enforce and the Use of assert-enforce

enforce for throwing exceptions #

Not every unexpected situation is an indication of a program error. Programs may also experience unexpected inputs and unexpected environmental states. For example, the data that is entered by the user should not be validated by an assert check because invalid data has nothing to do with the correctness of the program itself. In such cases, it is appropriate to throw exceptions like we have been doing in previous programs.

std.exception.enforce is a convenient way of throwing exceptions. For example, let’s assume that an exception must be thrown when a specific condition is not met:

if (count < 3) {
    throw new Exception("Must be at least 3."); 
}

enforce() is a wrapper around the condition check and the throw statement. The following is the equivalent of the previous code:

import std.exception; 
// ...
    enforce(count >= 3, "Must be at least 3.");

Note: Observe how the logical expression is negated compared to the if statement. It now spells out what is being enforced.

How to use #

assert is for catching programmer errors. The conditions that assert guards against in the monthDays() function and the menuTitle variable above are all about mistakes.
Sometimes it is difficult to decide whether to rely on an assert check or to throw an exception. The decision should be based on whether the unexpected situation is due to a problem with how the program has been coded.

Otherwise, the program must throw an exception when it is not possible to accomplish a task. enforce() is expressive and convenient when throwing exceptions.

Another point to consider is whether the unexpected situation can be remedied in some other way. If the program can not do anything special, such as displaying an error message about the problem with input data, then it is appropriate to throw an exception. That way, callers of the code that threw the exception can catch it to do something special to recover from the error condition.

---

In the next lesson, you will find a coding challenge to test the concepts covered in this chapter.