Related
I have some psuedocode for a function to display a value and get a value
int getValue(){
int value;
// open file
// read line into "value"
if(error occurs){
// if file doesn't open or line was not an integer
/* Normally I would return something such as -1
but -1 in this case would be a valid value*/
value = ?
}
return value;
}
void displayValue(){
int value = getValue();
if(value is valid)
display(value);
}
As described in the code above, I would like to return that there was an error and let displayValue know that there was an error. But i want to accept negative,positive, and 0 from getValue.
Is there a better way to go about this? Does anyone have any advice?
Throw an exception. One of the advantages of C++ over C is that, when you have an error, you don't have to smuggle error codes out of the function, you can just throw an exception. If it's a truly exceptional case, most of the time the caller won't have anything useful to do with it anyway, so forcing them to check for it manually, then pass the error up the call chain is pointless. If they do know what to do with it, they can catch it.
This solution is also more composable. Imagine a scenario where A returns int, B calls A and returns a std::string based on it, and C calls B and returns class Foo based on that. If A has an exceptional condition that requires it to return an error, you can either:
Come up with some way to smuggle the error out of A as an int (or std::optional<int> or std::pair<bool, int> or whatever), then check for and convert that smuggled error to a different smuggled error for B, then check for and convert that to yet another smuggled error for C, then the caller of C still needs to check for that smuggled error and all three layers have to pay the price of the checks every time, even when all three layers succeeded, or...
You throw an exception in A, neither B nor C have anything useful to do with it (so they don't write any additional code at all), and the caller of C can choose to catch the exception and produce a friendlier error message if they so choose.
On modern architectures, the cost in the success case for #2 should be pretty negligible; the failure case might be more costly than the "check at every level case", but for something like "file doesn't exist" or "file contains corrupt data", it hardly matters if performance suffers, since you're probably about to exit the program (so speed doesn't count) or pop a dialog the user needs to respond to (the user is slower than the computer by many orders of magnitude).
There are several error handling approaches in C++:
The traditional way popular in C API's (also used by std::string algorithms) is to reserve at least one value as "invalid", which when returned would signal that there was an error, or that the value represents "no result". In case of error, the C API's would use the global errno to inform what error happened.
One of the features C++ introduced over the C language is exceptions. You can simply throw when error occurs. This is most appropriate for unexpected errors or pre/post-condition violations, rather than "no result exists" type situations.
Yet another way is to return both the value, and information about whether the result is valid. Old fashioned approach might be to return a pair of integer and boolean (or a named class that achieves the same). Alternatively, either value or error state can written into object passed through indirection. std::optional has been introduced into the standard library just for this kind of situation an is a great way of representing lack of result.
Latter approach can be further extended to not only return a boolean, but actual information about the error in similar way to the way exceptions do. The error information can also be wrapped with the value in a "variant" type so that they can share the space, as only one of them can exist at any time. This approach is similar to Maybe type in Haskell. There is a proposal to introduce a template for this purpose into the standard library.
Each of these approaches have their benefits and drawbacks. Choose one that is appropriate for your use case.
One option is to throw an exception when an error occurs. It's highly dependent on the rest of your project. Are Exceptions used all around ? Personally, I prefer more conventional old-school approaches. Mostly because people will start throwing exception everywhere, where it's not really exceptional and then it makes debugging much harder as the debugger keeps stopping for non-exceptional situations.
Another option is to return a pair std::pair<bool, int>. Some people love it, some people hate it.
My preference would be bool attemptGetValue(int& outValue). You return false if there's an error, in which case you don't touch outValue. Your return true otherwise and modify outValue
You can also use std::optional, but old timers might not be familiar wiht it.
Other than throwing an exception, returning a std::optional, or a std::pair, there is a precedent here: std::string::npos is normally set to a particularly large std::string::size_type value, normally -1 (wrapped around of course) and is used by some std::string functions to indicate a failure.
If you're willing to give up one legitimate return value then you could do something similar in your case. In reality though, typical (perhaps all) strings will be significantly smaller than npos; if that's not the case for you then perhaps one of the alternatives already mentioned would be better.
I am designing a cross-platform application protection library to communicate with a dongle. The dongle is able to send and receive blocks of data, to create and delete directories and files inside the dongle's internal file system, etc. To put it simple, the main dongle object has methods to implement all of these functionalities. Unfortunately, any of these methods may fail. Right now I am thinking about what to do if they do :)
The most obvious solution is to return a boolean type result from all of these methods to indicate if they fail or success. However, this leads to a loss of details about the nature of an error.
The next possible solution is to return an error code, but there will be more then 30 types of errors. Also, I will need to design a separate function to convert all of these return codes to a more readable string representation.
The third solution is to return a boolean type result, but to keep an error state object in the main dongle object so it will be possible to retrieve it using GetLastError() like method. I am considering about using this one.
Now is my question. Are there any other reasonable error representation and handling patterns and what do you suggest me to use in my case?
Thanks,
Ilya
Of the options you have presented, the best is to use return codes. Typically a return code of zero indicated success, and each other return code has its own positive integral value. 30 error conditions isn't many. As you've said, you'll have to write code to translate these codes to something human-readable, but you have to do this anyway.
I would consider writing an exception hierarchy to do this instead of error codes, however. Exceptions can be more expressive than return codes, and the code can be cleaner if done properly. You would typically design your library so that there is a different exception class for each type of error return condition, each derived ultimately from std::exception. The what() method gives you a place to put the human-message building, and the type of the exception itself describes the error.
There are those who will tell you that exceptions are "only" for exceptional circumstances, like your computer caught fire, or something. This is a hotly debated assertion. I'll tell you that is nonsense. Just because it is named "exception" doesn't mean your computer has to catch fire in order to use it. Exceptions give you a lot of benefits, one of the biggest being stack unwinding. Preventing yourself from using them simply because of some arbitrary line where errors are "bad enough" is silly.
You could also try using std::exception. It allows you to say what the error is and the nature behind it.
Very often you have a function, which for given arguments can't generate valid result or it can't perform some tasks. Apart from exceptions, which are not so commonly used in C/C++ world, there are basically two schools of reporting invalid results.
First approach mixes valid returns with a value which does not belong to codomain of a function (very often -1) and indicates an error
int foo(int arg) {
if (everything fine)
return some_value;
return -1; //on failure
}
The scond approach is to return a function status and pass the result within a reference
bool foo(int arg, int & result) {
if (everything fine) {
result = some_value;
return true;
}
return false; //on failure
}
Which way do you prefer and why. Does additional parameter in the second method bring notable performance overhead?
Don't ignore exceptions, for exceptional and unexpected errors.
However, just answering your points, the question is ultimately subjective. The key issue is to consider what will be easier for your consumers to work with, whilst quietly nudging them to remember to check error conditions. In my opinion, this is nearly always the "Return a status code, and put the value in a separate reference", but this is entirely one mans personal view. My arguments for doing this...
If you choose to return a mixed value, then you've overloaded the concept of return to mean "Either a useful value or an error code". Overloading a single semantic concept can lead to confusion as to the right thing to do with it.
You often cannot easily find values in the function's codomain to co-opt as error codes, and so need to mix and match the two styles of error reporting within a single API.
There's almost no chance that, if they forget to check the error status, they'll use an error code as if it were actually a useful result. One can return an error code, and stick some null like concept in the return reference that will explode easily when used. If one uses the error/value mixed return model, it's very easy to pass it into another function in which the error part of the co-domain is valid input (but meaningless in the context).
Arguments for returning the mixed error code/value model might be simplicity - no extra variables floating around, for one. But to me, the dangers are worse than the limited gains - one can easily forget to check the error codes. This is one argument for exceptions - you literally can't forget to handle them (your program will flame out if you don't).
boost optional is a brilliant technique. An example will assist.
Say you have a function that returns an double and you want to signify
an error when that cannot be calculated.
double divide(double a, double b){
return a / b;
}
what to do in the case where b is 0;
boost::optional<double> divide(double a, double b){
if ( b != 0){
return a / b;
}else{
return boost::none;
}
}
use it like below.
boost::optional<double> v = divide(a, b);
if(v){
// Note the dereference operator
cout << *v << endl;
}else{
cout << "divide by zero" << endl;
}
The idea of special return values completely falls apart when you start using templates. Consider:
template <typename T>
T f( const T & t ) {
if ( SomeFunc( t ) ) {
return t;
}
else { // error path
return ???; // what can we return?
}
}
There is no obvious special value we can return in this case, so throwing an exception is really the only way. Returning boolean types which must be checked and passing the really interesting values back by reference leads to an horrendous coding style..
Quite a few books, etc., strongly advise the second, so you're not mixing roles and forcing the return value to carry two entirely unrelated pieces of information.
While I sympathize with that notion, I find that the first typically works out better in practice. For one obvious point, in the first case you can chain the assignment to an arbitrary number of recipients, but in the second if you need/want to assign the result to more than one recipient, you have to do the call, then separately do a second assignment. I.e.,
account1.rate = account2.rate = current_rate();
vs.:
set_current_rate(account1.rate);
account2.rate = account1.rate;
or:
set_current_rate(account1.rate);
set_current_rate(account2.rate);
The proof of the pudding is in the eating thereof. Microsoft's COM functions (for one example) chose the latter form exclusively. IMO, it is due largely to this decision alone that essentially all code that uses the native COM API directly is ugly and nearly unreadable. The concepts involved aren't particularly difficult, but the style of the interface turns what should be simple code into an almost unreadable mess in virtually every case.
Exception handling is usually a better way to handle things than either one though. It has three specific effects, all of which are very good. First, it keeps the mainstream logic from being polluted with error handling, so the real intent of the code is much more clear. Second, it decouples error handling from error detection. Code that detects a problem is often in a poor position to handle that error very well. Third, unlike either form of returning an error, it is essentially impossible to simply ignore an exception being thrown. With return codes, there's a nearly constant temptation (to which programmers succumb all too often) to simply assume success, and make no attempt at even catching a problem -- especially since the programmer doesn't really know how to handle the error at that part of the code anyway, and is well aware that even if he catches it and returns an error code from his function, chances are good that it will be ignored anyway.
In C, one of the more common techniques I have seen is that a function returns zero on success, non-zero (typically an error code) on error. If the function needs to pass data back to the caller, it does so through a pointer passed as a function argument. This can also make functions that return multiple pieces of data back to the user more straightforward to use (vs. return some data through a return value and some through a pointer).
Another C technique I see is to return 0 on success and on error, -1 is returned and errno is set to indicate the error.
The techniques you presented each have pros and cons, so deciding which one is "best" will always be (at least partially) subjective. However, I can say this without reservations: the technique that is best is the technique that is consistent throughout your entire program. Using different styles of error reporting code in different parts of a program can quickly become a maintenance and debugging nightmare.
There shouldn't be much, if any, performance difference between the two. The choice depends on the particular use. You cannot use the first if there is no appropriate invalid value.
If using C++, there are many more possibilities than these two, including exceptions and using something like boost::optional as a return value.
C traditionally used the first approach of coding magic values in valid results - which is why you get fun stuff like strcmp() returning false (=0) on a match.
Newer safe versions of a lot of the standard library functions use the second approach - explicitly returning a status.
And no exceptions aren't an alternative here. Exceptions are for exceptional circumstances which the code might not be able to deal with - you don't raise an exception for a string not matching in strcmp()
It's not always possible, but regardless of which error reporting method you use, the best practice is to, whenever possible, design a function so that it does not have failure cases, and when that's not possible, minimize the possible error conditions. Some examples:
Instead of passing a filename deep down through many function calls, you could design your program so that the caller opens the file and passes the FILE * or file descriptor. This eliminates checks for "failed to open file" and report it to the caller at each step.
If there's an inexpensive way to check (or find an upper bound) for the amount of memory a function will need to allocate for the data structures it will build and return, provide a function to return that amount and have the caller allocate the memory. In some cases this may allow the caller to simply use the stack, greatly reducing memory fragmentation and avoiding locks in malloc.
When a function is performing a task for which your implementation may require large working space, ask if there's an alternate (possibly slower) algorithm with O(1) space requirements. If performance is non-critical, simply use the O(1) space algorithm. Otherwise, implement a fallback case to use it if allocation fails.
These are just a few ideas, but applying the same sort of principle all over can really reduce the number of error conditions you have to deal with and propagate up through multiple call levels.
For C++ I favour a templated solution that prevents the fugliness of out parameters and the fugliness of "magic numbers" in combined answers/return codes. I've expounded upon this while answering another question. Take a look.
For C, I find the fugly out parameters less offensive than fugly "magic numbers".
You missed a method: Returning a failure indication and requiring an additional call to get the details of the error.
There's a lot to be said for this.
Example:
int count;
if (!TryParse("12x3", &count))
DisplayError(GetLastError());
edit
This answer has generated quite a bit of controversy and downvoting. To be frank, I am entirely unconvinced by the dissenting arguments. Separating whether a call succeeded from why it failed has proven to be a really good idea. Combining the two forces you into the following pattern:
HKEY key;
long errcode = RegOpenKey(HKEY_CLASSES_ROOT, NULL, &key);
if (errcode != ERROR_SUCCESS)
return DisplayError(errcode);
Contrast this with:
HKEY key;
if (!RegOpenKey(HKEY_CLASSES_ROOT, NULL, &key))
return DisplayError(GetLastError());
(The GetLastError version is consistent with how the Windows API generally works, but the version that returns the code directly is how it actually works, due to the registry API not following that standard.)
In any case, I would suggest that the error-returning pattern makes it all too easy to forget about why the function failed, leading to code such as:
HKEY key;
if (RegOpenKey(HKEY_CLASSES_ROOT, NULL, &key) != ERROR_SUCCESS)
return DisplayGenericError();
edit
Looking at R.'s request, I've found a scenario where it can actually be satisfied.
For a general-purpose C-style API, such as the Windows SDK functions I've used in my examples, there is no non-global context for error codes to rest in. Instead, we have no good alternative to using a global TLV that can be checked after failure.
However, if we expand the topic to include methods on a class, the situation is different. It's perfectly reasonable, given a variable reg that is an instance of the RegistryKey class, for a call to reg.Open to return false, requiring us to then call reg.ErrorCode to retrieve the details.
I believe this satisfies R.'s request that the error code be part of a context, since the instance provides the context. If, instead of a RegistryKey instance, we called a static Open method on RegistryKeyHelper, then the retrieval of the error code on failure would likewise have to be static, which means it would have to be a TLV, albeit not an entirely global one. The class, as opposed to an instance, would be the context.
In both of these cases, object orientation provides a natural context for storing error codes. Having said that, if there is no natural context, I would still insist on a global, as opposed to trying to force the caller to pass in an output parameter or some other artificial context, or returning the error code directly.
I think there is no right answer to this. It depends on your needs, on the overall application design etc. I personally use the first approach though.
I think a good compiler would generate almost the same code, with the same speed. It's a personal preference. I would go on first.
If you have references and the bool type, you must be using C++. In which case, throw an exception. That's what they're for. For a general desktop environment, there's no reason to use error codes. I have seen arguments against exceptions in some environments, like dodgy language/process interop or tight embedded environment. Assuming neither of those, always, always throw an exception.
Well, the first one will compile either in C and C++, so to do portable code it's fine.
The second one, although it's more "human readable" you never know truthfully which value is the program returning, specifying it like in the first case gives you more control, that's what I think.
I prefer using return code for the type of error occured. This helps the caller of the API to take appropriate error handling steps.
Consider GLIB APIs which most often return the error code and the error message along with the boolean return value.
Thus when you get a negative return to a function call, you can check the context from the GError variable.
A failure in the second approach specified by you will not help the caller to take correct actions. Its different case when your documentation is very clear. But in other cases it will be a headache to find how to use the API call.
For a "try" function, where some "normal" type of failure is reasonably expected, how about accepting either a default return value or a pointer to a function which accepts certain parameters related to the failure and returns such a value of the expected type?
Apart from doing it the correct way, which of these two stupid ways do you prefer?
I prefer to use exceptions when I'm using C++ and need to throw an error, and in general, when I don't want to force all calling functions to detect and handle the error. I prefer to use stupid special values when there is only one possible error condition, and that condition means there is no way the caller can proceed, and every conceivable caller will be able to handle it.. which is rare. I prefer to use stupid out parameters when modifying old code and for some reason I can change the number of parameters but not change the return type or identify a special value or throw an exception, which so far has been never.
Does additional parameter in the
second method bring notable
performance overhead?
Yes! Additional parameters cause your 'puter to slow down by at least 0 nanoseconds. Best to use the "no-overhead" keyword on that parameter. It's a GCC extension __attribute__((no-overhead)), so YMMV.
I have an application with some bunch of code like this:
errCode = callMainSystem();
switch (errCode){
case FailErr: error("Err corresponding to val1\n");
case IgnoreErr: error("Err corresponding to val2\n");
...
...
default: error("Unknown error\n");
}
The values are enum constants. Will it make some sense to have something like:
// Error* callMainSystem()
... Some code
return FaileErr(); // or some other error
// handling code
Error* err = callMainSystem();
err->toString();
The Error class may be made singleton as it only has to print error messages.
What are the pros and cons of above methods,size is an important criteria as the application needs to be supported on embedded devices as well.
P.S: I don't want to use exception handling because of portability issues and associated overheads.
When you say:
I don't want to use exception handling
because of portability issues and
associated overheads.
There really aren't any, assuming an even moderately up-to-date compiler. All you are doing is reproducing in your code what the C++ compiler will do for you anyway.
IMHO polymorphism would be overkill here, as you don't actually need different behaviour, only use different data per error code. I would use a simple map<ErrorCode, string> instead to store the mappings from error codes to messages.
The polymorphic solution would require the extra overhead of creation and destruction of Error instances. I can't see how to make Error a singleton; however, you could implement it as a sort of typesafe enum like in Java (i.e. a fixed number of constant instances). However, hardwiring their behaviour (e.g. printing error messages to stderr) can make it awkward to unit test your code.
I've programmed C and C++ for a long time and so far I've never used exceptions and try / catch. What are the benefits of using that instead of just having functions return error codes?
Possibly an obvious point - a developer can ignore (or not be aware of) your return status and go on blissfully unaware that something failed.
An exception needs to be acknowledged in some way - it can't be silently ignored without actively putting something in place to do so.
The advantage of exceptions are two fold:
They can't be ignored. You must deal with them at some level, or they will terminate your program. With error code, you must explicitly check for them, or they are lost.
They can be ignored. If an error can't be dealt with at one level, it will automatically bubble up to the next level, where it can be. Error codes must be explicitly passed up until they reach the level where it can be dealt with.
The advantage is that you don't have to check the error code after each potentially failing call. In order for this to work though, you need to combine it with RAII classes so that everything gets automatically cleaned up as the stack unwinds.
With error messages:
int DoSomeThings()
{
int error = 0;
HandleA hA;
error = CreateAObject(&ha);
if (error)
goto cleanUpFailedA;
HandleB hB;
error = CreateBObjectWithA(hA, &hB);
if (error)
goto cleanUpFailedB;
HandleC hC;
error = CreateCObjectWithA(hB, &hC);
if (error)
goto cleanUpFailedC;
...
cleanUpFailedC:
DeleteCObject(hC);
cleanUpFailedB:
DeleteBObject(hB);
cleanUpFailedA:
DeleteAObject(hA);
return error;
}
With Exceptions and RAII
void DoSomeThings()
{
RAIIHandleA hA = CreateAObject();
RAIIHandleB hB = CreateBObjectWithA(hA);
RAIIHandleC hC = CreateCObjectWithB(hB);
...
}
struct RAIIHandleA
{
HandleA Handle;
RAIIHandleA(HandleA handle) : Handle(handle) {}
~RAIIHandleA() { DeleteAObject(Handle); }
}
...
On first glance, the RAII/Exceptions version seems longer, until you realize that the cleanup code needs to be written only once (and there are ways to simplify that). But the second version of DoSomeThings is much clearer and maintainable.
DO NOT try and use exceptions in C++ without the RAII idiom, as you will leak resources and memory. All your cleanup needs to be done in destructors of stack-allocated objects.
I realize there are other ways to do the error code handling, but they all end up looking somewhat the same. If you drop the gotos, you end up repeating clean up code.
One point for error codes, is that they make it obvious where things can fail, and how they can fail. In the above code, you write it with the assumption that things are not going to fail (but if they do, you'll be protected by the RAII wrappers). But you end up paying less heed to where things can go wrong.
Exception handling is useful because it makes it easy to separate the error handling code from the code written to handle the function of the program. This makes reading and writing the code easier.
return an error code when an error condition is expected in some cases
throw an exception when an error condition is not expected in any cases
in the former case the caller of the function must check the error code for the expected failure; in the latter case the exception can be handled by any caller up the stack (or the default handler) as is appropriate
Aside from the other things that were mentioned, you can't return an error code from a constructor. Destructors either, but you should avoid throwing an exception from a destructor too.
I wrote a blog entry about this (Exceptions make for Elegant Code), which was subsequently published in Overload. I actually wrote this in response to something Joel said on the StackOverflow podcast!
Anyway, I strongly believe that exceptions are preferable to error codes in most circumstances. I find it really painful to use functions that return error codes: you have to check the error code after each call, which can disrupt the flow of the calling code. It also means you can't use overloaded operators as there is no way to signal the error.
The pain of checking error codes means that people often neglect to do so, thus rendering them completely pointless: at least you have to explicitly ignore exceptions with a catch statement.
The use of destructors in C++ and disposers in .NET to ensure that resources are correctly freed in the presence of exceptions can also greatly simplify code. In order to get the same level of protection with error codes you either need lots of if statements, lots of duplicated cleanup code, or goto calls to a common block of cleanup at the end of a function. None of these options are pleasant.
Here's a good explanation of EAFP ("Easier to Ask for Forgiveness than Permission."), which I think applies here even if it's a Python page in Wikipedia. Using exceptions leads to a more natural style of coding, IMO -- and in the opinion of many others, too.
When I used to teach C++, our standard explanation was that they allowed you to avoid tangling sunny-day and rainy-day scenarios. In other words, you could write a function as if everything would work ok, and catch the exception in the end.
Without exceptions, you would have to get a return value from each call and ensure that it is still legitimate.
A related benefit, of course, is that you don't "waste" your return value on exceptions (and thus allow methods that should be void to be void), and can also return errors from constructors and destructors.
Google's C++ Style Guide has a great, thorough analysis of the pros and cons of exception use in C++ code. It also indicates some of the larger questions you should be asking; i.e. do I intend to distribute my code to others (who may have difficulty integrating with an exception-enabled code base)?
Sometimes you really have to use an exception in order to flag an exceptional case. For example, if something goes wrong in a constructor and you find it makes sense to notify the caller about this then you have no choice but to throw an exception.
Another example: Sometimes there is no value your function can return to denote an error; any value the function may return denotes success.
int divide(int a, int b)
{
if( b == 0 )
// then what? no integer can be used for an error flag!
else
return a / b;
}
The fact that you have to acknowledge exceptions is correct but this can also be implemented using error structs.
You could create a base error class that checks in its dtor whether a certain method ( e.g. IsOk ) has been called. If not, you could log something and then exit, or throw an exception, or raise an assert, etc...
Just calling the IsOk on the error object without reacting to it, would then be the equivalent of writing catch( ... ) {}
Both statement would display the same lack of programmer good will.
The transport of the error code up to the correct level is a greater concern. You would basically have to make almost all methods return an error code for the sole reason of propagation.
But then again, a function or method should always be annotated with the exceptions it can generate. So basically you have to same problem, without an interface to support it.
As #Martin pointed out throwing exceptions forces the programmer to handle the error. For example, not checking return codes is one of the biggest sources of security holes in C programs. Exceptions make sure that you handle the error (hopefully) and provide some kind of recover path for your program. And if you choose to ignore an exception rather than introduce a security hole your program crashes.