添加标志以引发异常是不好的做法吗？

Question

I have some methods in a class that I think could be useful for testing stuff, but also in some cases the program may want to halt completely if the check fails. Originally I was wrapping the method calls in if conditions and then throwing an exception, however, I ended up having the same if conditions in many methods and it seemed wasteful, so I added a boolean flag to the check method to have it throw an exception if the check failed. An example:

public function isValidDirection($direction, $throwException = false) {
    if(!in_array($direction, $this->getDirections()) && $throwException) {
        throw new \InvalidArgumentException(sprintf('Invalid direction value. Valid directions are: "%s"', implode(", ", $this->getDirections())));
    }    

    return in_array($direction, $this->getDirections());
}

Is this a bad idea? I've not come across this kind of pattern before and I'm wondering are there any pitfalls to it?

Answer 1

An exception should be thrown in exceptional circumstances. That means when your code is in a situation that it is not prepared to handle. If your function is a validation function whose job it is to confirm the validity of data, there should hardly be any exceptional error possible. The job of the function is simple: take input, return true or false depending on whether it's valid. If you want to handle the case of invalid input by throwing an exception and you want to DRY that repetitive check, create another function which wraps your validation function. So you'd have two functions: isValidDirection($input) and assertIsValidDirection($input) , the latter of which throws an exception if it's not valid and otherwise does nothing.

function isValidDirection($input) {
    return ...; // true or false
}

function assertIsValidDirection($input) {
    if (!isValidDirection($input)) {
        throw new InvalidArgumentException;
    }
}

This keeps both functions' responsibilities clear and their implementation simple.

Answer 2

Yeah, it's a bad idea. If the method gets into a state where it can't continue or otherwise shouldn't get into it should throw an exception. It's up to the calling code to handle the exception as it feels is necessary or let the program crash. Otherwise the caller is oblivious to the program failing and will continue and usually make the situation worse/harder to debug.

If, in the case that you don't want to halt your program you can catch the exception and log an error, or handle it some more graceful way depending on your situation. If you don't expect the error at all the program will crash (ideally during testing) and the error won't propagate; you then know exactly where to look to fix it.

So, if I understand it correctly, your code should throw an exception if the $direction isn't in getDirections .

eg:

public function isValidDirection($direction) {
    if(!in_array($direction, $this->getDirections())) {
        throw new \InvalidArgumentException(sprintf('Invalid direction value. Valid directions are: "%s"', implode(", ", $this->getDirections())));
    }    

    return in_array($direction, $this->getDirections());
}

(Also, you don't need to call getDirections 3 times, just call it once and store it in a variable)