Simple C++ program to check win in TicTacToe
Computing a square-root is unnecessary; further, it only works when kBoardSize is a perfect square. Prefer

const int kBoardLength = 3;
const int kBoardSize = kBoardLength * kBoardLength;

Your test for unreachability is not complete. You verify that O has made no more moves than X, but you really need to check that the winner made the last move. If X won, then should have numberOfX = numberOfO + 1; if O won then should have numberOfO = numberOfX. It should be clear how to add these checks to your CheckTicTacToeBoard method.
A more overarching tension in this code is: should the board size really be a constant? The code is written so abstractly as to support any board size. But then the board size is fixed to a constant. I would choose one of the following directions to proceed in:


Commit to a board size of 9. In this case, I would recommend hard-coding the win conditions in winDetection. There are only 8 ways to win, so the resulting code would be a lot cleaner.
Allow arbitrary board sizes. In this case, you should accept a boardLength argument. Further, you will have to refine the notion of an UnreachableState. For example, in 5-by-5 tic tac toe, the following state is unreachable, your code would return Xwins:

x x x x x
o . o . o
x x x x x
o . o . o
o o x o o