LNK2019 error: output protected vector member of class template via ostream friend method

[bardicer]

Hey everyone, sorry for bugging y'all again, but I've been unable to figure this one out after an hour or so. And I have a little less hair up top...

I've never worked with vectors before, and I'm trying to do a list class with a vector to hold the list items. However, when I try to overload the << operator as a friend of the class, I get an error. Here are the relevant parts of my code:

//-------------------------------myClass.h Header File
template <class T>
class myClass {

  protected:
    vector<T> items;
//------ operator << declared as friend in class
  friend ostream& operator <<(ostream&, const myClass<T>&);
};

//----------------------operator << defined as friend in myClass.h file
template <class T>
ostream& operator <<(ostream& out, const myClass<T>& list) {
  items.iterator it;
  for(it = items.begin();it <= items.end();++it) {
    if(items(it) == items.begin())
       out << items(it);
    else
       out << " " << items(it);
  }
  return out;
}

The error that is being generated is:

1>Driver.obj : error LNK2019: unresolved external symbol "class std::basic_ostream<char,struct std::char_traits<char> > & __cdecl
operator<<(class std::basic_ostream<char,struct std::char_traits<char> > &,class myClass<int> const &)"
(??6@YAAAV?$basic_ostream@DU?$char_traits@D@std@@@std@@AAV01@ABV?$myClass@H@@@Z) referenced in function _main

In my Driver file, I have:

//---------------------------Driver.cpp
#include <iostream>
#include <vector>
#include "myClass.h"
  using namespace std;


int main() {
  myClass<int> list;
  int num;

  for(int i = 0;i < 10; i++) {
    cout << "\nEnter a number to insert into the vector";
    cin >> num;
    list.insert(num);
  }

  cout << list;
  return 0;

}

Any ideas as to why this is occurring? I appreciate any feedback.

[hamsterman]

Is operator << defined in "myClass.h" ? Template functions should always be in headers.

[bardicer]

Yes it is. I edited the original post to better convey this information.

[hamsterman]

I just noticed that your function is pretty wrong. I wonder if that could be the cause (If the compiler can't compile a function, the linker won't find it).
Instead of items it should be list.items, vector has no member "iterator". it should be vector<T>::iterator it;
condition in the for loop should be <, not <=
What did you mean by "items(it)" it seems like you just want "it" or "*it".

[bardicer]

Instead of items it should be list.items, vector has no member "iterator". it should be vector<T>::iterator it;
condition in the for loop should be <, not <=

So, where I have, for instance, items.begin(), it should be list.items.begin()?

What did you mean by "items(it)" it seems like you just want "it" or "*it".

I am wanting to output the element in that particular spot in the vector - kind of like iterating through an array and outputting what is in the sub-scripted location during each iteration.

[hamsterman]

1. yes

2. look, there are two ways to iterate through an array:

int array[5];

/*1*/
for(int i = 0; i < 5; i++) array[i]++;

/*2*/
for(int* it = &array[0]/*or just array*/; it < &array[5]/*or preferably array+5, since array[5] doesn't exist*/; it++ ) (*it)++;

vectors try to look like arrays, so the same two methods exist. If "array" is a vector, in the first case change 5 to array.size() and in the second change "int*" to vector<int>::iterator, "&array[0]" to array.begin() and "&array[5]" to array.end(). The rest remains.

[bardicer]

Sorry for the delay in responding. I went back through the program and driver file, and had a whole slew of issues crop up. Namely, I was attempting to use vector operations (like items.end()) and such that my computer just really did NOT like. However, I have gotten those resolved (I hope...I only have a warning comparing int i to items.size() ). I'm just iterating through the vector as if it were an array.

Now, I am back to the original issue - 1 unresolved external symbol.

I have the following line declared in my class, before all other declarations

friend ostream& operator <<(ostream&, const myClass<T>&)


At the end of the header file, since I'm defining my functions in the same file instead of a separate .cpp file, I have

template <class T>
ostream& operator <<(ostream& out, const myClass<T>& list) {
  
  for(int i = 0;i < list.items.size();++i) {
    if(i == 0)
       out << list.items[i];
    else
       out << " " << list.items[i];
  }
  return out;
}

In the driver program, I have the following line of code:

cout << list

Where "list" is an object of class myClass.

I'm under the impression that a friend function can access protected members of the class, and items is a protected member of the class. I know I have to pass an ostream object by reference, as well as an object of what I want outputted. I believe I have kept both of these standards intact...however I have spent the better part of the evening running from funnel clouds...

[bardicer]

Um...Okay, I'm not sure WHY this worked, so if someone could explain it to me, I would be extremely grateful.

In the declaration of the friend function within the class, I changed it to:

template <class U>
friend ostream& operator <<(ostream&, myClass<U>&);

And in the method definition, I modified it to:

template <class U>
ostream& operator <<(ostream& out, const myClass<U>& list) {
  
  for(int i = 0;i < list.items.size();++i) {
    if(i == 0)
       out << list.items[i];
    else
       out << " " << list.items[i];
  }
  return out;
}

Now it works...we get along like old pals now... but for some reason, before I changed "template T" to "template U" on the ostream method - had it like alllll the other methods - it threw the unresolved external error. I'm boggled. :(

[jsmith]

Because your friend declaration originally was a non-template function, but the ostreamer you wrote was a template function. When you use the keyword "friend", you are not declaring a function. You are declaring the function to be a friend of the class. Said "function" is not a member of the class, either; it is a free function.

Therefore, when you actually declared and implemented the function below, the function you implemented was a template free function. But the friend declaration was not a template function.