Sign out from Game Centre via Coding

I want to implement sign in and out functionality for my game. For android version, I have used Google Play Game services and put sign in/out button for that.

For iOS version, I have implemented Game Centre and put sign in/out button for that. What is actual code for sign out from Game Centre?

I have easily found a way for Android version but for iOS I can't found any thing useful. Please give some suggestion over here.

Sign out code for GPG, I want similar code for iOS Game Centre.

 // sign out
 PlayGamesPlatform.Instance.SignOut();