Operator overloading for multiplication of two different classes

[Perotto]

Could you, please, help me with operator overloading for multiplication of two different class pointers? I'd like to program this operation like in MATLAB, without extra subroutine.

Class definition:

class Vector
{
public:
double* V;
unsigned int Size;

Vector(unsigned int Elems, bool ZeroV = true)
{
Size = Elems;

if (Size < 1) return;

V = new double[Size];

if (ZeroV)
{
for (int i = 0; i < Size; i++)
V[i] = 0.0;
}
}

~Vector()
{
delete[] V;
}
};

class Matrix
{
public:
double** M;
unsigned int Rows;
unsigned int Cols;

Matrix(unsigned int Row, unsigned int Col, bool ZeroM = true)
{
Rows = Row; Cols = Col;

if (Rows < 1 || Cols < 1) return;

M = new double*[Rows];

for (int i = 0; i < Rows; i++)
M[i] = new double[Cols];

if (ZeroM)
{
for (int i = 0; i < Rows; i++)
for (int j = 0; j < Cols; j++)
M[i][j] = 0.0;
}
}

~Matrix()
{
for (int i = 0; i < Rows; i++)
delete[] M[i];

delete[] M;
}
};

My attempts to code this:

Vector* operator* (const Matrix *M)
{
Vector* Result = new Vector(M->Cols);

if (this->Size != M->Rows)
return Result;

for (int i = 0; i < M->Cols; i++)
for (int j = 0; j < M->Rows; j++)
Result->V[i] += this->V[j] * M->M[j][i];

return Result;
}

main function:

void main()
{
Vector* V = new Vector(5, true);
Matrix* M = new Matrix(5, 2, true);

Vector* Vnew = V * M;
}

[Ganado]

It's possible to call C/C++ in Matlab https://www.mathworks.com/help/simulink/slref/coder.ceval.html

I assume vector here means column vector. If so, multiplying a column vector of length 5 by a 5x2 matrix isn't possible (V * M, [5x1]*[5x2]). Can we see the Matlab code?

Anyway,
(1) use int main, not void main
(2) Don't dynamically allocate the Vector or Matrix objects themselves.

Prefer to do:

Matrix M = Matrix(2, 3);
Vector V = Vector(3, true);

Vector V_result = M * V;

and go from there. Of course you'll need to change the operator* to take in a const reference to a matrix instead of a const pointer.

Edit: Wait a sec what's your actual issue? What errors are you getting, runtime or compile time?

[Ganado]

class Vector
{
  public:
    double* V;
    unsigned int Size;
    
    Vector(unsigned int Elems, bool ZeroV = true)
    {
        Size = Elems;
        
        if (Size < 1) return;
        
        V = new double[Size];
        
        if (ZeroV)
        {
            for (unsigned int i = 0; i < Size; i++)
                V[i] = 0.0;
        }
    }
    
    ~Vector()
    {
        delete[] V;
    }
};

class Matrix
{
  public:
    double** M;
    unsigned int Rows;
    unsigned int Cols;
    
    Matrix(unsigned int Row, unsigned int Col, bool ZeroM = true)
    {
        Rows = Row; Cols = Col;
    
        if (Rows < 1 || Cols < 1) return;
       
        M = new double*[Rows];
        
        for (unsigned int i = 0; i < Rows; i++)
            M[i] = new double[Cols];
        
        if (ZeroM)
        {
            for (unsigned int i = 0; i < Rows; i++)
            for (unsigned int j = 0; j < Cols; j++)
                M[i][j] = 0.0;
    
        }
    }
    
    ~Matrix()
    {
        for (unsigned int i = 0; i < Rows; i++)
            delete[] M[i];
        
        delete[] M;
    }
};

Vector operator*(const Matrix& mat, const Vector& vec)
{   
    Vector result(mat.Rows, true);
    
    for (unsigned int i = 0; i < mat.Rows; i++)
        for (unsigned int j = 0; j < mat.Cols; j++)
            result.V[i] += mat.M[i][j] * vec.V[j];
        
    return result;
}

#include <iostream>
int main()
{
    Matrix M = Matrix(2, 3);
    M.M[0][0] = 1; // row 0, column 0
    M.M[0][1] = 2; 
    M.M[0][2] = 3; 
    M.M[1][0] = 4; // row 1, column 0
    M.M[1][1] = 5; 
    M.M[1][2] = 6;

    Vector V = Vector(3);
    V.V[0] =  7;
    V.V[1] =  9;
    V.V[2] = 11;
    
    Vector Vnew = M * V;;
    std::cout << Vnew.V[0] << '\n'
              << Vnew.V[1] << '\n';
    
}

Sloppy but here's an example of what I'm talking about, based on your code.
This might no be what you were looking for, so feel free to say so.

PS: To simplify construction, look into std::initializer_list
https://en.cppreference.com/w/cpp/utility/initializer_list
So that you could do something like Matrix m = { {1, 2}, {3, 4 } };