Safety concerns about short circuit evaluation [duplicate]

Answer 1

/* do some work that may change value of ptr */
if ( 0!=ptr && ptr->some_flag ) {
    /* do something */
}

Is it safe to use this kind of idiom?

Yes.

Answer 2

Or is there any chance that the compiler may generate code that evaluates ptr->some_flag before making sure that ptr is not a zero pointer?

No, zero-chance. It is guaranteed by the standard that ptr->some_flag will not be evaluated if the first operand is false.

6.5.13-4

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

Answer 3

The evaluation order of short-circuit operators (|| and &&) is guaranteed by the standard to be left to right (otherwise they would lose part of their usefulness).

§6.5.13 ¶4

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

§6.5.14 ¶4

Unlike the bitwise | operator, the || operator guarantees left-to-right evaluation; there is a sequence point after the evaluation of the first operand. If the first operand compares unequal to 0, the second operand is not evaluated.

Answer 4

It is safe in the above example, however anyone maintaining such code may not realise that there is a dependency on the order of evaluation and cause an interesting bug.