Q:

How can I multiply matrices by overloading the * operator, when the operator doesn't match the operands?

I have two matrices which should multiply together by overloading * operator in the constructor class, but the problem here is that no operator [] matches these operands. Why? 
I saw videos and asked my classmates multiple times and tried my own way, but I can't make it work. I only get this error!
This is the code I have a problem with:
The Constructor Code:
I made two ways to make this code works. The result should store at cell matrix or the new matrix:
Matrix operator*(const Matrix &matrix1, const Matrix &matrix2)
  {
    if (matrix1.Cols != matrix2.Rows) {
        throw("Error");
    }
    cell.resize(matrix2.Cols); // one way to call 
    Matrix res(matrix1.Rows, matrix2.Cols, 1.0); // second way to call
    for (int i = 0; i < matrix1.Rows; i++) {
        cell[i].resize(matrix1.Rows);
        for (int j = 0; j < matrix2.Cols; j++) {
            double value_of_elements;
            for (int k = 0; k = matrix1.Cols; k++) {
                res[i][j] += matrix1[i][k] * matrix2[i][j];// 
   1. metod
                value_of_elements += matrix1[i][k] * 
    matrix2[i][j];// 2. metod
            }
            cell[i][j]+=value_of_elements;
        }
    }
    return res;
   }

The Header Code:
The header code normally I don't have unless some modification should be made.
friend Matrix operator*(const Matrix &matrix1, const Matrix &matrix2);

The source Code:
This is where the code is tested:
try {

        Matrix m1(3, 3, 1.0);
        Matrix m2(3, 4, 1.0);

        std::cout << "m1*m2:" << m1 * m2 << std::endl;// this si where the matrix should be multiplied here;

    }
    catch (std::exception &e) {
        std::cout << "Exception: " << e.what() << "!" << std::endl;
    }
    catch (...) {
        std::cout << "Unknown exception caught!" << std::endl;
    }
   system("pause");
   return 0;
}

The result:
The result should be this:
m1*m2:[3, 3, 3, 3
3, 3, 3, 3
3, 3, 3, 3]

What I get is an error; the cause of error are that res[i][j], matrix1[i][k] etc. have operators [] wont work on these operands:
Error   C2065   'cell': undeclared identifier 71  matrix.cpp
Error   C2065   'cell': undeclared identifier 74  matrix.cpp
Error   C2065   'cell': undeclared identifier 81  matrix.cpp
Error   C2088   '[': illegal for class 79   matrix.cpp 
Error   C2088   '[': illegal for class 78   matrix.cpp  
Error   C2676   binary '[': 'Matrix' does not define this operator or a conversion to a type acceptable to the predefined operator  78  matrix.cpp
Error   C2676   binary '[': 'const Matrix' does not define this operator or a conversion to a type acceptable to the predefined operator    78  matrix.cpp
Error   C2676   binary '[': 'const Matrix' does not define this operator or a conversion to a type acceptable to the predefined operator    79  matrix.cpp
Error (active)  E0020   identifier "cell" is undefined  71  Matrix.cpp
Error (active)  E0349   no operator "[]" matches these operands 78  Matrix.cpp
Error (active)  E0349   no operator "[]" matches these operands 78  Matrix.cpp
Error (active)  E0349   no operator "[]" matches these operands 78  Matrix.cpp
Error (active)  E0349   no operator "[]" matches these operands 79  Matrix.cpp
Error (active)  E0349   no operator "[]" matches these operands 79  Matrix.cpp  

A:

Assuming that the class matrix has a member vector<vector<double>> cell, here is a sample that multiply matrices:
Matrix operator*(const Matrix &matrix1, const Matrix &matrix2)
  {
    if (matrix1.Cols != matrix2.Rows) {
        throw("Error");
    }
    Matrix res(matrix1.Rows, matrix2.Cols, 1.0);
    for (int i = 0; i < matrix1.Rows; i++) {
        for (int j = 0; j < matrix2.Cols; j++) {
            double value_of_elements=0;
            for (int k = 0; k = matrix1.Cols; k++)
                value_of_elements += matrix1.cell[i][k] * matrix2.cell[k][j];
            res.cell[i][j]=value_of_elements;
        }
    }
    return res;
}

There was three problems. First the class Matrix doesn't have an operator[]. The problem was solved by accessing the member cell directly. Second, the variable value_of_elements was not initialized, making the result undefined. Third, the matrix multiplication was not done correctly. You multiply one column from matrix1 with one column from matrix2, whereas you should multiply a row by a column.

