double question mark in dart code example

Example 1: dart double question mark

// This means a equals b, but if b is null then a equals 'hello'.

String a = b ?? 'hello';


// This means if b is null then set it equal to hello. Otherwise, don't change it.
b ??= 'hello';

Example 2: question mark in dart

childCount: snapshot.data?.length ?? 0,