Question

Does an if-statement with an && operator check for the second parameter if the first one is false / NO?

Would the following be able to crash?

NSDictionary *someRemoteData = [rawJson valueForKey:@"data"];
if( [someRemoteData isKindOfClass:[NSDictionary class]] && someRemoteData.count > 0 ){
    //..do something
}

Please no simple yes or no answer, but explain why.

Answer 1

No, it does not evaluate the expression after learning that the answer is going to be NO. This is called short-circuiting, and it is an essential part of evaluating boolean expressions in C, C++, Objective C, and other languages with similar syntax. The conditions are evaluated left to right, making the evaluation scheme predictable.

The same rule applies to the || operator: as soon as the code knows that the value is YES, the evaluation stops.

Short-circuiting lets you guard against invalid evaluation in a single composite expression, rather than opting for an if statement. For example,

if (index >= 0 && index < Length && array[index] == 42)

would have resulted in undefined behavior if it were not for short-circuiting. But since the evaluation skips evaluation of array[index] when index is invalid, the above expression is legal.