData;
}

Code:

// linktest.cpp

#include "link.h"

int main()
{
  Link<int> myLink(4);
}

I'm trying to compile this with "g++ linktest.cpp link.cpp -o linktest" and am getting undefined reference errors:

/tmp/ccUiS6RY.o(.text+0x26): In function `main':
/home/nick/C++/linktest.cpp:7: undefined reference to `Link<int>::Link(int)'
collect2: ld returned 1 exit status

If I move the definition of the constructor into link.h, i.e.

Code:

// link.h
...
 public:
  Link(T d) { data = d; }

...

then it compiles and links with no errors.

What am I doing wrong when I'm trying to use separate files?

Thanks!

ntubski · 04-15-2007, 12:50 PM

see http://www.parashift.com/c++-faq-lit...html#faq-35.12

Quote:

Originally Posted by Nylex

I have a really simple problem (I think)

simple is a word that does not apply to C++

dmail · 04-15-2007, 01:33 PM

Ye parashift faqs have this nailed, although I would like to comment on something.
Your constructor should be using the initialiser list, otherwise the constructor creates an object of type T and then copies the input value into it.

Code:

  Link(T const& d):data(d){}

It would also be better to return references and constant references, additionally making functions which do not change the class constant functions.

Nylex · 04-15-2007, 01:42 PM

Thanks for the link! I did try with the initialiser list as well, but changed it.