The default assignment is used to assign a value to a variable if the current value of the variable is logically false. The default assignment operator is:
?=
And the syntax is:
variable ?= value expression
When executed the driver tests the logical value of variable. If it is false, it assigns the result of value expression, otherwise it is untouched. Example:
len ?= 79;
Will assign 79 to len if it is false (for instance, if it is zero). This operator is equivalent to a combination of the Simple Assignment and the if-else Conditional Expression:
len = len ? len : 79;
Simple | Scatter | Dual | Default