Is there a general consensus in the C++ community on when exceptions should be used? [closed]

Answer 1

No consensus, but as the replies indicate, many people hold to the view to throw exceptinos only in exceptional circumstances. I don't much like that advice, because then the question becomes "what is an exceptional circumstance?". I prefer the following advice

A function must throw an exception if, and only if, the alternative would be failure to meet a post-condition (including all invariants as implicit post-conditions).

The implementation decision of whether to write throw in your code is then tied to the design decisions of the post-conditions for the function.

See C++ Coding Standards by Herb Sutter and Andrei Alexandrescu, items 70 (Distinguish between errors and non-errors) and 72 (Prefer to use exceptions to report errors).

Answer 2

There is most certainly no consensus. At a high level, my mantra is that exceptions should be used in "exceptional cases" - that is, cases that are not a result of programmer error but of unpredictable conditions in the execution environment.

There are volumes of debate surrounding this issue. Depending on other practices in which you engage when writing programs - RAII for example - exceptions may become more or less idiomatically useful.

Answer 3

Exceptions are easier to use than error codes, because they can be thrown by an deeply nested subroutine and intercepted only at the level where it can be handled.

Error codes need to be passed up the chain so every function that calls another has to pass the error code back to its callers.

An error code doesn't have any clear functional advantages over exceptions since an exception can bundle an error code within it. There may have been a time when error codes might be more efficient than exceptions, but I think the cost of the extra code and the difficulty in maintaining it outweighs any possible advantage there.

However, error codes exist in many applications because they are written in, or were ported from, a language that didn't have exceptions, so it makes sense to keep using a uniform approach to error handling.

Answer 4

Even if there were a common consensus, it wouldn't mean it is valid. Your primary consideration here should be the implementation and performance costs. These are both real constraints on the project and the resulting program. In this respect, here are some things to consider.

At runtime, exceptions are more expensive to propagate than simple return values. This is usually the source of the in exceptional cases argument. If you are throwing a lot of exceptions all the time you are suffering a performance penalty. Don't take this to mean exceptions are slow, they are still implemented very efficiently, but nonetheless costlier than error codes.

Exceptions can carry more information than error codes. Something like the boost::exception library allows tagged information which can provide a wealth of useful information up the chain of the exception. Unlike a simple error code saying file not found an exception can carry the filename, the module that tried to load it, and of course the underlying error code. This type of information is very hard to propagate via error codes up the stack.

Error code propagation can be cumbersome at implementation time. Any function that doesn't want to deal with an error has to pass the value up higher. Quite often you'll find code that simply ignores the error since the programmer couldn't deal with it at the time, or he didn't want to refactor his function signatures to pass along the error.

Exception catch syntax is bulky. It is often far easier to check a return code in an if statement than having to write a catch block. If a function has to catch too many differing exceptions at different points, the meaning of the code will be lost in a sea of braces and try/catch clauses. Here is the likely root of the notion that if a function call can normally fail a return code is likely better than an exception.

Understanding clearly how exceptions work will help you in making a decision. There are performance considerations but for many projects this will be negligible. There are concerns about exception safety, you have to know where it is safe and unsafe to use exceptions. Error codes also have the same safety concerns if people start doing short returns from functions.

Understanding the background of your team can also help out; consider if you have two new C++ programmers, one from a C background and the other coming from Java. Some people are more comfortable with error codes, others with exceptions. How far you push either in each direction will impact your project and contribute to the overall quality.

In the end there is no clear answer. While there are some situations where one may clearly win over the other, it highly depends on the project. Look at your code and continue to refactor when appropriate. Sometimes you won't even have an option as to what you use. Other times it'll make no difference. It is highly project specific.

Answer 5

I like Boost's Error and Exception Handling guidelines. Basically, use exceptions when stack unwinding is desirable. This is simple and unambiguous.

Answer 6

I don't think this is a discussion which is exclusive to the C++ community, but here are two high-level guidelines which have helped me:

1. Throw exceptions only in exceptional circumstances. It sounds obvious, but many APIs get built with exceptions being thrown about 50% of the time they are called (and a boolean return status would have been more appropriate).
2. In your catch clauses, know when to consume exceptions, when to rethrow them as-is and when to throw a different type of exception instead. I can't give you a one-size-fits-all rule for this because it's so dependent on your application's needs, but if you take one thing away from this it should be that the silent consumption of an exception might be the worst thing your code could do. Each frame should have some knowledge of what the calling frame(s) expects when things go wrong.