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Exceptions vs. status returns
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Created 15 September 2003, last updated 14 October 2003
Broadly speaking, there are two ways to handle errors as they pass from layer to layer in software: throwing exceptions and returning status codes. Almost everyone agrees that exceptions are the better way to do it, but some people still prefer status returns. This article shows why exceptions are better.
The examples here are in C++, because that is the primary battlefield for this argument. Older languages like C don't have exceptions as a real possibility, and newer languages like Java, Python, Ruby, and even Perl provide facilities for exceptions, without the cultural heritage that encouraged status returns in the first place.
Exceptions let you leave error handling code out of much of your code. Exceptions are transmitted automatically through layers that have no knowledge of them, so you can write useful code that has no error handling logic at all. This helps keep the code straightforward and usable.
For example, compare two ways of writing the same simple procedure. With status returns:
And then with exceptions:
Even if the first version is rewritten with macros to hide much of the status return scaffolding, it's uglier and more cluttered than with exceptions:
If the code were more complex than this, the extra noise from the error handling would be much worse. Exceptions keep the code clean.
With status returns, a valuable channel of communication (the return value of the function) has been taken over for error handling. Some methods are so simple, and conceptually return a value, so human temptation takes over, and the method is written to return the value rather than a status code. "It's a simple function, it can't fail, this will be more convenient". Over time, the code grows, and the method gets larger, calling more helper functions, and pretty soon it can fail, but it has no way to express it.
So the return value is overloaded: "If it fails, it returns NULL". Now we have more than one convention in the code (probably more, because your numeric getters will return -1 if they fail), and everything still has to be checked. Your previously failsafe function now has to have all of its call sites updated to check for the new error value.
Why use a technique that begs to be subverted in the first place? Exceptions stay in the background, leaving the most useful tools for the successful cases. Functions can return values, and still have a useful way to fail.
Status returns are typically an integer. A few bits are reserved for flags (for example, a severity indication), and the rest are a large set of enumerated failures. This is a fairly impoverished error value. For example, suppose the failure is that a file could not be found. Which file? A status return can't convey that much information.
Other channels can be developed to carry supplemental information, but typically they are not. In a status return world, the best you can hope for is for the failure site to log a message, and then to return the status. This is simplistic: perhaps the caller knows that it is OK for a file to be missing. If the file opener logs a message, the log will be incorrect (an error will be printed when nothing is wrong). If the file opener doesn't log a message, the caller (or his caller) may have no way of getting the detail on the error needed to print a useful message.
Exceptions are instances of classes, and as such can carry as much information as they need to accomplish their task. Because they can be subclassed, different exceptions can carry different data, allowing for a very rich zoology of error messages.
As an example of the richness exceptions can express, Java defines exceptions as containing a reference to another exception, so that chains of effect can be constructed and retained as the exception moves up through layers of handling. This allows for rich diagnostic information: exception B occurred here, and was caused by exception A occurring there.
Status returns can't even be used with some functions. For example, constructors don't have an explicit return type, and so cannot return a status code. Destructors may not even be explicitly called, never mind that they don't have a return value. In C++, operator overloading and implicit casting (controversial though they are) are other forms of implicit function calls that cannot be checked for return values.
None of these functions can be given status returns. If you don't use exception handling, you have to either come up with some other way of marking errors within them, or pretend that they cannot fail. Simple code may be fail-safe, but code always grows, adding opportunities for failure. Without a way to express the failure, your system will only grow more error-prone and mysterious.
Consider what happens in each technique when a coder slips up.
When a status return goes unchecked, a failure in the called routine will be undetected. The code will continue executing as if that operation had succeeded. There's no way to characterize what might happen at that point, but it is clear that no one will know that an error had occurred. Perhaps the code will visibly fail later on, but that could be many operations later. How will you trace the problem back to the original failure?
If an exception goes uncaught, the exception will travel upward in the call stack either to a higher catch block, or to the uppermost frame where the operating system will do something with it, usually present it to the user. This is not good behavior for the system, but it is visible. You will see an exception, you will be able to diagnose where it was thrown, and where it should have been caught, and you will be able to fix the code.
I'm not covering here the problem of failing to announce a problem (either by returning a failure code or throwing an exception), because that case is a wash for the two techniques. Both are prone to it, and both will fail in similar ways.
So for human error, it comes down to this: human error with status returns results in invisible problems, human error with exceptions results in visible problems. Which would you rather have?
Joel Spolsky has argued that status returns are better. His main argument is that exceptions "are significantly worse than gotos":
This seems like a reasonable argument until you work out what the code would look like with status returns. We aren't arguing here whether functions should be able to fail or not, just what should happen when they do. So all of those possible exit points for a function are still possible exit points, but you have to check the status returns explicitly, and return from the function. So you've traded implicit complexity for explicit complexity, which may not be a good trade. With explicit complexity, you can't see the forest for the trees. Your code is cluttered with the explicit handling of error statuses.
When presented with this explicit complexity, programmers will strive to reduce it. They have two primary ways to do it: hide the error handling, or omit the error handling.
Hiding the error handling is what we did above with the TRY macro. This simply turns your explicit code paths back into implicit code paths, but with the annoying noise of TRY littered all over the place. This is certainly no victory for the "explicit code paths are better" argument for status returns. If you're going to use implicit code paths, at least use exceptions to create them so you have some modern tools at your disposal.
The other solution to the overwhelming explicit complexity in code paths is to simply avoid checking the error returns. Developers will convince themselves (through code inspection or system-level understanding or just plain bad logic) that a certain function always succeeds. This leads to errors going unchecked, which leads to invisible problems.
Status returns are difficult to use. There are places where they are impossible to use. They hijack a useful channel of communication. For all of these reasons, it is easy and tempting to not use them. When not used, they produce silent failures in your system.
Status returns are inferior to exceptions. All modern programming systems provide tools for exception handling. Use them.