[C++] Undefined Reference to...

[smiddy]

I seem to always get this error when I start a new assignment, and I never remember how I fixed it the last time. This time it's driving me bat s- crazy. I've been googling for a few hours now with no luck so I've decided it's time to post for help and go to bed.

Here's some snippets of the code first:

LinkedList.h:

#include "ListNode.h"
#include <string>
using std::string;

#ifndef LINKEDLIST_H_
#define LINKEDLIST_H_

template <class T>
class LinkedList
{
public:
	LinkedList();
	virtual ~LinkedList();

	T push(T);
	T get(int position);
	T remove(int position);

	ListNode<T>* getNode(int position);
	ListNode<T>* lastNode();

	string toString();
private:
	ListNode<T> *firstNode;
	int length;
};

#endif /*LINKEDLIST_H_*/

LinkedList.cpp: (the first few lines of it anyway -- all of the functions declared in the .h are implemented below the empty constructor/destructor)

#include "LinkedList.h"

template <class T>
LinkedList<T>::LinkedList()
{
	length = 0;
}
template <class T>
LinkedList<T>::~LinkedList()
{
}

Main.cpp:

#include <iostream>
#include "LinkedList.h"

int main(int argc, char *argv[]) {
	std::cout << "Entry Point Reached" << std::endl;
	LinkedList<int> linkedList;
	linkedList.push(124);
}

So that's the effective source, now here's what's being run to compile:

g++ -pedantic-errors -c ListNode.cpp
g++ -pedantic-errors -c LinkedList.cpp
g++ -pedantic-errors -c FileFacade.cpp
g++ -pedantic-errors -c RecordFacade.cpp
g++ -pedantic-errors -o cpps3a1.bin Main.cpp ListNode.o LinkedList.o FileFacade.o RecordFacade.o

Which gives me this error:

/tmp/ccnecGST.o: In function `main':
Main.cpp:(.text+0xaf): undefined reference to `LinkedList<int>::LinkedList()'
Main.cpp:(.text+0xc2): undefined reference to `LinkedList<int>::push(int)'
Main.cpp:(.text+0xcd): undefined reference to `LinkedList<int>::~LinkedList()'
Main.cpp:(.text+0xe8): undefined reference to `LinkedList<int>::~LinkedList()'
collect2: ld returned 1 exit status

I can't figure it out. I'm really hoping it's just something really simple that I've missed over and over again, but who knows.

Thanks very much for the help.

[skistovel]

So that's the effective source, now here's what's being run to compile:

g++ -pedantic-errors -c ListNode.cpp
g++ -pedantic-errors -c LinkedList.cpp
g++ -pedantic-errors -c FileFacade.cpp
g++ -pedantic-errors -c RecordFacade.cpp
g++ -pedantic-errors -o cpps3a1.bin Main.cpp ListNode.o LinkedList.o FileFacade.o RecordFacade.o

Which gives me this error:

/tmp/ccnecGST.o: In function `main':
Main.cpp:(.text+0xaf): undefined reference to `LinkedList<int>::LinkedList()'
Main.cpp:(.text+0xc2): undefined reference to `LinkedList<int>::push(int)'
Main.cpp:(.text+0xcd): undefined reference to `LinkedList<int>::~LinkedList()'
Main.cpp:(.text+0xe8): undefined reference to `LinkedList<int>::~LinkedList()'
collect2: ld returned 1 exit status

I don't know about GCC, not in the mood to test it(!), but VC++ compiler hates it when using templates the class/function implementation in another file.

Whenever I use template in a class I add the implementation in the header file as well.

Hope that helps.

[david13lt]

Yeah, you can not separate template class into several files. Template class is not ordinary class, it's just a template for compiler which makes special functions later and if you separate into several files it does not know that. More information in C++ FAQ:

http://www.parashift.com/c++-faq-lite/temp....html#faq-35.12

[rpgfan]

You can, however, #include the CPP file. Remove the #include "LinkedList.h" part from LinkedList.cpp, and #include "LinkedList.cpp" in LinkedList.h. Here is an example using something simpler:

rpgfan.h

#ifndef __RPGFAN_H__
#define __RPGFAN_H__

template <typename T>
class RpgFan
{
  protected:
	T* data;

	//nobody should be creating an empty object without a way to set the data
	RpgFan (void);

  public:
	RpgFan (T);
	virtual ~RpgFan (void);

	T
	getData (void) const;
};

#include "rpgfan.cpp" /**** this is the important part ****/

#endif //__RPGFAN_H__

rpgfan.cpp

#ifndef __RPGFAN_CPP__
#define __RPGFAN_CPP__

/**** notice there is no #include "rpgfan.h" here ****/

template <typename T>
RpgFan<T>::RpgFan (void)
{
  data = 0;
}

template <typename T>
RpgFan<T>::RpgFan (T t)
{
  data = new T(t);
}

template <typename T>
RpgFan<T>::~RpgFan (void)
{
  delete data;
  data = 0;
}

template <typename T>
T
RpgFan<T>::getData (void) const
{
  return *data;
}

#endif //__RPGFAN_CPP__

I hope this helps!

[smiddy]

Fantastic! Thanks very much Rpgfan, that worked.

[david13lt]

Basically it's still makes one file using pre-processor command "include". I still suggest to put everything in one file.

[rpgfan]

Basically it's still makes one file using pre-processor command "include". I still suggest to put everything in one file.

Yeah, that is true. Then again, everything gets #included anyway, so why not just put everything into a single file? It is possible to do for some of the largest projects. It just gets really complicated, and loading a 23475-line source file tends to be a bit tedious for some of the best source code editors. That is why we have the freedom to #include files, right? ^_^