My 2D maze solver recurses infinitely and I get a stack overflow - why?

Question

The Problem :-

I am trying to solve a 2d maze navigation problem in C++ using 2-dimensional array. To give a concise idea about the problem itself, I intend to navigate from node 'S' in the array to node 'G' by walking through free spaces denoted by '.' The nodes '#' are obstacles. One is not allowed to move on spaces denoted as obstacles. Care must also be taken to make all moves as legal moves (within configuration space). I denote the valid move with a '+' after replacement of the '.' If you like to know more about this problem (not necessary) then please refer this link.

What is the issue ?

I coded a recursive algorithm for this problem where we receive an array and a start node position, and then try to navigate to the goal node using recursion. However, I am getting a stack overflow error. It seems like my recursion never stops. I strongly believe there is some problem in my play() function or my check() function. I am not sure what actually is the problem.

What did I try ?

I am reproducing my code below :

void spawn(std::string (&board)[6]) {
    for (int i = 0; i <= 6; i++) {
        std::cout << board[i] << std::endl;
    }
}

bool check(size_t a, size_t b, const std::string (&board)[6]) {
    if (a < board[1].size() && a >= 0 && b < board[1].size() && b >= 0) {
        if (board[a][b] == '#' || board[a][b] == '+')
            return false;
        else if (board[a][b] == '.')
            return true;
    }
    return false;
}

void play(std::string (&board)[6], size_t a, size_t b) {
    auto status = check(a, b, board);
    if (board[a][b] == 'G' || board[a][b] == 'g') {
        spawn(board);
        return;
    }
    if (status) {
        board[a][b] = '+';
        play(board, ++a, b);
        play(board, --a, b);
        play(board, a, ++b);
        play(board, a, --b);
    }
}

int main() {
    std::string grid[6] = {{"S#####"},
                           {".....#"},
                           {"#.####"},
                           {"#.####"},
                           {"...#.G"},
                           {"##...#"}};
    play(grid, 0, 0);
    return 0;
}

Answer 1

The check function prevents recursion because it sees the 'S' in the grid at the starting location. Changing:

else if (board[a][b] == '.')

to

else if (board[a][b] == '.' || board[a][b] == 'S')

got it to work for me.