Conditionals with Omitted Operands
The middle operand in a conditional expression may be omitted. Thenif the first operand is nonzero, its value is the value of the conditionalexpression.
Therefore, the expression
x ?: y
has the value of x if that is nonzero; otherwise, the value of
y.
This example is perfectly equivalent to
x ? x : y
In this simple case, the ability to omit the middle operand is not especially useful. When it becomes useful is when the first operand does,or may (if it is a macro argument), contain a side effect. Then repeating the operand in the middle would perform
the side effect twice. Omitting the middle operand uses the value already computed without the undesirable effects of recomputing it.
x? : y is equal to x?
x:y