In the case of voluntary fasting, if I forget and eat is the fasting correct or not?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad, sallallaahu 'alayhi wa sallam, is His slave and Messenger.

Whoever eats or drinks out of forgetfulness while fasting should complete his fast and his fast is valid. He is not required to do anything for this. Besides, there is no difference between a voluntary or obligatory fast concerning the mentioned ruling. The Prophet said: “Whoever forgets that he is fasting, and eats or drinks should complete his fast, as it was Allah who fed him and gave him something to drink”. [Al-Bukhari and Muslim]

Allah knows best.