How does this CheckDraw method work for TicTacToe- C++

Question

I have to determine how exactly the CheckDraw method works but after reviewing the code for hours, I still haven't figured out anything.

bool CheckDraw(bool GameOver)
{
   int n = 1;
   int i = 0;
   int j = 0;
   int counter = 0;

   for( i = 0; i < 3; i++ ) {
      for( j = 0; j < 3; j++ )
         if(table[i][j] == '0'+n)
            counter++;
      n++;
   }
   if( counter < 1 )
   {
      cout << "It's a draw!\n\n";
      GameOver = true;
   }
   return GameOver;
}

Answer 1

I think I've found out how it works...So there's this method in the code where if the player enters a number, that number is replaced with the character assigned to a player (X or O). So this part of the code:

for( i = 0; i < 3; i++ ) {
  for( j = 0; j < 3; j++ )
     if(table[i][j] == '0'+n)

scans through the board for numbers that have not been changed to X or O.

The code below the nested loops starting from "counter++" verifies the number of times the nested loop looped. The nested loop should continue to loop for 3 times, so the counter variable should also increment 2 times and if it looped less than 1 then the game is considered a draw.

This is how I think it should work, if I omitted any mistake, feel free to correct me

Huge thanks to @AlanBirtles for his comment, I suddenly got an idea.