?: ternary conditional operator behaviour when leaving one expression empty

This is a GNU C extension (see ?: wikipedia entry), so for portability you should explicitly state the second operand.

In the 'true' case, it is returning the result of the conditional.

The following statements are almost equivalent:

a = x ?: y;
a = x ? x : y;

The only difference is in the first statement, x is always evaluated once, whereas in the second, x will be evaluated twice if it is true. So the only difference is when evaluating x has side effects.

Either way, I'd consider this a subtle use of the syntax... and if you have any empathy for those maintaining your code, you should explicitly state the operand. :)

On the other hand, it's a nice little trick for a common use case.

This is a GCC extension to the C language. When nothing appears between ?:, then the value of the comparison is used in the true case.

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 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.