This is used to access the procs and vars of a prototyped object. It is just like the . operator, but if the object is null, the access does not happen and there will be no runtime error. (A runtime error can still happen if the object is valid but is a different type that doesn’t have the property available.)
When used in an expression to read a value or call a proc from a null object, the result of the expression is null. When used for assignment, the assignment will not happen, and the expression being assigned will not be evaluated, if the object is null.
var/mob/M // M is null by default
M?.name = "futz" // assignment is skipped because M is null
world << M?.name // M?.name reads as null because M is null
M?.Move(loc) // call Move() mob proc; again nothing happens
M = new
M?.name = "futz" // assignment is made because M is valid now
world << M?.name // outputs "futz"
M?.Move(loc) // call Move() mob proc for MWhen reading A?.B, it’s roughly equivalent to A && A.B except that A is only evalulated once, even if it’s a complex expression like a proc call. Making an assignment to A?.B is the same: A is evalulated only once, and if it’s not null then an assignment is made to its B var.
For a version of this operator that doesn’t check at compile time if the property is available, use the ?: operator instead.
If ?. is used after a proc call, a list lookup, or a complex expression where the type can’t be known, it will act like ?: instead.