Disclosing before filing would indeed, I believe, preclude obtaining a patent in most of the world. In the US, though, there is a one year grace period after the inventor publicly discloses the invention during which that disclosure does not automatically make the invention ineligible for a patent.

Of course it would still be better to keep it private until you file, so as to not lose the possibility of patents outside the US, and so as to not subject yourself to the risk that it might take more than a year to get to the point that you are ready to file.