What is the Output of 'cout

The Enigma of "cout << a << a": Unmasking Undefined Behavior

A puzzling interview question recently sparked a debate: what is the correct output for the following statement?

Some candidates confidently selected "01," while others reasoned that the behavior was undefined—an answer that initially surprised some.

The crux of the matter lies in the evaluation order and side effects of the statement. To comprehend this, let's expand it into its function call equivalent:

C guarantees that side effects of preceding evaluations will be performed at sequence points. However, there are no sequence points between argument evaluations, leaving the order of argument evaluation uncertain—either a can be evaluated before a or vice versa.

This ambiguity in evaluation order renders the behavior of the statement undefined. However, in the case of a and a, the result of the former is used as the argument for the latter, making the output contingent on the order of evaluation.

C 17 Update: Clarity Amidst the Ambiguity

In C 17, the rules have evolved. A specific statement regarding shift operators was introduced, stipulating that the evaluation and side effects of the left-hand operand must occur before those of the right-hand operand.

By this mandate, the expression "a < a" must produce the output "01" in C 17, resolving the previously undefined behavior.

This refinement in semantics ensures clarity and consistency in the evaluation of expressions involving shift operators, aligning with idiomatic C practices.

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