Design by contract using assertions or exceptions? [closed]

The rule of thumb is that you should use assertions when you are trying to catch your own errors, and exceptions when trying to catch other people's errors. In other words, you should use exceptions to check the preconditions for the public API functions, and whenever you get any data that are external to your system. You should use asserts for the functions or data that are internal to your system.

Disabling assert in release builds is like saying "I will never have any issues whatsoever in a release build", which is often not the case. So assert shouldn't be disabled in a release build. But you don't want the release build crashing whenever errors occur either, do you?

So use exceptions and use them well. Use a good, solid exception hierarchy and ensure that you catch and you can put a hook on exception throwing in your debugger to catch it, and in release mode you can compensate for the error rather than a straight-up crash. It's the safer way to go.

The principle I follow is this: If a situation can be realistically avoided by coding then use an assertion. Otherwise use an exception.

Assertions are for ensuring that the Contract is being adhered to. The contract must be fair, so that client must be in a position to ensure it complies. For example, you can state in a contract that a URL must be valid because the rules about what is and isn't a valid URL are known and consistent.

Exceptions are for situations that are outside the control of both the client and the server. An exception means that something has gone wrong, and there's nothing that could have been done to avoid it. For example, network connectivity is outside the applications control so there is nothing that can be done to avoid a network error.

I'd like to add that the Assertion / Exception distinction isn't really the best way to think about it. What you really want to be thinking about is the contract and how it can be enforced. In my URL example above that best thing to do is have a class that encapsulates a URL and is either Null or a valid URL. It is the conversion of a string into a URL that enforces the contract, and an exception is thrown if it is invalid. A method with a URL parameter is much clearer that a method with a String parameter and an assertion that specifies a URL.