Segmentation fault when passing values to dynamically allocated array - c++

Question

I am working on an assignment that requires me to simulate Langton's ant. I have dynamically allocated memory for a 2D array in the constructor of class Ant. The pointer to this array is a member of class Ant. Additionally, I have defined get functions for the rows and columns which I am using to pass these values to my array.
Ant.hpp
#ifndef ANT_HPP
#define ANT_HPP
enum Direction {UP, RIGHT, DOWN, LEFT};
class Ant
{
private:
char** board;
char spaceColor;
Direction facing;
int rows, cols, steps, stepNumber;
int startRow, startCol, tempCol, tempRow;
int lastRow, lastCol;
int thisRow, thisCol;
public:
Ant();
Ant(int, int, int, int, int);
void print();
void makeMove();
};
Ant.cpp
Ant::Ant()
{
rows = 5;
cols = 5;
steps = 5;
startRow = 0;
startCol = 0;
stepNumber = 0;
facing = LEFT;
setThisRow(5);
setThisCol(5);
setLastRow(5);
setLastCol(5);
setSpaceColor(' ');
board = new char*[rows];
for(int i = 0; i < rows; ++i){
board[i] = new char[cols];
}
for(int i = 0; i < rows; ++i){
for(int i = 0; i < rows; ++i){
board[i][j] = ' ';
}
}
board[startRow][startCol] = '*';
}
char Ant::getSpaceColor()
{
return spaceColor;
}
void Ant::makeMove()
{
if(getSpaceColor() == ' '){
board[getLastRow()][getLastCol()] = '#';
}
}
int Ant::getLastRow()
{
return lastRow;
}
int Ant::getLastCol()
{
return lastCol;
}
main.cpp
#include "Ant.hpp"
#include <iostream>
using std::cout;
using std::endl;
int main()
{
Ant myAnt;
myAnt.print();
myAnt.makeMove();
return 0;
}
Gdb has reported a segmentation fault at this line of code:
board[getLastRow()][getLastCol()] = '#';
Gdb is able to print accurate values for getLastRow() & getLastCol(), but cannot access memory for board[getLastRow()][getLastCol()].
I am not sure what I am doing wrong, any help would be greatly appreciated

Answer 1

Assuming board[getLastRow()][getLastCol()] translates to board[5][5], you go beyond the buffer. Your board is 0..4.

Answer 2

You are invoking undefined behavior by accessing array elements out of bounds. Your setLastRow(5); and
setLastCol(5); functions cause both of your getLastRow() and getLastCol() functions to return the value of 5 but since arrays are zero indexed that would imply you are accessing 6th element. So with:
board[getLastRow()][getLastCol()] = '#';
you are effectively calling:
board[5][5] = '#'; // undefined behavior because there are no 6 X 6 elements
whereas you can only have a maximum index of 4:
board[4][4] = '#'; // OK as there are 5 X 5 elements
One solution is to have your functions return lastRow - 1 and lastCol - 1 respectively.

Answer 3

Segmentation fault usually refers that the program is accessing an unallocated address.
board[getLastRow()][getLastCol()]
You might want to check the starting index and ending index of the arrays.
I think you might allocating the 2D array with index starting from 0
getLastRow()/Col might return the size of the row / Col
So when the index starts from 0, your last row index would be getLastRow()-1 and the same thing would apply for the Column
which yields ==> board[getLastRow()-1][getLastCol()-1]