Overloading operator* for a matrix class

2
votes

I have a matrix:

#ifndef MATRIX_H
#define MATRIX_H

class Matrix
{   
    public:
        Matrix(int rows, int columns);
        Matrix(int, int, int** matrix); 
        Matrix(Matrix* copy);
        ~Matrix();

        void Set(int, int, int);
        void SetMatrix(int, int, int** matrix);             
        void Print();
        void ZeroMatrix(int,int,int** matrix);      
        void Add(Matrix* B);
        void Subtract(Matrix* B);
        void Copy(Matrix* B);

        int** Multiply(Matrix* B);
        int** Create(int,int);
        int** Get();
        int** Transpose();
        int** Scalar(int);

        int Get(int,int);
        int Rows();
        int Columns();

        Matrix operator*(int);

    private:
        int** _matrix;
        int _rows;
        int _columns;
};

#endif

Here's the implementation:

Matrix Matrix::operator*(int scale)
{
    return Matrix(_rows, _columns, Scalar(scale));
}

And for a school assignment we have to overload the multiple operator to work with integer scalar. The problem is I keep getting this error:

main.cpp: In function ‘int main(int, char*)’: main.cpp:18:15: error: no match for ‘operator’ in ‘4 * B’

Breaking code:

#include "Matrix.h"
#include <fstream>
#include <iostream>

int main(int argc, char *argv[])
{   
    Matrix* A = new Matrix(4,2);

    A->Set(0,0,1);  
    A->Set(0,1,2);
    A->Set(1,0,3);
    A->Set(1,1,4);  
    A->Print();

    Matrix B(A);
    B.Print();

    Matrix C(4 * B); //this line
    C.Print();


    delete A;

    return 0;
}

Any ideas?

edit # 1:

the code:

Matrix operator*(int); 
        Matrix operator* (int, const Matrix &);

generates:

In file included from main.cpp:1:0:
Matrix.h:31:40: error: ‘Matrix Matrix::operator*(int, const Matrix&)’ must take either zero or one argument
In file included from matrix.cpp:1:0:
Matrix.h:31:40: error: ‘Matrix Matrix::operator*(int, const Matrix&)’ must take either zero or one argument
matrix.cpp:207:50: error: ‘Matrix Matrix::operator*(int, const Matrix&)’ must take either zero or one argument
2
c++matrixoperator-overloading
By the way, you don't need either fstream or iostream in the main file, as nothing from either is actually used in main. Best to just include what you need on a per-file basis. – chris
homework should have the homework tag – Jasper
@chris: it was for something i was testing earlier. – MintGrowth

2 Answers

4
votes

When you specify your member function, your class must be the left hand side.

B * 4 is equivalent to B.operator* (4). When you say 4 * B, this does not work.

To remedy this, simply use B * 4 instead of 4 * B, or provide an external overload 

Matrix operator* (int, const Matrix &);

Then, the 4 * B will match this overload.

2
votes

This will work in either direction...

#include <iostream>

class Matrix
{
public:
  Matrix(int x) // This works as a convert constructor
    : _x(x) { } // if you don't use the explicit keyword

  friend Matrix operator*(const Matrix& left, const Matrix& right);

  int _x;
};

Matrix operator*(const Matrix& left, const Matrix& right)
{
  return Matrix(left._x * right._x);
}

int main()
{
  Matrix m(3);
  int a = 4;

  Matrix m1(m * a);
  Matrix m2(a * m);

  std::cout << m._x  << endl  // 3
            << a     << endl  // 4
            << m1._x << endl  // 12
            << m2._x << endl; // 12
}