What's the point of NSAssert, actually?

Question

I have to ask this, because: The only thing I recognize is, that if the assertion fails, the app crashes. Is that the reason why to use NSAssert? Or what else is the benefit

Answer 1

To fully answer his question, the point of any type of assert is to aid debugging. It is more valuable to catch errors at their source, then to catch them in the debugger when they cause crashes.

For example, you may pass a value to a function expects values in a certain range. The function may store the value for later use, and on later use the application crashes. The call stack seen in this scenario would not show the source of the bad value. It's better to catch the bad value as it comes in to find out who's passing the bad value and why.

Answer 2

NSAssert make app crash when it match with the condition. If not match with the condition the next statements will execute. Look for the EX below:

I just create an app to test what is the task of NSAssert is:

    - (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    [self testingFunction:2];
}

-(void)testingFunction: (int)anNum{
    // if anNum < 2 -> the app will crash
    // and the NSLog statement will not execute
    // that mean you cannot see the string: "This statement will execute when anNum < 2"
    // into the log console window of Xcode
    NSAssert(anNum >= 2, @"number you enter less than 2");
    // If anNum >= 2 -> the app will not crash and the below 
    // statement will execute
    NSLog(@"This statement will execute when anNum < 2");
}

into my code the app will not crash.And the test case is:

• anNum >= 2 -> The app will not crash and you can see the log string:"This statement will execute when anNum < 2" into the outPut log console window
• anNum < 2 -> The app will crash and you can not see the log string:"This statement will execute when anNum < 2"

Answer 3

NSAssert gives you more than just crashing the app. It tells you the class, method, and the line where the assertion occurred. All the assertions can also be easily deactivated using NS_BLOCK_ASSERTIONS. Thus making it more suitable for debugging. On the other hand, throwing an NSException only crashes the app. It also does not tell about the location of the exception, nor can it be disabled so simply. See the difference in the images below.

The app crashes because an assertion also raises an exception, as the NSAssert documentation states:

When invoked, an assertion handler prints an error message that includes the method and class names (or the function name). It then raises an NSInternalInconsistencyException exception.

NSAssert:

NSException:

Answer 4

It's worthwhile to point out that aside from run time checking, assert programming is a important facility used when you design your code by contract.

More info on the subject of assertion and design by contract can be found below:

Assertion (software development)

Design by contract

Programming With Assertions

Design by Contract, by Example [Paperback]

Answer 5

Apart from what everyone said above, the default behaviour of NSAssert() (unlike C’s assert()) is to throw an exception, which you can catch and handle. For instance, Xcode does this.

Answer 6

Assertions are commonly used to enforce the intended use of a particular method or piece of logic. Let's say you were writing a method which calculates the sum of two greater than zero integers. In order to make sure the method was always used as intended you would probably put an assert which tests that condition.

Short answer: They enforce that your code is used only as intended.