Why is `cout

Undefined Behavior: Understanding cout << a << a

In C , it's imperative to understand the concept of undefined behavior. Undefined behavior refers to situations where the behavior of the program is not specified by the C standard, resulting in unpredictable outcomes.

Consider the following statement:

cout << a++ << a;

Here, we have a sequence of operations involving the increment operator ( ) and the insertion operator (<<). When applying these operations to variable a, we encounter undefined behavior due to the absence of sequence points.

Sequence points are specific points in the code where the compiler guarantees that all side effects have occurred. However, in the statement above, there is no sequence point between the evaluation of a and a. This means that the order in which these operations are performed is not specified by the standard.

As a result, the output of this statement could vary depending on the implementation. In certain cases, the output might be "01", while in others, it could be "10". However, the standard does not guarantee which case will occur, making the behavior undefined.

C 17 Update

It's important to note that the rules governing expression evaluation in C have been updated in C 17. In C 17, the following rule applies to shift operator expressions like << and >>:

"In a shift operator expression E1<>E2, every value computation and side-effect of E1 is sequenced before every value computation and side effect of E2."

This rule eliminates the undefined behavior in the original statement. In C 17, the statement cout << a << a; will always produce the output "01", as the side effects of a (incrementing a) are guaranteed to occur before the evaluation of a.

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