Exceptions and error handling

Part of C++ FQA Lite. To see the original answers, follow the FAQ links.

This page is about C++ exceptions - an error handling facility which may be worse than dereferencing a null pointer upon error.

[17.1] What are some ways try / catch / throw can improve software quality?

FAQ: You'll have less if statements in your code: you won't have to check for errors each time you call a function. Conditional statements are known to contain more bugs than other statements. With less if tests, you'll ship a better product, faster.

FQA: This is cargo cult programming. Conditional statements are error-prone because they are used to handle complicated scenarios, where an action can result in many different outcomes, which affect the next actions. In order to make errors less probable, one has to simplify the model of the desired behavior of the software. The problem is the complexity that leads to if statements, not the if keyword, and using different keywords is not going to solve the problem by itself.

Exceptions are supposed to simplify the error handling model based on the assumption that in most cases, a function that detected an error can't handle it, and has to propagate it to the caller. Finally, a "high-level" enough caller is reached and actually makes a decision (pops up an error message, tries a different action, etc.).

Despite its promises, this approach has inherent problems. There's a "social" problem - with exceptions, people are not aware of the different errors that may happen in the code because most of the code doesn't deal with errors. And when people rarely think about a particular aspect of an application, ultimately this aspect is unlikely to be handled well. There's a more "technical" problem - functions essentially doing nothing upon error except for propagating errors to the caller still can't be completely unaware of errors. That's because they may need to release the resources they acquired before returning to the caller, which may lead to more errors, which must also be handled. Finally, in practice exception support has run-time overhead, and very significant code size overhead, even if exceptions are never raised at run time, and even if they are not mentioned in your code. C++ devotees may claim otherwise; you can check by compiling your code with and without exception support (if your compiler doesn't have such a flag, compile code as C and as C++ instead). This is unacceptable in resource-constrained systems.

Still, in many cases, the benefits of exceptions are more important than their problems. For example, if your language manages memory automatically, the problem of releasing acquired resources becomes a small one (you only have to care about files, etc., which are a tiny part of the "resources" used by a program - most of the "resources" are memory). If your language throws exceptions when you violate its rules (for example, upon out-of-bounds array access), these exceptions will help you find lots of bugs, especially if you can get the call stack from an exception. If the purpose of an application is automated testing, and/or it's used as a quick-and-dirty internal tool as opposed to a product for an end user, this kind of exceptions is all you need to handle errors of almost all kinds. In some languages, you can even resume the execution from the point where the exception was raised after fixing the problem at the point where it was caught.

C++ exceptions offer none of these features. "Exception-safe" C++ code can't handle errors which happen when it tries to release resources; "exception-unsafe" C++ code will leak resources, most frequently memory; and once you throw an exception, the call stack is lost. This means that even separating your code to several processes and executing code like *(int*)0 = 0; upon error is a better way to handle errors than C++ exceptions: at least the memory is going to be reclaimed by the operating system, and you can typically have it save a snapshot of the process, so that you can open it in a debugger and see where the error happened. A recommendation to "ban" exceptions is probably over the edge, but think a lot before using C++ exceptions, or a feature that implicitly depends on them, such as constructors and overloaded operators, which have no other way to report an error. What C++ calls "exceptions" is as unlikely to give you the benefits people get from exceptions in other languages as what C++ calls "classes" is unlikely to give you the benefits of OO.

[17.2] How can I handle a constructor that fails?

FAQ: As you'd guess from the location of this question in the FAQ, the answer is "by throwing an exception". Alternatively, you can mark the object as a "zombie" by using some kind of validity flag. You can then check that flag in the calling code and maybe in the member functions of the object. The latter solution tends to "get ugly".

FQA: The inability to gracefully handle errors in C++ constructors is one good reason to avoid constructors that do more than nothing, and use initialization functions instead. And C++ exceptions are not a graceful way to handle errors, especially in constructors. If your member object constructor throws an exception, and you want to catch it in your constructor, the normally ugly colon syntax gets much uglier.

By the way, the C++ standard library doesn't throw exceptions in constructors (except for the ones thrown by operator new). For example, you are supposed to test whether ofstream objects are zombies when you pass them a filename in the constructor.

[17.3] How can I handle a destructor that fails?

FAQ: Actually you can't - not beyond logging the problem to a file or the like. In particular, do not throw an exception. The problem is that destructors are called when exceptions are thrown so that functions propagating errors to their callers can clean up resources. Your destructor can also be called in such a situation. And when an exception is already thrown, throwing another one will result in a call to terminate(), killing your process. Because you see, what else could C++ do? There's an ambiguity: which exception out of the two do you want caught now?

Strictly speaking, you can make that "do not throw exceptions in a destructor unless you are sure that it won't be called as a result of an exception already thrown", but you can rarely be sure of that.

FQA: That's right, terminate(). Solomon-style conflict resolution carried to the end. See? Exceptions are not a graceful way to handle errors.

And "don't throw exceptions in destructors" actually means "don't call functions in destructors unless you are sure they don't throw an exception". The C++ compiler won't check for you, because it can't: the language doesn't force a function to declare whether it throws exceptions.

This is one good reason to avoid destructors doing more than nothing: like constructors and operators, they can't handle errors.

[17.4] How should I handle resources if my constructors may throw exceptions?

FAQ: Well, the destructor of your class will not get called, but the destructors of the successfully constructed sub-objects will get called. Conclusion: you should have all the resources allocated by your constructors assigned to sub-objects. For example, if you call new in a constructor, don't use a bare member pointer to hold the result - use a smart pointer, like std::auto_ptr. You can also define your own smart pointer classes to point to things like disk records! Groovy!

And you can use typedef to make the syntax of using smart pointers easier.

FQA: WARNING - cyclic dependency between C++ features detected! You see, exceptions are a must in this language so that you can handle errors in all the functions which fail to look like functions, such as constructors & operators. Then it turns out that you need constructors to work with exceptions - unless each and every piece of memory you acquire is not immediately assigned to some smart pointer, your code is not exception safe. This is known as "Resource Allocation Is Initialization" (RAII) in the C++ community; it's supposed to be a good thing.

And smart pointers are no picnic, as are virtually all automatic devices with something like "smart", "simple" or "fast" in their name. Sure, you can use typedef to simplify the syntax. So can someone else; you'll end up with many different type names for the same thing. This may annoy people, but it's perfectly OK with the compiler - when it spits an error message, it simply substitutes the full type names for all typedef names. But you can write a program to filter the error messages...

Seriously, the syntax of smart pointers is the small problem. The big problem is their semantics. When you see a bare pointer, you know how it works. But a smart pointer can work in a lot of ways. The boost libraries allow you to instantiate hundreds of different smart pointer classes from a single template (which made it to TR1, so we're going to see it in the next version of the C++ standard). How are you going to figure out whether your program manages resources correctly or not when it's littered with smart pointers of different kinds, especially in case there's any non-trivial scenario there, like the cases when "ownership" (the right & duty to dispose a resource) is passed from object to object, or there are cyclic references in your code, or whatever?

When every single piece of software is "smart", and you can't trust things like *p and p->x, the software becomes unmanageable.

[17.5] How do I change the string-length of an array of char to prevent memory leaks even if/when someone throws an exception?

FAQ: If you want to work with strings, use something like std::string instead of char*. Otherwise, there's lots of exceptions to catch, and lots of code to manage memory.

FQA: The FAQ is right about one thing - char* is a nasty kind of string, and using it for text processing is very tedious. If you're doing anything not entirely trivial with strings, std::string is better than char*; using a different language than C++ for text processing, one with a good built-in string type, is still better.

However, the part with exceptions really comes from operator new, not from char*. You can use malloc instead, or configure your compiler to disable exceptions.

[17.6] What should I throw?

FAQ: C++ allows you to throw objects of arbitrary types; however, you probably shouldn't throw objects of built-in types. For example, you can derive all your exception classes from std::exception, and throw temporary objects of your classes.

FQA: Yep, C++ allows to throw anything. Too bad you can't really catch it later. The only way to catch an arbitrary exception is to use catch(...), which doesn't let you find out what was thrown from where, and will even catch illegal memory access on some systems. This makes finding code like throw "C++ is so grand - you can throw anything!!"; a lot of fun (you have to find it on occasions when the uncaught exception crashes your program).

The FAQ's advice is thus a good one, as opposed to the language decision to allow to throw anything - a typical example of the twisted notion of "generality" used throughout the language design. This decision is completely incomprehensible unless you realize that there's a basic axiom in C++: the language must not force the compiler writer to treat any class specially. For example, having a common base class for all user-defined classes which have at least one virtual function could be quite handy, but it's incompatible with this implicit axiom. What did the C++ designers gain from following this bizarre rule? Apparently nothing, except for an illusion of "generality", whatever that means.

[17.7] What does throw; (without an exception object after the throw keyword) mean? Where would I use it?

FAQ: It means "throw the last caught exception". It may be handy to catch an exception object, add some context information and rethrow it; this way you get something like a stack trace. This feature also allows you to factor out several exception handlers into a function called from a catch(...) block. Inside the function, you list the handlers for various special cases and prefix them with try { throw; }.

FQA: Rethrowing the last exception is a useful feature, and many languages have it. It would be equally useful in C++ if C++ exceptions were any good. In particular, having to use this kind of feature throughout the code to get a call stack is an insult to the language user. Unless it's some kind of "logical" call stack (context information not equivalent to the list of C++ functions you'd see in a debugger at the point where the exception was thrown), call stacks should be provided by the language.

If you are using C++ and want to figure out the current call stack, it may be better to rely on platform-specific tricks (reading the frame pointer using inline assembly and traversing the linked list pointed by it, then translating the instruction pointers using a symbol table) than to litter your code with statements duplicating the information that's already there.

[17.8] How do I throw polymorphically?

FAQ: Suppose you have a BaseEx exception class and a DerivedEx exception class, which is inherited from BaseEx. Than the following code might not work as you expect:

void f(BaseEx& e)
  throw e;
void g()
  DerivedEx e;
  try {
  catch(DerivedEx&) {
    std::cout << "derived exception caught" << std::endl;

The program will not enter the catch block because you didn't throw polymorphically. That is, the statement throw e; throws the object e as a BaseEx, because that's the type of e in that context; once an object is thrown as a BaseEx, it will not get caught as a DerivedEx. If you prefer the other behavior, you can "easily get it" by having a virtual void raise() { throw *this; } in your base class and your derived class, and calling e.raise(); instead of throw e;. This way DerivedEx::raise() is called, and in the context of that function e is of type DerivedEx.

FQA: Let's see. You use C++ exceptions. Moreover, you have a hierarchy of exception classes. Moreover, you pass exception objects to functions, in a way relying on an implicit upcast. Looks like you have lots of confidence in your knowledge of C++ features. But along comes C++ and beats your common sense once again. The startling inconsistency of the language is almost a virtue: maybe this time you will learn the value of simplicity and write something readable.

The behavior of throw, which looks at the static type of its argument expression, is somewhat surprising considering the behavior of catch, which does "respect" inheritance (to the extent made possible by throw). In practice, it is probably better to remove some of the complexity in the example rather than add more complexity by mixing the dynamic binding of virtual with the static binding of throw. A human might need to understand the code, you know.

If you do want to memorize the quirks of C++, try to warp your mind to think in terms used by the compiler construction peanut gallery. From this perspective, the behavior of throw and catch is consistent: both only look at things known at compile time (the relationships between classes), and ignore things only known at run time (the actual type of an object). Basically all of C++ behaves this way except for virtual, dynamic_cast and typeid. I think.

[17.9] When I throw this object, how many times will it be copied?

FAQ: Zero or more. There's no universal answer. The compiler has to make sure a thrown object provides a copy constructor, even if it doesn't actually copy anything.

FQA: If you care about performance, C++ exceptions are probably no good for you. Exception support translates to a huge mountain of code in your executable, and slows down function calls throughout your program. If you didn't care about performance, you wouldn't ask this question. If you think that you care about performance, but never actually measure it or look at the performance implications of the techniques you use in your code, feel free to entertain yourself with any fake answer that suits your emotional needs.

[17.10] Exception handling seems to make my life more difficult; clearly I'm not the problem, am I??

FAQ: Of course you can be the problem!

Here are some habits that may prevent you from utilizing the power of C++ exception handling:

There are other wrong mindsets as well.

FQA: Yeah, you know how it is with those humans. They always fail to realize they are the problem, and keep asking the wrong questions. You give them a helpful and powerful language, and all they do is shooting themselves in the feet. Clearly it's their flawed minds that must be fixed.

Let's look a little closer at the impressive list of "wrong mindsets" compiled by the FAQ:

[17.11] I have too many try blocks; what can I do about it?

FAQ: Maybe you have a "return codes mindset" even though syntactically you use exceptions. There are many special cases of this problem in which you can organize the code differently to reduce the amount of try blocks (the FAQ lists several cases). If you can't solve the problem yourself, get a mentor.

FQA: Alternatively, you can stop throwing C++ exceptions so you won't have to catch them.

Copyright © 2007-2009 Yossi Kreinin
revised 17 October 2009