Undefined reference when using template [duplicate] - c++

Question

This question already has answers here:
Why can templates only be implemented in the header file?
(17 answers)
Closed 7 years ago.
I have a templated class named DataHandler
#ifndef DATAHANDLER_H
#define DATAHANDLER_H
#include <string>
#include <vector>
#include <iostream>
#include <fstream>
#include <set>
#include "constants.h"
template <typename T>
using Car = std::pair< T, T>;
template <typename T>
using SparseMatrix = std::vector< Car<T> >;
template <class T>
class DataHandler
{
public:
// initializes a new DataHandler only if none has been created,
// otherwise return the living instance
static DataHandler<T>* getInstance()
{
if(!dataHandler)
dataHandler = new DataHandler();
return dataHandler;
}
void readFile();
SparseMatrix<T>* getSparseBlue(){ return &sparseBlue; }
SparseMatrix<T>* getSparseRed(){ return &sparseRed; }
virtual ~DataHandler();
private:
// static DataHandler to ensure only one instance can be created
static DataHandler<T> *dataHandler;
// private constructor to use DataHandler as a Singleton
DataHandler();
int numElem = 0;
int m_rows, m_cols = -1;
#endif // DATAHANDLER_H
The source file is:
#include "data_handler.h"
#include <fstream>
#include <algorithm>
#include <omp.h>
#include <chrono>
using namespace std;
using namespace constants;
// Global static pointer used to ensure a single instance of the class.
template<typename T>
DataHandler<T>* DataHandler<T>::dataHandler = NULL;
template<typename T>
DataHandler<T>::DataHandler()
{
//ctor
}
template<typename T>
DataHandler<T>::~DataHandler()
{
//dtor
}
template<typename T>
void DataHandler<T>::readFile()
{
// do some stuff
}
// Instantiation of relevant templates
template class DataHandler<unsigned char>;
template class DataHandler<unsigned short int>;
In the last two lines I instantiate the templates which I define in main.cpp:
#include <iostream>
#include <chrono>
#include <fstream>
#include <algorithm>
#include "data_handler.h"
#include "dense_traffic_handler.h"
#include "sparse_traffic_handler.h"
#include "constants.h"
using namespace std;
// Check the number of rows/cols to choose between char or short int for the sparse case
bool matrixIsSmall()
{
return true;
}
void integerCase()
{
typedef unsigned char T;
DataHandler<T> *dh = DataHandler<T>::getInstance();
dh->readFile();
DenseTrafficHandler dth(dh); // ****** ERROR HERE *****
}
void charCase()
{
typedef unsigned char T;
DataHandler<T> *dh = DataHandler<T>::getInstance();
dh->readFile();
DenseTrafficHandler dth(dh); // ****** ERROR HERE *****
SparseTrafficHandler<T> sth;
set<unsigned short int> step = dh->getstep();
int currentStep = 0;
set<unsigned short int>::const_iterator stepToSave = step.begin();
}
int main(int argc, char *argv[])
{
if(matrixIsSmall())
charCase();
else
integerCase();
return 0;
}
Compiler gives me an error: undefined reference to DenseTrafficHandler::DenseTrafficHandler<unsigned short>(DataHandler<unsigned short>*)
DenseTrafficHandler header is like that:
#ifndef TRAFFICHANDLER_H
#define TRAFFICHANDLER_H
#include "constants.h"
#include "data_handler.h"
class DenseTrafficHandler
{
public:
template<typename T>
DenseTrafficHandler(DataHandler<T> *dh);
virtual ~DenseTrafficHandler();
private:
int m_cols, m_rows;
char* data;
char ** dense = NULL;
};
#endif // TRAFFICHANDLER_H
DenseTrafficHandler source is:
#include "dense_traffic_handler.h"
using namespace std;
using namespace constants;
template <typename T>
DenseTrafficHandler::DenseTrafficHandler(DataHandler<T> *datah)
{
DataHandler<T> *dh = datah;
dense = dh->getDense();
m_rows = dh->getm_rows();
m_cols = dh->getm_cols();
}
DenseTrafficHandler::~DenseTrafficHandler()
{
//dtor
}
So I have two questions:
Why do I receive this error and how can I manage it?
Is there a way in DataHandler source to not specify
template <typename T>
DataHandler<T>::functionName() for every function? (I mean something like using namespace Datahandler<T>)

Answer 1

You receive this error because compiler did not generate the code for this template type. One of solutions is to tell the compiler to do this explicitly by template instantiation:
add to your DenseTrafficHandler.cpp:
template class DenseTrafficHandler<unsigned short>;
Yes, just implement it in the header file. Reading more about it here.