g++/make failing while linking object files - c++

Question

So, I am trying to learn c++ and a little bit about makefiles. However, for some reason I cannot get my files to link properly. The makefile is the following:
OBJS = stl_test.o src/t_stack.o src/matrix_w.o src/matrix.o
stl_test: $(OBJS)
g++ -o stl_test $(OBJS) -lm
.cpp.o:
g++ -c -O -I. $< -o $# -std=c++0x
stl_test.o: include/t_stack.h
src/matrix.o: include/matrix.h
src/matrix_w.o: include/matrix_w.h
src/t_stack.o: include/t_stack.h
include/matrix_w.h: include/matrix.h
touch include/matrix_w.h
include/t_stack.h: include/matrix_w.h include/matrix.h
touch include/t_stack.h
The problem that i am currently having is the following:
touch include/matrix_w.h
touch include/t_stack.h
g++ -c -O -I. stl_test.cpp -o stl_test.o -std=c++0x
g++ -c -O -I. src/t_stack.cpp -o src/t_stack.o -std=c++0x
g++ -c -O -I. src/matrix_w.cpp -o src/matrix_w.o -std=c++0x
g++ -c -O -I. src/matrix.cpp -o src/matrix.o -std=c++0x
g++ -o stl_test stl_test.o src/t_stack.o src/matrix_w.o src/matrix.o -lm
src/t_stack.o: In function `T_stack::pop()':
t_stack.cpp:(.text+0xc9): undefined reference to `Matrix<double>::display() const'
t_stack.cpp:(.text+0xd7): undefined reference to `Matrix<double>::~Matrix()'
src/t_stack.o: In function `T_stack::push_translation(double, double, double)':
t_stack.cpp:(.text+0x1af): undefined reference to `Matrix<double>::Matrix(int, int, double*)'
t_stack.cpp:(.text+0x239): undefined reference to `Matrix<double>::multiply(Matrix<double>*) const'
src/t_stack.o: In function `T_stack::push_scaling(double, double, double)':
t_stack.cpp:(.text+0x33f): undefined reference to `Matrix<double>::Matrix(int, int, double*)'
t_stack.cpp:(.text+0x3c9): undefined reference to `Matrix<double>::multiply(Matrix<double>*) const'
src/t_stack.o: In function `T_stack::T_stack()':
t_stack.cpp:(.text+0x4f4): undefined reference to `Matrix<double>::Matrix(int, int, double*)'
t_stack.cpp:(.text+0x516): undefined reference to `Matrix<double>::copy() const'
src/t_stack.o: In function `T_stack::~T_stack()':
t_stack.cpp:(.text+0x64c): undefined reference to `Matrix<double>::display() const'
t_stack.cpp:(.text+0x65a): undefined reference to `Matrix<double>::~Matrix()'
t_stack.cpp:(.text+0x74f): undefined reference to `Matrix<double>::display() const'
collect2: ld returned 1 exit status
make: *** [stl_test] Error 1
I am not entirely sure what the problem is. I tried changing the order of the files in the g++ command but that didnt work either. I have tried checking the headers of the files and I think they are correct but I have posted fragments here anyways:
stltest.cpp
#include <iostream>
#include "include/t_stack.h"
using namespace std;
int main(void) {
......
matrix.h
#ifndef _MATRIX_
#define _MATRIX_
#include <iostream>
using namespace std;
......
matrix_w.h
#ifndef _MATRIX_W_
#define _MATRIX_W_
#include "include/matrix.h"
class Matrix_w {
......
t_stack.h
#ifndef _T_STACK_H_
#define _T_STACK_H_
#include <list>
#include <stack>
#include <iostream>
#include "include/matrix.h"
#include "include/matrix_w.h"
using namespace std;
class T_stack {
private:
// matrix stack
stack<Matrix_w*, list<Matrix_w* > >* m_stack;
// inverse transform list
list<Matrix<double>* >* t_list;
public:
T_stack();
void pop();
void push_translation(double tx, double ty, double tz);
void push_scaling(double sx, double sy, double sz);
int size() const;
~T_stack();
};
#endif
Any ideas what the problem might be? Thanks!
Just in case, here is the implementation of the methods that it complains about (the file is called matrix.cpp)..
#include "include/matrix.h"
using namespace std;
template <typename T>
Matrix<T>::~Matrix() {
delete data;
}
template <typename T>
Matrix<T>::Matrix(int width, int height) {
this->height = height;
this->width = width;
this->data = new T[height*width];
}
template <typename T>
Matrix<T>::Matrix(int width, int height, T* data) {
this->height = height;
this->width = width;
this->data = new T[height*width];
int i;
//may be able to speed this up by using memcpy
// Not sure whether this is a good idea anyways
for(i=0; i < height*width; i++) {
this->data[i] = data[i];
}
}
template <typename T>
T Matrix<T>::at(int x, int y) const {
#ifdef __DEBUG
if(x < width && y < height) {
return data[y*width + x];
} else {
throw 1;
}
#else
return data[y*width + x];
#endif
}
template <typename T>
void Matrix<T>::set(int x, int y, T val) {
#ifdef __DEBUG
if(x < width && y < height) {
data[y*width + x] = val;
} else {
throw 1;
}
#else
data[y*width + x] = val;
#endif
}
//this function is just for convenience but it should only work
//when T is some number type ----ASK!!
template <typename T>
void Matrix<T>::display() const {
int i, j;
cout << "[" << endl;
for(i=0; i<height; i++) {
for(j=0; j<width; j++) {
cout << " " << data[i*width + j];
}
cout << endl;
}
cout << "]" << endl;
}
template <typename T>
Matrix<T>* Matrix<T>::multiply(Matrix<T>* other) const {
#ifdef __DEBUG
if(other->height != width) {
throw 1;
}
#endif
T* res = new T[other->width*height];
int i,j,k;
T sum;
for(i=0; i<height; i++) {
for(j=0; j<other->width; j++) {
sum = 0;
for(k=0; k<width; k++) {
sum += other->data[k*other->width+j]*data[i*width+k];
}
res[i*other->width + j] = sum;
}
}
return new Matrix<double>(other->width, height, res);
}
template <typename T>
Matrix<T>* Matrix<T>::copy() const {
return new Matrix<T>(width, height, data);
}

Answer 1

When you have a template class or functions, the functions should be inline, and the body should be in the header file, in the class or outside with inline keyword in the class.
The difference is that, unlike regular class, the compiler is compiling the class for each type you are using as the template type.
If you are not using it at all, the file won't be compiled even once.
In several compilers your code will work even if it split to h and cpp. but you can't count on it.

Answer 2

I see, it would be typical to implement your template as inline functions. You can, however, use an explicit instantiation of the template in your matrix.cpp file. At the bottom, after the definitions, you could try to add:
template class Matrix<double>;
If you're not using precompiled headers, this actually might make a lot of sense. Also, it can go pretty much anywhere I think, but you can only have one of those statements per program, if you instantiate a Matrix in two places it may or may not work.