Is the "shortcut" for evaluating "A && B" part of one of the ISO standards?

Question

If A is false, then A && B is false, so there is no need to check B. So very often, the compiler writes assembly in such a way that if A && B is to be evaluated and A is evaluated as false, then B is not evaluated.

Can I assume that that all compilers which adhere to ISO standard (let's say C99 or more modern), will behave like this?

The reason I ask is that in my program, I have the statement if (i > -1 && x[i] > 3). So if i is negative but x[i] > 3 still gets evaluated, this will lead to a segmentation fault.

Yes, you can definitely rely on this behavior (Boolean short-circuiting) though I don't have a reference handy. — 500 - Internal Server Error, May 02 '22 at 22:21
Yes, but careful not to also assume that other operators (like `+` or `-` for example) will evaluate left-to-right serially. — BadZen, May 02 '22 at 22:22
Try to create a nested if-else statement in which x[i] > 3 will only be evaluated inside of i <= -1 block. — Ivan Silkin, May 02 '22 at 22:24
@IvanSilkin Yes, I was just hoping to avoid this so the code is cleaner. — user56202, May 02 '22 at 22:25

score 4 · Answer 1 · answered May 02 '22 at 22:22

Yes, the ISO C standard does guarantee short-circuit evaluation for the && operator.

§6.5.13 ¶4 of the ISO C11 standard states the following:

Unlike the bitwise binary & operator, the && operator guarantees left-to-right evaluation; if the second operand is evaluated, there is a sequence point between the evaluations of the first and second operands. If the first operand compares equal to 0, the second operand is not evaluated.

Is the "shortcut" for evaluating "A && B" part of one of the ISO standards?

1 Answers1