This is the null-conditional list index operator. It is used to access an element of a list, IF that list is not null. If the list is null, then the access will not happen and the index expression inside the brackets won’t be evaluated. If this is the left-hand side of an assignment operator, such as list?[index] = rhs, then rhs is also not evaluated when the list is null.
var/list/good = list(5,6,7)
var/list/bad
var/idx = 1
// prints null because bad is null; idx is not changed
world << json_encode(bad?[idx++])
// prints 5 because good is not null, and good[1] is 5; idx changes to 2
world << json_encode(good?[1])
idx = 1 // reset idx
// bad is null, so idx does not advance and myproc() is not called
bad?[idx++] = myproc()
// good is not null, so idx advances after read, and myproc() is called
good?[idx++] = myproc()This operator cannot be overloaded, but overloads to the [] operator will apply to this operator as well.