Linker Errors

Hi Guys

Just Started assignment 2 today , getting a couple of linker errors when i try and compile the project:

[Linker error] undefined reference to `linkedListType<int>::linkedListType()'

The individual files i.e. linkedlist.h , linkedlist.cpp and the main file all compile fine on their own, which leads me to believe there is something wrong specifically with the linker.

I created a new project (console application and selected C++)

I have #include"LinkedList.h" in the .cpp and the main file.

Have any of you come across these types of errors and manged to solve them.

Thanks

BigSteve i've been struggling with this for some time now .. still can't figure this out (getting the same error ) and it frustrating me ..cause i want to see whether my functions required are doing what they're supposed to.I'll try again and report back BUT if someone comes up with a solution ... please don't keep it to yourself !!

OK , after struggling with this error for many hours i finally decided to call Unisa and ask someone.
It seems that when you use templates you have to include the code for the header file and the implementation file in one file and not split the two up i.e. do not try and create the LinkedList.cpp and a LinkedList.h in two separate files but rather include the code for both in the LinkedList.h file.

Also is anyone getting the error regarding the friend function for the overloaded << operator ?

[Warning] friend declaration `stdstream& operator<<(stdstream&, const linkedListType<Type>&' declares a non-template function

Not sure if its me or if there is something we have to do differently ?

Hops this helps

Yip i also got that error ..if you look at the bottom you can see a whole list of errors of which this is one of them !

I combined LinkedList.cpp and LinkedList.h into one file: LinkedList.h, but I still get those damn link errors!

I also tried that ... still getting those errors and with the friend function !!

You're working with templates here. You must realise that when it comes to templates, you're not working with real classes, only instructions on how to create classes. The compiler creates the real classes when it compiles them.

For this reason, the entirety of the class - the declaration and the function definitions - must at all times be seen by the compiler when it needs to compile.

The above may or may not make sense. The short version is to put your class and the function definitions - basically everything - inside the header that gets included in whatever file that needs the template.

That will solve your problem.

Sorry BigSteve, looks like you answered that one already.

As far as your warning goes, you need to make your friend function a template too.

Language: C++
template <typename Type>
class MyClass {
 
  friend std::ostream& operator<< <Type>(std::ostream &sout, const linkedListType<Type> &list);
 
public:
  /*
  omitted code
  */
};
 
template <typename Type>
std::ostream& operator<< <Type>(std::ostream &sout, const linkedListType<Type> &list){
  /*
  omitted code
  */
  return sout;
}

Basically, if one of a function's parameters is a template class, that function must be a template too, and as such must be defined completely in the header. This is so the compiler knows how to create the real function/class once it knows what the types are.

if I do that I get compile error: 'operator <<' : illegal use of explicit template arguments

Oh Sorry. I just copy-pasted the function declaration to the function itself:

The friend should be:

Language: C++
... operator << <Type> (....)

The function template itself should be:

Language: C++
... operator<< (.....)

I was playing around with it here at work and found that there's some circular referencing going on in this thing so to sort it out, I had to prototype the template class and template operator function so they knew about each other before they were defined. The following code compiles and runs with the Unisa-supplied MinGW compiler:

Language: C++
#include <iostream>
using namespace std;
 
// class and function prototypes
template <typename Type> class MyClass;
template <typename Type> std::ostream& operator<<(std::ostream &sout, const MyClass<Type> &list);
 
// class template definition
template <typename Type>
class MyClass {
 
  friend std::ostream& operator<< <Type>(std::ostream &sout, const MyClass<Type> &list);
 
public:
 MyClass(Type value):m_value(value){}
 
private:
  Type m_value; 
};
 
// function template definition
template <typename Type>
std::ostream& operator<<(std::ostream &sout, const MyClass<Type> &list){
  sout << list.m_value;  
 
  return sout;
}
 
// main program entry point
int main(int argc, char **argv){
  MyClass<int> list(7);  // Compiler instantiates "MyClass<int>" in the background using the "MyClass<Type>" template to work from and replacing "Type" with "int"
 
  cout << list << endl;  // Compiler instantiates "ostream &operator<< (ostream &, MyClass<int> &)" in the background using the "ostream &operator <<(ostream &, MyClass<Type>)" template
 
  return 0;
}

Yes! it works! thanks robanaurochs

I got warnings from the compiler but mine still compiled ... BUT after adjusting it slightly with your suggestion things are working perfectly and really making sense ..
Thumbs up to Rob ..

PS! You've been rather quiet these days.. wazzzuuupp ???

Working on a project Wiseguy. Started my own little venture part-time so I usually only have time for the fora at lunch time.

Seems that this problem is an old problem -- > COS211X_2008_phorum_post