Undefined symbols for architecture x86_64 error

Question

I have this error:

Undefined symbols for architecture x86_64:
  "my::Queue<int>::Queue()", referenced from:
      _main in ccdwI88X.o
ld: symbol(s) not found for architecture x86_64
collect2: ld returned 1 exit status

for this code 'main.cpp':

#include "Queue.hpp"

int main()
{
  my::Queue<int> myqueue;
  return 0;
}

'Queue.hpp':

#ifndef QUEUE_HH__
#define QUEUE_HH__

namespace my
{
  template <typename T>
  class Queue
  {
  public:
    Queue();     
  };
}

#endif

and 'Queue.cpp':

#include "Queue.hpp"

template <typename T>
my::Queue<T>::Queue() 
{
}

Please read at least the first question in the "related" list on the right of this page. — Mat, Apr 21 '12 at 22:54
I read it, but it's not the same problem that I have. Thanks anyway. — Bebeoix, Apr 21 '12 at 23:03
He recommend to put the definitions of the class to a header file, but I did that (Queue.hpp) and it doesn't work anyway... — Bebeoix, Apr 21 '12 at 23:05
The code you have in your post does not have the complete definition of your class template constructor in the header. It only has a declaration. — Mat, Apr 21 '12 at 23:06
possible duplicate of [C++ Linking Errors: Undefined symbols using a templated class](http://stackoverflow.com/questions/312115/c-linking-errors-undefined-symbols-using-a-templated-class) — Troubadour, Apr 21 '12 at 23:10

score 5 · Accepted Answer · edited May 23 '17 at 12:00

5

The answer posted here: https://stackoverflow.com/a/312402/700926 is what I think you need.

If I edit your Queue.cpp file to this:

#include "Queue.hpp"

template <typename T>
my::Queue<T>::Queue() 
{

}

template class my::Queue<int>;

.. it compiles fine.

For the detailed explanation, please consult the URL i mentioned at first.

edited May 23 '17 at 12:00

Community

1
1

answered Apr 21 '12 at 23:06

Lasse Christiansen

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Ok it works. But I was thinking that the principle of template is that it can take a type we don't know in advance... What if I want to create a lib with my queue class and I want use it in the future without knowing wich type T I will use ? – Bebeoix Apr 21 '12 at 23:10
As mentioned here: http://stackoverflow.com/a/312402/700926 : "If you don't know with which template parameters the template will be used, you have to put all the definitions into the header". That should be what you are looking for :) – Lasse Christiansen Apr 21 '12 at 23:13
Ok it works thanks. But it's like in the main.cpp I include the .hpp followed by the .cpp. Kind of weird. – Bebeoix Apr 21 '12 at 23:26

score 1 · Answer 2 · answered Apr 21 '12 at 23:27

The easiest and safest thing to do when using templates is always to put the class functions definition (implementation) inside the .hpp file and not in a separate .cpp file.

All the details are also here: http://www.parashift.com/c++-faq-lite/templates.html#faq-35.12

Undefined symbols for architecture x86_64 error

2 Answers2