#include "Matrix.h" gives an error but "matrix.cpp" doesn't?

[PeteyPii]

hello, this is my first post so forgive me if i did anything wrong.

so i tried to create a simple templated class to store a 2d dynamic array. all the code is good when its stored in the same file but i wanted to try using class declarations and definitions via header and .cpp files. to my dismay my compiler gave me an error: LNK2019 unresolved external symbol. i have no clue what that means as im fairly new to C++ but the error goes away when i use #include "Matrix.cpp" instead of #include "Matrix.h". this is not the way i was taught to use header files so im wondering what i am doing wrong. im using microsoft visual c++. any help is appreciated and thx in advance. here is my code:

Matrix.h:

#ifndef MATRIX_H
#define MATRIX_H

template <typename T>
class Matrix
{
private:
	int mGridWidth;
	int mGridHeight;
	T *mGrid;
	Matrix();

public:
	Matrix(int gridWidth, int gridHeight);
	~Matrix(void);
	T& operator ()(int column, int row);
	int GetWidth();
	int GetHeight();
};

#endif 

Matrix.cpp:

#include "Matrix.h"

template <typename T>
Matrix<T>::Matrix(int gridWidth = 1, int gridHeight = 1)
	: mGridWidth(gridWidth), mGridHeight(gridHeight)
{
	mGrid = new T [mGridWidth * mGridHeight];
}

template <typename T>
Matrix<T>::~Matrix(void)
{
	delete[] mGrid;
}

template <typename T>
T& Matrix<T>::operator ()(int column, int row)
{
	return mGrid[column * mGridWidth + row];
}

template <typename T>
int Matrix<T>::GetWidth()
{
	return mGridWidth;
}

template <typename T>
int Matrix<T>::GetHeight()
{
	return mGridHeight;
}

main.cpp:

#include "Matrix.h"
//#include "Matrix.cpp" //use this and the error goes away
#include <iostream>

int main()
{
	using namespace std;

	Matrix<double> Foo(8,8); //this throws the first error

	Foo(4,3) = 9;

	cout << Foo(4,3) << endl;

	cout << "Press Enter to continue...";
	cin.get();

	return 0;
}

this is the exact error of the first LNK2019 error, there are actually 3 of them

error LNK2019: unresolved external symbol "public: double & __thiscall Matrix<double>::operator()(int,int)" (??R?$Matrix@N@@QAEAANHH@Z) referenced in function _main

i hope thats all the information necessary, if not just ask

[kbw]

The template functions defined in Matrix.cpp should be inlined in Matrix.h

[Disch]

Function bodies for templates need to exist in the header file.

Matrix shouldn't have a cpp file. Put everything in the header.

It has to do with instantiation and whatnot. (too lazy to explain in detail)

[PeteyPii]

ok as long as its the correct way to do it. should i define the functions in the class our outside of it? thx for the fast replies

[Disch]

It's the correct way for templates and inline functions. For non-inline, non-templated functions, you'd put them in a cpp file like you originally tried. Templates are just weird like that.


It doesn't matter whether your put them in the class or afterward. Whichever you prefer.

[PeteyPii]

alright, thx again for the help

[kbw]

I've added a link to a previous thread on preinstantiated templates that you may find relevant.
http://www.cplusplus.com/forum/unices/7252/