Possible segmentation fault: Am I using the "this->" operator correctly?

Question

I am doing a homework problem that I have a question about. If you don't feel comfortable assisting with a homework problem, I should say that my instructor has encouraged us to ask for help on this site when we are completely stumped. Also, I have completed the basic portion of the assignment on my own, and am now doing an optional challenge problem. Anyway, on to the problem!

Being new to OOP and C++ in general, I am having trouble understanding the "this->" operator. We haven't covered it in class, but I have seen it elsewhere and I am sort-of guessing how it is meant to be used.

For the assignment, I have to create a console based Tic-Tac-Toe game. Only the challenge portion of the assignment wants us to create an AI opponent, and we don't get any extra credit for doing the challenge, I just want to know how to do it. I am studying things like minimax and game trees, but for now I just wanted to create a "pick a random, open spot" function.

I have a class called TicTacToe which is basically the entire program. I will post it below with the parts that are relevant to the question, but part that is giving me an error is this subroutine:

void TicTacToe::makeAutoMove(){     srand(time(NULL));     int row = rand() % 3 + 1;     int col = rand() % 3 + 1;     if(this->isValidMove(row, col)){         this->makeMove(row, col);     }else{         this->makeAutoMove();     } } 

The only thing that this function is meant to do is make a move on the board, assuming that it is open. The board is set up like:

char board[4][4]; 

and when I print it, it looks like:

   1  2  3 1  -  -  -  2  -  -  - 3  -  -  - 

The problem, is that on occasion a move is made by the computer that gives me an error that is difficult to track down because of the random nature of the function. I believe it is a segfault error, but I can't tell because I can't replicate it in my debugger.

I think that the "this->" operator functions as a pointer, and if a pointer is NULL and it is accessed it could give me this problem. Is this correct? Is there a way to fix this?

I understand that this may be a very low-level question to many of the members of the community, but I would appreciate your help as long as it doesn't come with snide remarks about how trivial this is, or how stupid I must be. I'm LEARNING, which means that I am going to have some silly questions sometimes.

Here is more of my .cpp file if it helps:

TicTacToe::TicTacToe() {     for(int row = 0; row < kNumRows; row++){         for(int col = 0; col < kNumCols; col++){             if(col == 0 && row == 0){                 board[row][col] = ' ';             }else if(col == 0){                 board[row][col] = static_cast<char>('0' + row);             }else if(row == 0){                 board[row][col] = static_cast<char>('0' + col);             }else{                 board[row][col] = '-';             }         }     }     currentPlayer = 'X'; }  char TicTacToe::getCurrentPlayer(){     return currentPlayer; }  char TicTacToe::getWinner(){     //Check for diagonals (Only the middle square can do this)     char middle = board[2][2];     if(board[1][1] == middle && board[3][3] == middle && middle != '-'){         return middle;     }else if(middle == board[3][1] && middle == board[1][3] && middle != '-'){         return middle;     }      //Check for horizontal wins     for(int row = 1; row < kNumRows; row++){         if(board[row][1] == board[row][2] && board[row][2] == board[row][3] && board[row][1] != '-'){             return board[row][1];         }     }      //Check for vertical wins     for(int col = 1; col < kNumCols; col++){         if(board[1][col] == board[2][col] && board[2][col] == board[3][col] && board[1][col] != '-'){             return board[1][col];         }     }      //Otherwise, in the case of a tie game, return a dash.     return '-'; }  void TicTacToe::makeMove(int row, int col){     board[row][col] = currentPlayer;     if(currentPlayer == 'X'){         currentPlayer = 'O';     }else if(currentPlayer == 'O'){         currentPlayer = 'X';     } }  //TODO: Make sure this works after you make the make-move function bool TicTacToe::isDone(){     bool fullBoard = true;     //First check to see if the board is full     for(int col = 1; col < kNumCols; col++){         for(int row = 1; row < kNumRows; row++){             if(board[row][col] == '-'){                 fullBoard = false;             }         }     }      //If the board is full, the game is done. Otherwise check for consecutives.     if(fullBoard){         return true;     }else{         //Check for diagonals (Only the middle square can do this)         char middle = board[2][2];         if(board[1][1] == middle && board[3][3] == middle && middle != '-'){             return true;         }else if(middle == board[3][1] && middle == board[1][3] && middle != '-'){             return true;         }          //Check for horizontal wins         for(int row = 1; row < kNumRows; row++){             if(board[row][1] == board[row][2] && board[row][2] == board[row][3] && board[row][1] != '-'){                 return true;             }         }          //Check for vertical wins         for(int col = 1; col < kNumCols; col++){             if(board[1][col] == board[2][col] && board[2][col] == board[3][col] && board[1][col] != '-'){                 return true;             }         }     }     //If all other tests fail, then the game is not done     return false; }  bool TicTacToe::isValidMove(int row, int col){     if(board[row][col] == '-' && row <= 3 && col <= 3){         return true;     }else{         //cout << "That is an invalid move" << endl;         return false;     } }  void TicTacToe::print(){     for(int row = 0; row < kNumRows; row++){         for(int col = 0; col < kNumCols; col++){             cout << setw(3) << board[row][col];         }         cout << endl;      } } 

Answer 1

A general preface: you almost never need to use this explicitly. In a member function, in order to refer to member variables or member methods, you simply name the variable or method. As with:

class Foo {   int mN; public:   int getIt()   {     return mN; // this->mN legal but not needed   } }; 

I think that the "this->" operator functions as a pointer, and if a pointer is NULL and it is accessed it could give me this problem. Is this correct? Is there a way to fix this?

this is a pointer, yes. (Actually, it's a keyword.) If you call a non-static member function of a class, this points to the object. For instance, if we were to call getIt() above:

int main() {   Foo f;   int a = f.getIt(); } 

then this would point to f from main().

Static member functions do not have a this pointer. this cannot be NULL, and you cannot change the value of this.

There are several cases in C++ where using this is one way to solve a problem, and other cases where this must be used. See this post for a list of these situations.