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Conditionals
with omitted operands
The middle operand in a
conditional expression may be omitted. Then if the first operand is nonzero,
its value is the value of the conditional expression. Therefore, the following
expression has the value of ‘x’
if that is nonzero; otherwise, the value of ‘y’.
x ?:y
This example is perfectly equivalent
to the following.
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.