Unresolved external symbol with custom blocking queue implementation - c++

Question

I have created my own blocking queue and I'm having some trouble figuring out why I get a linker error (note this is a Qt app in Visual Studio 2010):
#ifndef BLOCKING_QUEUE_H
#define BLOCKING_QUEUE_H
#include <QObject>
#include <QSharedPointer>
#include <QWaitCondition>
#include <QMutex>
#include <queue>
namespace TestingNS
{
template<typename Data>
class BlockingQueue
{
private:
std::queue<QSharedPointer<Data>> _queue;
QMutex _mutex;
QWaitCondition _monitor;
volatile bool _closed;
public:
BlockingQueue();
void Close();
size_t Size();
void Empty();
bool IsClosed();
bool Enqueue(QSharedPointer<Data> data);
bool TryDequeue(QSharedPointer<Data>& value, unsigned long time = ULONG_MAX);
};
}
#endif //BLOCKING_QUEUE_H
The implementation is a bit longer, so I have a pastie for it: http://pastie.org/5368660
The program entry point looks like this:
#include <QtCore/QCoreApplication>
#include <QTimer>
#include <iostream>
#include "BlockingQueue.h"
using namespace std;
using namespace TestingNS;
class Item
{
};
int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);
BlockingQueue<Item> queue;
cout << "Press any key to exit!" << endl;
char in;
cin.get(in);
QTimer::singleShot(0, &a, SLOT(quit()));
return a.exec();
}
The linker error I get is:
Error 1 error LNK2019: unresolved external symbol "public: __thiscall TestingNS::BlockingQueue<class Item>::BlockingQueue<class Item>(void)" (??0?$BlockingQueue#VItem###TestingNS##QAE#XZ) referenced in function _main
I don't understand why the linker can't find the constructor (nor any other method from BlockingQueue). Any ideas?

Answer 1

It's template, you have to put the implementation inside BlockingQueue.h
For a while, the standard did
provide the keyword export to allow such a separate implementation file. But not many
vendors implemented it. C++11 discontinues that use of export but reserves the export
keyword for possible future use.)
Templates have to be used in conjunction with requests for particular
instantiations of templates.