Unresolved Externals with Templates

Question

Possible Duplicate:
Why do I get “unresolved external symbol” errors when using templates?

I'm trying to implement a generic Queue using templates.

I have the following code in my header:

template<class Item>
class Queue{
protected:
    struct linked_list;
    int size;
public:
    Queue();
    Queue(Item T);
};

I have a Queue.cpp:

template<class Item>
Queue<Item>::Queue()
{

}
template<class Item>
Queue<Item>::Queue(Item T)
{

}

but every time I compile, I get a linker error because of unresolved externals.

I reinstalled VS2012 twice (thinking the linker was broken), but the problem keeps appearing.

I read that there is some problem with the function implementations being in a separate file when working with templates, but I haven't seen any solution except putting the implementation in the header.

Is there a more elegant way to do so?

See http://stackoverflow.com/questions/3749099 – aschepler Dec 22 '12 at 13:07 — aschepler, Dec 22 '12 at 13:07

billz · Answer 1 · 2012-12-22T13:10:35.547

2

Template doesn't support a definition is provided elsewhere and creates a reference (for the linker to resolve) to that definition

You need to use the inclusion model, put all Queue.cpp definition into Queue.h file. Or in the bottom of Queue.h

#include "Queue.cpp"

edited Dec 22 '12 at 13:10

answered Dec 22 '12 at 13:04

billz

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Really? Usually when working with headers, all I do is create a header and a source with the same name and include the header in the source, and it works just fine. – Moshe Magnes Dec 22 '12 at 13:08
1

but this is template, it's different – billz Dec 22 '12 at 13:09

Andrei Tita · Answer 2 · 2012-12-22T13:16:19.637

Template declarations must be included in your source code in their entirety. If you want to split them, one way I prefer to use is:

on the bottom of queue.h:

#define QUEUE_H_IMPL
#include "queue_impl.h"

and in queue_impl.h

//include guard of your choice, eg:
#pragma once

#ifndef QUEUE_H_IMPL
#error Do not include queue_impl.h directly. Include queue.h instead.
#endif

//optional (beacuse I dont like keeping superfluous macro defs)
#undef QUEUE_H_IMPL

//code which was in queue.cpp goes here

Actually now after I've looked at it you don't need an include guard at all if you #undef QUEUE_H_IMPL.

Unresolved Externals with Templates

2 Answers2