C++ Overloading << operator in Matrix class - c++

Question

I am trying to overload the ostream << operator in my Matrix class, but I keep getting the following error:
Expected constructor, destructor, or type conversion before token &
Matrix::ostream& operator<<(const Matrix& matrix)
{
for (int r = 0; r < matrix.getNumrows(); r++)
{
cout << matrix.getPoint(r, 0);
for (int c = 0; c < matrix.getNumcolumns(); c++)
{
cout << " " << matrix.getPoint(r,c);
}
cout << endl;
}
return stream;
}
This is the rest of my class
#include <iostream>
#include <string>
#include <fstream>
#include <sstream>
#include "Matrix.h"
using namespace std;
Matrix::Matrix()
{
}
Matrix::Matrix(int rows, int cols) {
numRows=rows;
numCols=cols;
//col=new double[cols];
mx=new double*[rows];
for ( int i=0; i < rows; i++ ) {
mx[i] = new double[cols];
// initalize each element of the new row.
for ( int c=0; c < cols; c++ ) {
mx[i][c] = 0.0;
}
}
}
Matrix::Matrix(const Matrix &theMatrix) {
int rows=theMatrix.numRows;
int cols=theMatrix.numCols;
numRows = rows;
numCols = cols;
mx=new double*[rows];
for ( int r=0; r < rows; r++ ) {
mx[r] = new double[cols];
// copy each element of the new row.
for ( int c=0; c < cols; c++ ) {
mx[r][c] = theMatrix.mx[r][c];
}
}
}
void Matrix::setMatrix(string file)
{
/* read the file */
fstream inputStream(file.c_str());
if(inputStream.is_open() )
{
string line;
stringstream ss;
getline(inputStream, line);
ss.clear();
ss.str(line);
ss >> numRows >> numCols;
mx=new double*[numRows];
for ( int i=0; i < numRows; i++ ) {
mx[i] = new double[numCols];
// initalize each element of the new row.
for ( int c=0; c < numCols; c++ ) {
mx[i][c] = 0.0;
}
}
//now loop to get values
for(int row=0; row<numRows; row++)
{
getline(inputStream, line);
ss.clear();
ss.str(line);
//now get every value in the line
for(int col=0; col<numCols; col++)
{
double current;
ss >> current;
mx[row][col] = current;
}//end reading values of row
}//end reading rows
}
//close the file
inputStream.close();
}
int Matrix::getNumrows()
{
return numRows;
}
int Matrix::getNumcolumns()
{
return numCols;
}
void Matrix::printPoint()
{
for ( int r=0; r < numRows; r++ )
{
for ( int c=0; c < numCols; c++ )
{
cout << mx[r][c] << " ";
}
cout << endl;
}
cout << endl;
}
bool Matrix::getIsSquared()
{
if( numRows == numCols )
{
return true;
}
else
{
return false;
}
}
double Matrix::det()
{
double det=0.0;
if(numRows!=numCols)
{
cout << "Number Rows must be same as number Colums\n";
}
if(numRows==2)
{
det=(mx[0][0]*mx[1][1])-(mx[0][1]*mx[1][0]);
}
else
{
for(int i=0 ; i<numCols ; i++)
{
Matrix temp(numRows-1,numCols-1);
for(int j=0 ; j<numRows-1 ; j++)
{
for(int k=0 ; k<numCols-1 ; k++)
{
if(k<i)
temp.mx[j][k]=mx[j+1][k];
else
temp.mx[j][k]=mx[j+1][k+1];
}
}
det+=pow(-1.0,i)*mx[0][i]*temp.det();
}
}
return det;
}
double Matrix::getPoint(int row, int col)
{
return mx[row][col];
}
Matrix Matrix::operator +(const Matrix &right) const
{
Matrix result(numRows,numCols);
if ( right.numRows != numRows || right.numCols != numCols )
{
cout << "\nError while adding matricies, the two must have the same dimentions.\n";
}
else
{
for ( int r=0; r < numRows; r++ )
{
for ( int c=0; c < numCols; c++ )
{
result.mx[r][c] = (this->mx[r][c]) + (right.mx[r][c]);
}
}
}
return result;
}

Answer 1

If you want to overload the ostream operator<< for your class, you need to use either a friend function or a non-member function because the ostream object appears on the left-hand side of the expression (it's written as os << my_matrix).
std::ostream& operator<<(std::ostream& os, const Matrix& matrix) { /* ... */ }
It looks like you are trying to implement it as a member function, but that should actually look like:
std::ostream& Matrix::operator<<(const Matrix& matrix) { /* ... */ }
This won't work because when you implement an operator overload as a member function, the type of the object on the left hand side of the expression is the same as the type of the class of which the overload is a member (so, in this case, you'd have to write my_matrix1 << my_matrix2, which isn't what you want).
Inside of the overload, you shouldn't write to cout directly; you should write to the ostream object that is passed as an argument to the function.

Answer 2

Write it as:
ostream& operator<<(ostream& os, const Matrix& matrix)
{
for (int r = 0; r < matrix.getNumrows(); r++)
{
os << matrix.getPoint(r, 0);
for (int c = 0; c < matrix.getNumcolumns(); c++)
{
os << " " << matrix.getPoint(r,c);
}
os << endl;
}
return os;
}
and it will work. It does not have to be a member function of Matrix.

Answer 3

Just change
Matrix::ostream& operator<<(const Matrix& matrix)
to
std::ostream& operator<< (std::ostream &stream, const Matrix &matrix)
It'll be a stand-alone function .. and should work just fine.