friend and template in C++ - c++

Question

I have a big problem in my C++ Code example. There is something wrong with 'friend' and the 'template'.
Error Messages:
Matrix.h:26:79: warning:
friend declaration 'std::ostream&
matrixClass::operator<<(std::ostream&, const matrixClass::Matrix&)'
declares a non-template function [-Wnon-template-friend]
friend std::ostream &operator<<(std::ostream&, const Matrix &matrix);
Matrix.h:26:79: note:
(if this is not what you intended, make sure the function template
has already been declared and add <> after the function name here)
Matrix.h:28:77: warning:
friend declaration 'matrixClass::Matrix<T>*
matrixClass::operator*(const matrixClass::Matrix&, const
matrixClass::Matrix&)' declares a non-template function
[-Wnon-template-friend]
friend Matrix* operator*(const Matrix &m1, const Matrix &m2);
Matrix.cpp:1:0:
C:\Users\Peter\CLionProjects\PK\untitled76\Matrix.h:26:79: warning:
friend declaration 'std::ostream&
matrixClass::operator<<(std::ostream&, const matrixClass::Matrix&)'
declares a non-template function [-Wnon-template-friend]
friend std::ostream &operator<<(std::ostream&, const Matrix &matrix);
Matrix.h:26:79: note:
(if this is not what you intended, make sure the function template
has already been declared and add <> after the function name here)
Matrix.h:28:77: warning:
friend declaration 'matrixClass::Matrix<T>*
matrixClass::operator*(const matrixClass::Matrix&, const
matrixClass::Matrix&)' declares a non-template function
[-Wnon-template-friend]
friend Matrix* operator*(const Matrix &m1, const Matrix &m2);
CMakeFiles\untitled76.dir/objects.a(main.cpp.obj): In function `main':
main.cpp:8: undefined reference to
main.cpp:8: undefined reference to matrixClass::Matrix<int>::Matrix(int)'<br>
main.cpp:10: undefined reference tomatrixClass::Matrix::set(int, int, int)'
main.cpp:11: undefined reference to matrixClass::Matrix<int>::set(int, int, int)'<br>
main.cpp:12: undefined reference tomatrixClass::Matrix::set(int, int, int)'
main.cpp:13: undefined reference to matrixClass::Matrix<int>::set(int, int, int)'<br>
main.cpp:15: undefined reference tomatrixClass::operator<<(std::ostream&, matrixClass::Matrix const&)'
main.cpp:15: undefined reference to matrixClass::operator<<(std::ostream&, matrixClass::Matrix<int> const&)'<br>
main.cpp:8: undefined reference tomatrixClass::Matrix::~Matrix()'
main.cpp:8: undefined reference to `matrixClass::Matrix::~Matrix()'
Code:
Matrix.h
#ifndef MATRIX_H_
#define MATRIX_H_
#include <iostream>
namespace matrixClass {
template<class T>
class Matrix {
private:
int dimension;
T **m;
public:
Matrix(int d);
Matrix(const Matrix &original);
~Matrix();
void set(int x, int y, T value);
T get(int x, int y) const;
int getDimension() const;
friend std::ostream &operator<<(std::ostream&, const Matrix<T> &matrix);
friend Matrix<T>* operator*(const Matrix<T> &m1, const Matrix<T> &m2);
};
}
#endif
Matrix.cpp
#include "Matrix.h"
using namespace matrixClass;
template<class T>
Matrix<T>::Matrix(int d)
: dimension{d}, m{new T *[d]} {
//m = new T*[d];
for (int i = 0; i < d; i++) {
m[i] = new T[d];
}
}
// COPY-CONSTRUCTOR
template<class T>
Matrix<T>::Matrix(const Matrix &original)
: dimension{original.dimension},
m{new T *[original.dimension]} {
for (int i = 0; i < dimension; i++) {
*(m + i) = *(original.m + i);
}
}
// DESTRUCTOR
template<class T>
Matrix<T>::~Matrix() {
for (int i = 0; i < dimension; i++) {
delete[] m[i];
}
delete[] m;
}
template<class T>
void Matrix<T>::set(int x, int y, T value) {
m[x][y] = value;
}
template<class T>
T Matrix<T>::get(int x, int y) const {
return m[x][y];
}
template<class T>
int Matrix<T>::getDimension() const {
return dimension;
}
template<class T>
std::ostream& operator<<(std::ostream& output, const Matrix<T>& matrix) {
int dimension = matrix.getDimension();
for(int x = 0; x < dimension; x++) {
for(int y = 0; y < dimension; y++) {
output << matrix.get(x, y) << " ";
}
return output;
}
}
template<class T>
Matrix<T>* operator*(const Matrix<T>& m1, const Matrix<T>& m2) {
int dimension = m1.getDimension();
Matrix<T>* m = new Matrix<T>(dimension);
for(int x = 0; x < dimension; x++) {
for(int y = 0; y < dimension; y++) {
T value = 0;
for(int i = 0; i < dimension; i++) {
value += m1.get(x, i) * m2.get(i, y);
}
m->set(x, y, value);
}
}
return m;
}
main.cpp
#include <iostream>
#include "Matrix.h"
using namespace matrixClass;
using namespace std;
int main() {
Matrix<int> m(2);
m.set(0, 0, 1);
m.set(0, 1, 2);
m.set(1, 0, 3);
m.set(1, 1, 4);
cout << m << "*" << endl << m << "=" << endl;
return 0;
}

Answer 1

In friend declaration operator<< refers to a non-template function, while its definition says it's a template function; they don't match.
You can define it inline with the friend declaration (as non-template function):
template<class T>
class Matrix {
... ...
friend std::ostream& operator<<(std::ostream& output, const Matrix<T>& matrix) {
int dimension = matrix.getDimension();
for(int x = 0; x < dimension; x++) {
for(int y = 0; y < dimension; y++) {
output << matrix.get(x, y) << " ";
}
return output;
}
}
... ...
};
Or make the friend declaration referring to the function template:
// class declaration
template<class T>
class Matrix;
// function declaration
template<class T>
std::ostream& operator<<(std::ostream& output, const Matrix<T>& matrix);
// class definition
template<class T>
class Matrix {
... ...
friend std::ostream& operator<< <T>(std::ostream& output, const Matrix<T>& matrix);
... ...
};
// function definition
template<class T>
std::ostream& operator<<(std::ostream& output, const Matrix<T>& matrix) {
int dimension = matrix.getDimension();
for(int x = 0; x < dimension; x++) {
for(int y = 0; y < dimension; y++) {
output << matrix.get(x, y) << " ";
}
return output;
}
}
And about the undefined reference error, see Why can templates only be implemented in the header file?

Answer 2

This answer addresses the problem with your non-member operator<<() and operator*() using Friend Templates, which is slightly different from making an instance of a function template a friend (the second solution presented by #songyuanyao). The difference is that with a Friend Template, all instances of the template function are friends of the class Matrix<>. In this case, I don't think there is any practical difference, but in others, there may be. Therefore, I figured I'd present it anyway.
I think about it like this. Both operators are non-member functions which means they are independent from the class Matrix<>, so think of their prototypes independently:
template<class T>
std::ostream& operator<<(std::ostream& output, const Matrix<T>& matrix);
template<class T>
Matrix<T>* operator*(const Matrix<T>& m1, const Matrix<T>& m2);
Now, replace T with U because to make all instances of them friends of Matrix<>, their prototype needs to be included in the class definition for Matrix<> which is also a template, and it is already using T as its template parameter.
template<class U>
std::ostream& operator<<(std::ostream& output, const Matrix<U>& matrix);
template<class U>
Matrix<U>* operator*(const Matrix<U>& m1, const Matrix<U>& m2);
Now, you can make them friends of Matrix<>. All you need is the appropriate syntax:
template<class T>
class Matrix
{
...
template<class U>
friend std::ostream &operator<<(std::ostream&, const Matrix<U> &matrix);
template<class U>
friend Matrix<U>* operator*(const Matrix<U> &m1, const Matrix<U> &m2);
};
Finally, they all need to be within the matrixClass namespace, and their declarations and definitions need to be in the right order so that everyone know the others exists:
namespace matrixClass {
// BEGIN Forward declarations
template<class T>
class Matrix;
template<class U>
std::ostream &operator<<(std::ostream&, const Matrix<U> &matrix);
template<class U>
Matrix<U>* operator*(const Matrix<U> &m1, const Matrix<U> &m2);
// END Forward declarations
template<class T>
class Matrix
{
...
template<class U>
friend std::ostream &operator<<(std::ostream&, const Matrix<U> &matrix);
template<class U>
friend Matrix<U>* operator*(const Matrix<U> &m1, const Matrix<U> &m2);
};
...
template<class U>
std::ostream& operator<<(std::ostream& output, const Matrix<U>& matrix)
{
... // your implementation goes here
}
template<class U>
Matrix<U>* operator*(const Matrix<U>& m1, const Matrix<U>& m2)
{
... // your implementation goes here
}
} // end of namespace matrixClass
All this template code should be in the header file, Matrix.h, as it has already been mentioned.
Also, I think that you should change the prototype of operator*() to return Matrix<U> instead of Matrix<U>*. It will behave more like the normal operator*(), and it'll enable you to do things like: Matrix<int> m = m1*m2; or m = m1*m2*m3;. Additionally, the users of your class won't have to worry about deleting the memory allocated by operator*() or the performance cost of the dynamic allocation. Just create a local Matrix<U> variable in operator*() and return it by value; let Return Value Optimization (RVO) take care of the rest.