找不到方法：模板，虚方法，继承，多态

Question

I'm fairly new to c++ and I can't seem to find anyone else who has had the exact same problem as me. Basically, I'm trying to have an abstract class which I never directly instantiate, and several child classes. Also, I'm trying to keep a consistent template over all super/sub classes. Here's my source files. I have 3 utility files and one .cpp file for the main function.

abstract_matrix.h

#ifndef ABSTRACTMATRIX
#define ABSTRACTMATRIX

template<class T>
class DataMatrix {

 public:
  int numFeatures;
  int numPoints;

  T* data;
  T* classifications; 

  virtual void scale(T scalar) = 0;
};

#endif

Here's my subclass declaration of that abstract class, sparse_host_matrix.h

#ifndef SPARSEHOSTMATRIX
#define SPARSEHOSTMATRIX

#include <iostream>

template<class T>
class SparseHostMatrix : public DataMatrix<T> {

 public:

  void scale(T scalar);
};

#endif

And here's the implementation of those functions..

#include "sparse_host_matrix.h"
#include <iostream>


template<class T>
void SparseHostMatrix<T>::loadFromFile(char* filename) {
  std::cout << "Loading in sparseHostMatrix" << std::endl;
}

template<class T>
void SparseHostMatrix<T>::scale(T scalar) {
  std::cout << "Loading in sparseHostMatrix" << std::endl;
}

And when I run this main function...

#include <iostream>

using namespace std;
#include "abstract_matrix.h"
#include "sparse_host_matrix.h"

int main() {
  DataMatrix<double> *myMat = new SparseHostMatrix<double>;
  myMat->scale(.5);
}

I get the error undefined reference to `SparseHostMatrix::scale(double)

Sorry for the massive amount of code, I'm just pretty confused and have been stuck on this for a while without successfully finding a solution on SO or otherwise.

Answer 1

Implementation of template functions must be in the header. You cannot place it in a separate source file. The compiler needs to see the actual body of the function at the point where it is used and actual template parameters become known.