Question

I am writing a templatized Matrix class, and I include the declarations in Matrix.h, the implementation in Matrix.cpp. I have a test file testMatrix.cpp.

The beginning of the files look like this:

Matrix.h

#ifndef _MATRIX
#define _MATRIX
#include <string.h>
// Definition of Matrix class
#endif

Matrix.cpp

#include "Matrix.h"
#include <iostream>
using namespace std;
// ...Implementation

testMatrix.cpp

#include "Matrix.h"
#include <iostream>
using namespace std;
int main () {
  cout << "Test constructors...\n";
  cout << "Unitialized matrix (4, 4):\n";
  Matrix<int> mi1 (4, 4);
  mi1.print ();
  cout << "4*4 matrix initialized to -1:\n";
  Matrix<int> mi2 (4, 4, -1);
  mi2.print();                                                                                                                                    
  cout << "Constructing mi3 as a copy of mi2:\n";
  Matrix<int> mi3 (mi2);
mi3.print();                                                                                                                                    
  cout << "Assigning mi3 to mi1:\n";
  mi1 = mi3; 
mi1.print();                                                                                                                                    

  return 0;
}

The command line I used to compile:

g++ -Wall -lrt -g Matrix.cpp testMatrix.cpp -o testMatrix

The compiler keeps giving me the error:

/tmp/ccofoNiO.o: In function `main':
/afs/ir/users/t/i/tianxind/practice/Essential_C++/testMatrix.cpp:9: undefined reference to `Matrix<int>::print() const'
collect2: ld returned 1 exit status

Does anyone have an idea what went wrong here? Thanks so much!

Answer 1

The implementation of template functions/classes has to be visible to the compiler at the point of instantiation. This means that you should put the whole templates in the header file, not in a .cpp file.