Getting and Setting 2D Array Object Values (C++)

Question

I'm new to C++ and the way 2D arrays work is confusing me. I've been reading online and trying to understand what's causing my specific issue, but have come up with nothing.

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According to this Stack Overflow answer , I should be able to get a value in my 2D array by doing this: (*myArrayObject)[row][col] , but it throws the following error:

error: invalid types 'int[unsigned int]' for array subscript  
  return (*myArrayObject)[row][col];  
                                  ^  

And if I try to do myArrayObject[row][col] it throws the following error:

error: invalid initialization of non-const reference of types 'double&' from an rvalue of type 'double'  
  return myArrayObject[row][col];
                               ^  

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Here's the full (related/concise) code in question:

main.cpp

#include "matrix.h"

using namespace std;

typedef unsigned int uint;

int main() {

 Matrix * matrix; //This could be the problem, but not sure what else to do
 matrix = new Matrix(10, 1);

 for(uint i = 0; i < matrix->numRows(); ++i) {
   for(uint j = 0; j < matrix->numCols(); ++j) {
     cout << matrix->at(i,j) << " " << endl;
   }
 }  
 return 0;
}

matrix.h

typedef unsigned int uint;  

class Matrix {  
public:
    Matrix(uint rows, uint cols); //Constructor
    const uint numRows() const;                  
    const uint numCols() const;                  

    void setRows(const uint &);                  
    void setCols(const uint &);                 

    double & at(uint row, uint col);
private:
    uint rows, cols;
    int ** matrix; //This could also be the problem, but not sure what else to do

    void makeArray() {
      matrix = new int * [rows];
      for(uint i = 0; i < rows; ++i) {
        matrix[i] = new int [cols];
      }
    }  
};

matrix.cpp

#include "matrix.h"

typedef unsigned int uint;

Matrix::Matrix(uint rows, uint cols) {
  //Make matrix of desired size
  this->setRows(rows);
  this->setCols(cols);

  //Initialize all elements to 0
  for(uint i = 0; i < rows; ++i) {
    for(uint j = 0; j < cols; ++j) {
      this->matrix[i][j] = 0;
    }
  }
}

const uint Matrix::numRows() const {
  return this->rows;
}

const uint Matrix::numCols() const {
  return this->cols;
}

void Matrix::setRows(const uint & rows) {
  this->rows = rows;
}

void Matrix::setCols(const uint & cols) {
  this->cols = cols;
}

double & Matrix::at(uint row, uint col) {
  return matrix[row][col]; //NOT WORKING
}

---

SOLUTION:
Changes made to matrix.h:

double ** matrix;

void makeArray() {
  matrix = new double * [rows];
  for(uint i = 0; i < rows; ++i) {
    matrix[i] = new double [cols];
  }
}  

Changes made to matrix.cpp:
added makeArray() to constructor.

Answer 1

1. As WhozCraig and Rakete1111 said, your problem is, you can't return a reference to double , when your data is all int .

But you may have other problems even after fixing that.

2. You never call your makeArray function and/or in your constructor you never allocate ( new ) your array, like you did in makeArray.

Not a problem yet, but.

3. You don't need any setters for rows and cols values. Or, you need to change these setters in such a way, that a new matrix is reallocated to fit the new sizes.

4. #include is the same thing, as copy & pasting the contents of the named file. And you have a typedef for uint which simply get copy pasted from matrix.h to matrix.cpp and main.cpp, so it would work even, if you don't specify it again.

5. You have a using namespace std , but no std headers included. You may need that thing, eg if you #include <iostream> or <vector> or any other standard library header. Or if for some reason you wrote code in your own namespace std {...} block.

Answer 2

matrix[row][col] is the right way to do it, as matrix is an int** , and applying operator[] two times is like deferencing the pointer two times, and you'll get an individual int .

The reason though why you get an error because at returns a double& . Let me explain, matrix[row][col] returns an int , and this int gets promoted to a double . That double is a temporary variable, and you can't make a reference out of a temporary variable, and that's why the compiler complains.

Making at return an int& would obviously solve it :)