I'm trying to do:
try{
int * i = NULL;
*i = 3;
}catch(Exception &Err){
ShowMessage(Err.Message);
}
I though that this should catch access violation exception and handle it by displaying an error message.
But for some reason I get simple
Access Violation
message instead of full one
Access Violation XXX in module YYY. Writing at address ZZZ.
BTW, ExceptObject() routine returns NULL for some strange reason.
What am I missing here?
In BCB5, catching an EAccessViolation works, e.g.:
#define AV_TRY { try {
#define AV_CATCH } catch(EAccessViolation &av) {Application->MessageBox((("Access Violation caught: " + string(__FILE__) + "; " + string(__FUNC__) + "; " + IntToString(__LINE__) + "\n\n") + av.Message.c_str()).c_str(), ("Program Error in " + string(class_name.c_str())).c_str(), MB_OK);} }
Note that class_name is specific to this project and should probably be replaced by AnsiString(this->ClassName) or left out. Also I've switched this code from logging silently to a database, to showing a MessageBox. I just wrap code where I've observed AVs in an AV_TRY ... AV_CATCH.
See the MSDN blog entry on Mixing SEH and C++ Exceptions. These are two different types of exceptions. Trying to catch a structured exception, generated by the OS, as a C++ exception isn't the correct way out of the box.
Temper this bit with this posting on not doing that.
Catching access violations can be a nice goal -- but something you may want to do within the context of debugging only. Catching access violations (or other major exceptions) in production code and trying to handle them is seldom going to result in correct operation.
Standard C++ does not specify that dereferencing a NULL pointer throws an exception - it says it results in undefined behaviour. On Windows platforms, the water is muddied somewhat by Windows Structured Exception Handling. This has nothing to do with C++ exception handling, except that some C++ run-times may translate these excptions into C++ exceptions. However, code that depends on such translations is not portable.
try {
int * i = NULL;
*i = 3;
}
catch (...) {
// This would catch the access violation but you don't have any more
// information of what has gone wrong
}
You can however, use structured exception handling (SEH) to catch all C++ exceptions. Since C++ exceptions are just a class based implementation based on SEH.
Related
Suppose I have a supplier and some exception occurs while invoking get method of that supplier.
Supplier<> supplier = () -> getSomething();
ResilienceDecorator.executeSupplier( //Edit 1 - Could be ResilienceDecorator.executeCallable
supplier,
resilienceConfiguration,
throwable -> {
log.error("Exception occured", throwable);
return null;
});
Edit 1 - Also same for ResilienceDecorator.executeCallable.
I need a consistent API to know what went wrong during execution i.e. What was the checked exception (Edit 1 - or Unchecked exception) causing the failure so I can
handle business logic. The throwable is not root cause of exception or whatever the supplier had thrown.
If we do not provide a throwable function like above then
every exception is wrapped in a ResilienceRuntimeException and we need a chain of getCause().getCause() to know what went wrong. This is internal to sdk which might change. Again a consistent API is needed.
Is there any alternative & consistent way to know what exception the supplier is throwing? I might need to bubble it up or write a user friendly message based on the exception occured, depending upon buisness logic.
Edit 1-
An example of my current situation.
Supplier<MatDocItm> supplier = () -> {
ErpHttpDestination erpHttpDestination = DestinationAccessor.getDestination(S4_SYSTEM).asHttp().decorate(DefaultErpHttpDestination::new);
return new CustomCommand(erpHttpDestination, params).execute();
}
try {
MatDocItm = ResilienceDecorator.executeSupplier(
supplier,
configuration
);
} catch (ResilienceRuntimeException e) {
throw new ReadException("Failed to read from SAP S/4HANA", e);
}
The supplier can throw runtime exceptions and based on these exceptions I want to give a User friendly error message with proper description.
The supplier can throw a DestinationAccessException (which is runtime exception) if a destination cannot be accessed, is not configured correctly, or does not fulfill certain prerequisites.
Not only the exceptions occured in supplier, I have TimeLimiterConfiguration & if timeout occurs then a TimeoutException can be thrown.
For above MatDocItm example the ResilienceRuntimeException.getCause() will give com.sap.cloud.sdk.cloudplatform.thread.exception.ThreadContextExecutionException. ResilienceRuntimeException.getCause().getCause() will reveal exception that actually caused the error i.e DestinationAccessException. Now ThreadContextExecutionException is internal to sdk and can change if sdk implementation changes. Will this implementation detail remain frozen and never change in future or is there any alternative way to get the root cause?
I would recommend ExceptionUtils.throwableOfType for matching exception type and sub classes.
Alternatively ExceptionUtils.throwableOfThrowable for matching exact type only.
The utility class already provided by dependency org.apache.commons:commons-lang3:3.10
Example:
#Nullable
YourException cause = ExceptionUtils.throwableOfType(throwable, YourException.class);
You can handle the nullable result in an If statement.
As a fan of fluent APIs, I would rather go with java.util.Optional or io.vavr.control.Option.
I'm running CPLEX through the COIN/OSI interface (OsiCpxSolverInterface). For some large LPs I get an error message CPX0000 CPLEX Error 1001: Out of memory.
Despite the error message, no exception (CoinError) is thrown. Looking at the source code of OsiCpxSolverInterface it looks like the return code of CPXlpopt must have been 0.
To be clear: my question is not how to avoid the out-of-memory problem. I'm just looking for a way to detect it from my program.
I found a hack that works for me. Setting an error handlers log level to 0 only allows error messages through. Overwriting the MessageHandler's print method then allows to react to the error. This workaround is definitely a hack. If anyone has any better suggestions, I'd be happy to accept a different answer.
class ErrorCatchingCoinMessageHandler: public CoinMessageHandler {
public:
ErrorCatchingCoinMessageHandler()
: CoinMessageHandler() {
// Would be nice to also overwrite setLogLevel to avoid later changes
// but its not virtual
setLogLevel(0);
}
virtual int print() __attribute__((noreturn)) {
CoinMessageHandler::print();
abort(); // or throw a CoinError
}
};
// Use it like this
lp_solver->passInMessageHandler(new ErrorCatchingCoinMessageHandler());
First of all, I know the standard answer will be that exceptions are never to be used for flow control. While I perfectly agree with this, I've been thinking a long time about something I sometimes did, which I'll describe with the following pseudo-code:
try
string keyboardInput = read()
int number = int.parse(keyboardInput)
//the conversion succeeds
if(number >= 1000)
//That's not what I asked for. The message to display to the user
//is already in the catch-block below.
throw new NumberFormatException() //well, there IS something wrong with the number...
catch(NumberFormatException ex) //the user entered text
print("Please enter a valid number below 1000.")
First of all, take this example in a very abstract way. This does not necessarily have to happen. The situation simply is:
A user input needs to be constrained and can go wrong in 2 ways,
either
by a thrown exception the language defines, or by a check. Both errors
are reported by the user in the same way, because they do not need to know
the technical difference of what caused it.
I have thought of several ways to solve it. To begin with, it would be better to throw a custom made exception. The problem I then face is, if I catch it locally, what to do with the other exception? In se, the custom exception would be cause for a second catch-block, in which the message would be copied into just as well. My solution:
//number is wrong
throw new MyException()
catch(NumberFormatException ex)
throw new MyException()
catch(MyException ex) {
print("Please enter...")
The meaning of the exceptions' names is everything here. This application of custom-made exceptions is widely accepted, but essentially I didn't do anything different from the first way: I forced to go into a catch-block, albeit by throwing a custom exception rather than a standard-library one.
The same way applied to throwing the exception on to the calling method (thus not having a catch block for the custom exception) seems to make more sense. My method can go wrong in what is technically two ways, but essentially one way: wrong user input. Therefore, one would write a UserInputException and make the method throw this. New problem: what if this is the main method of an application?
I'm not currently struggling with a specific application to implement this kind of behaviour, my question is purely theoretical and non-language specific.
What is the best way to approach this?
I would consider the first exception to be low-level, and I would handle it (by translation in this case) at the point of call. I find that this leads to code that is easier to maintain and refactor later, as you have less types of exceptions to handle.
try
string keyboardInput = read()
try
int number = int.parse(keyboardInput)
catch(NumberFormatException ex)
throw MyException("Input value was not a number")
//the conversion succeeds
if(number >= 1000)
throw MyException("Input value was out of range")
catch(MyException ex) //the user entered text
print( ex.ToString() )
print("Please enter a valid number below 1000.")
I think you have essentially a few ways to go about it with minimal code duplication in mind:
Use a boolean variable/store the exception: If there was an error anywhere in the the general logic of the specific task you are performing, you exit on the first sign of error and handle that in a separate error handling branch.
Advantages: only one place to handle the error; you can use any custom exception/error condition you like.
Disadvantages: the logic of what you are trying to achieve might be hard to discover.
Create a general function that you can use to inform the user about the error (pre-calculating/storing all information that describes the general error, e.g. the message to display the user), so you can just make one function call when an error condition happens.
Advantages: the logic of your intent might be clearer for readers of the code; you can use anu custom exception/error conditon you like.
Disadvantages: the error will have to be handled in separate places (although with the pre-computed/stored values, there is not much copy-paste, however complex the informing the user part).
If the intent is clear, I don't think throwing exceptions from within your try block explicitly is a bad idea. If you do not want to throw one of the system provided exceptions, you can always create your own that derives from one of them, so you only need a minimal number (preferably one) of catch blocks.
Advantages: only one place to handle error condition -- if there is essentially only one type of exception thrown in try-block.
Disadvantages: if more than one type of exception is thrown, you need nested try-catch blocks (to propagate the exceptions to the most outward one) or a very general (e.g. Exception) catch block to avoid having to duplicate error reporting.
The way I see it is this:
Assuming there's no other way to parse your int that doesn't throw an exception, your code as it is now, is correct and elegant.
The only issue would be if your code was in some kind of loop, in which case you might worry about the overhead of throwing and catching unnecessary exceptions. In that case, you will have to compromise some of your code's beauty in favor of only handling exceptions whenever necessary.
error=false;
try {
string keyboardInput = read();
int number = int.parse(keyboardInput);
//the conversion succeeds
if(number >= 1000) {
//That's not what I asked for. The message to display to the user
//is already in the catch-block below.
error=true;
} catch(NumberFormatException ex) { //the user entered text
error=true;
}
if (error)
print("Please enter a valid number below 1000.");
Also you can think about why you're trying to aggregate two errors into one.
Instead you could inform the user as to what error they did, which might be more helpful in some cases:
try {
string keyboardInput = read();
int number = int.parse(keyboardInput);
//the conversion succeeds
if(number >= 1000) {
//That's not what I asked for. The message to display to the user
//is already in the catch-block below.
print("Please enter a number below 1000.");
} catch(NumberFormatException ex) { //the user entered text
print("Please enter a valid number.");
}
You do not need any exceptions in this particular example.
int number;
if (int.TryParse(keyboardInput, out number) && number < 1000) // success
else // error
However, the situation you describe is common in business software, and throwing an exception to reach a uniform handler is quite common.
One such pattern is XML validation followed by XSLT. In some systems, invalid XML is handled through catching validation exceptions. In these systems, it is pretty natural to reuse the existing exception handling in XSLT (which can naturally detect some classes of data errors that a particular validation language cannot):
<xsl:if test="#required = 'yes' and #prohibited = 'yes'>
<xsl:message terminate='yes'>Error message</xsl:message>
</xsl:if>
It is important to see that if such conditions are extremely rare (expected to occur only during early integration testing, and disappear as defects in other modules get fixed), most of the typical concerns around not using exceptions for flow control do not really apply.
What about approaching this validation problem by writing several validator classes that take in an input and return errors, or no errors. As far as your struggle with exceptions: put that logic into each validator and deal with it there on a case by case basis.
after that you figure out the correct validators to use for your input, collect their errors and handle them.
the benefits of this are:
Validators do one thing, validate a single case
Its up to the validation function to decide how to handle the errors. Do you break on first validation error or do you collect them all and then deal with them?
You can write your code is such a way that the main validation function can validate different types of input using the same code, just picking the correct validators using your favorite technique.
and disadvantages:
You will end up writing more code (but if you are using java, this should be put into the 'benefits' bucket)
here is some example pseudo-code:
validate(input):
validators = Validator.for(input.type)
errors = []
for validator in validators:
errors.push(validator.validate(input))
if errors:
throw PoopException
and some validators:
MaxValidator extends IntValidator:
validate(input):
errors = []
errors.push(super.validate(input))
if input > 1000:
errors.push("bleee!!!! to big!")
return errors
IntValidator:
validate(input):
try:
int.parse(input)
catch NumberFormatException:
return ['not an int']
return []
of course you would need to do some trickery to make the parent validator possibly return you a valid version of the input, in this case string "123" converted to an int so the max validator can handle it, but this can be easily accomplished by making the validators statefull or some other magic.
I can't see this answer anywhere in here, so I'll just post it as another point of view.
As we all know, you can actually break the rules if you know them well enough, so you can use throwing an Exception for flow control if you know it's the best solution for your situation. From what I've seen, it happens usually with some dumb frameworks...
That said, before Java 7 (which brought us the mighty multicatch construct), this was my approach to avoid code repetition:
try {
someOffendingMethod();
} catch (Exception e) {
if (e instanceof NumberFormatException || e instanceof MyException) {
System.out.println("Please enter a valid number.");
}
}
It's a valid technique in C#, too.
This is a general programming question, not pertaining to any specific language.
A new programmer typically will write a method that calculates some value, then returns the value:
public Integer doSomething()
{
// Calculate something
return something;
}
public void main()
{
Integer myValue = doSomething();
}
But when an exception occurs during the calculation of something, what is the best way to handle the exception, especially when giving the user feedback? If you do a try/catch of the calculation of something and if an exception is caught, what do you return? Nothing was calculated, so do you return null? And once you return it (whatever it may be), do you need to do another try/catch in the parent method that checks to see if a valid value was returned? And if not, then make sure the user is given some feedback?
I have heard arguments on both sides of the table about never returning values at all, but instead setting calculated values as pointers or global variables and instead returning only error codes from methods, and then (in the parent method) simply handling the error codes accordingly.
Is there a best practice or approach to this? Are there any good resources that one could access to learn more about the best way to handle this?
UPDATE for Clarification
Consider the following code:
public void main()
{
int myValue = getMyValue();
MyUIObject whatever = new MyUIObject();
whatever.displayValue(myValue); // Display the value in the UI or something
}
public Integer getMyValue()
{
try
{
// Calculate some value
} catch (exception e) {
// ??
}
return value;
}
I call the method to get some int value, then I return it. Back in main(), I do something with the value, like show it in the Log in this case. Usually I would display the value in the UI for the user.
Anyways, if an exception is caught in getMyValue(), so does value get returned but it's null? What happens in main() then? Do I have to test if it's a valid value in main() as well?
I need the program to handle the error accordingly and continue. Someone below suggested displaying the appropriate information in the UI from within the getMyValue() method. I see two potential issues:
It seems like I would be mixing the business logic with (in this case) the logic for the UI.
I would have to pass a reference of the MyUIObject to the getMyValue() or something so I could access it from within the function. In the above simple example that is no big deal, but if there is a BUNCH of UI elements that need to be updated or changed based off of what happens in getMyValue(), passing them all might be a bit much...
I've read a bunch about the fundamentals of all of this but I can't seem to find a straight answer for the above situation. I appreciate any help or insight.
I think you do not quite understand exceptions.
If you throw an exception, you do not return from the function normally:
public Integer doSomething()
{
throw new my_exception();
// The following code does NOT get run
return something;
}
public void main()
{
Integer myValue = doSomething();
}
The main advantages of exceptions are:
You can write your code as though everything is succeeding, which is usually clearer
Exceptions are hard to ignore. If an exception is unhandled, typically an obvious and loud error will be given, with a stack trace. This contrasts with error codes, where it is much easier to ignore the error handling than not.
I recommend this post by Eric Lippert, which discusses exceptions and when it is and is not appropriate to handle them.
UPDATE (in response to comment):
You can absolutely handle an exception and continue, you do this by catching the exception.
eg:
try
{
// Perform calculation
}
catch (ExceptionType ex)
{
// A problem of type 'ExceptionType' occurred - you can do whatever you
// want here.
// You could log it to a list, which will be later shown to the user,
// you could set a flag to pop up a dialog box later, etc
}
// The code here will still get run even if ExceptionType was thrown inside
// the try {} block, because we caught and handled that exception.
The nice thing about this is that you know what kind of thing went wrong (from the exception type), as well as details (by looking into the information in ex), so you
hopefully have the information you need to do the right thing.
UPDATE 2 in response to your edit:
You should handle the exception at the layer where you are able to respond in the way you want. For your example, you are correct, you should not be catching the exception so deep down in the code, since you don't have access to the UI, etc and you thus can't really do anything useful.
How about this version of your example code:
public void main()
{
int myValue = -1; // some default value
String error = null; // or however you do it in Java (:
try
{
getMyValue();
}
catch (exception e)
{
error = "Error calculating value. Check your input or something.";
}
if (error != null)
{
// Display the error message to the user, or maybe add it to a list of many
// errors to be displayed later, etc.
// Note: if we are just adding to a list, we could do that in the catch().
}
// Run this code regardless of error - will display default value
// if there was error.
// Alternatively, we could wrap this in an 'else' if we don't want to
// display anything in the case of an error.
MyUIObject whatever = new MyUIObject();
whatever.displayValue(myValue); // Display the value in the UI or something
}
public Integer getMyValue()
{
// Calculate some value, don't worry about exceptions since we can't
// do anything useful at this level.
return value;
}
Exceptions is a property of object oriented languages (OOL). If you use OOL, you should prefer exceptions. It is much better than to return error codes. You can find nice examples how the error-codes approach generates a vastly longer source code than exceptions based code. For example if you want to read a file and do something with it and save in a different format. You can do it in C without exceptions, but your code will be full of if(error)... statements, maybe you will try to use some goto statements, maybe some macros to make it shorter. But also absolutely nontransparent and hard to understand. Also you can often simply forget to test the return value so you don't see the error and program goes on. That's not good. On the other hand if you write in OOL and use exceptions, your source code focuses on "what to do when there is no error", and error handling is in a different place. Just one single error handling code for many possible file errors. The source code is shorter, more clear etc.
I personally would never try to return error codes in object oriented languages. One exception is C++ where the system of exceptions have some limitations.
You wrote:
I have heard arguments on both sides of the table about never returning values at all, but instead setting calculated values as pointers or global variables and instead returning only error codes from methods, and then (in the parent method) simply handling the error codes accordingly.
[EDIT]
Actually, excetion can be seen as error code, which comes along with the relative message, and you as the programmer should know where your exception must be caught, handled and eventually display to the user. As long as you let the exception to propagate (going down in the stack of called functions) no return values are used, so you don't have to care about handling related missed values. Good exception handling is a quite tough issue.
As jwd replied, I don't see the point to raise an exception in a method and then handle the excpetion in the same method just to return an error value. To clarify:
public Integer doSomething(){
try{
throw new my_exception();}
catch{ return err_value;}
}
is pointless.
This is a refactoring question.
try
{
string line = GetFirstLineFromFile(); //Gets first line from a text file, this line would be a number.
int value = ConvertToInteger(line); // Gets the integer value from the string.
int result = DivideByValue(value); // Divides some number with the value retrieved.
}
catch(Exception ex)
{
}
My main concern is, what is the best approach for exception handling in such situations. Certainly wrapping the whole thing in a single try catch is like saying I expect an exception about everything. There must be some place we catch a generic exception right?
Just don't catch a "generic exception".
How can you possibly handle ANY exception and know how to keep your application in a clean state ?
It hides bugs and it's a really bad idea.
Read this serie of posts on catch (Exception).
You need to think about what exceptions can be thrown from the methods in the try block, as well as which ones of those you can deal with at the current level of abstraction.
In your case, I'd expect that the getFirstLineFromFile methods, for example, can definitely throw exceptions you'd want to catch here. Now whether you wrap and rethrow the exception, or take other action, depends on whether you can actually deal with the exception at this level. Consider the case where you have a default file you can fall back to - the approach may just be to log a warning and continue with the default. Or if the whole application is based on reading a file supplied by the user, then this is more likely to be a fatal exception that should be propagated up to the top level and communicated to the user there.
There's no hard-and-fast rule like "always throw" or "never throw"; in general, I consider that one should throw exceptions whenever there's an exceptional-type situation that is not considered a normal result of the method, and consequently cannot be adequately described by the return type of the method. (For example, an isValidDbUser method returning boolean might be able to handle SQLExceptions as just return false; but a getNumProductsRegisteredInDB returning an int should almost certainly propagate an exception).
Don't listen to the (hordes) of people that will tell you that you should never catch multiple exceptions in one big general block. It's a perfectly reasonable way to do general error handling in some cases, which is why the ability to do so exists in the language.
There will be some exceptions that you can do something specific and useful about (i.e. recover from them in the catch block.) These are the kinds of exceptions that you want to catch individually, and as close to the place where they occur as possible.
But the majority of exceptions that you'll face in real life will be completely unexpected, unchecked exceptions. They are the result of programmer error (bugs), failed assertions, failing hardware, dropped network connections, etc.
You should design your software defensively, by designating specific "chokepoints" to handle these unpredictable exceptions with a minimum of disruption to the rest of the application. (Remember, in many cases, "handling" the exception often just means aborting the current operation and logging an error or otherwise telling the user that an unexpected error occurred.)
So for example, if your program saves a file to the disk, you could wrap the entire save operation in a try block to catch things that goes wrong during the save:
try {
// attempt to perform the save operation
doSave();
} catch (Throwable t) {
// tell the user that the save failed for unexpected reasons
// and log the error somewhere
informUser("save failed!");
log("save failed!", t);
} finally {
// last minute cleanup (happens whether save succeeded or failed)
...
}
Notice that we choose a nice chokepoint method here ( doSave() ) and then stop any unexpected errors from bubbling up any further than this point. The chokepoint represents a single, cancellable operation (a save). While that operation is obviously toast if you're getting an unexpected exception, the rest of the application will remain in a good state regardless of what happens on the other side of the chokepoint.
Also notice that this idiom does NOT stop you from handling some of your exceptions further down in doSave() somewhere. So if there are exceptions that might get thrown that you can recover from, or that you want to handle in a special way, go ahead an do so down in doSave(). But for everything else, you have your chokepoint.
You might even want to set up a general uncaught exception handler for your entire program in your main method:
public static void main(String [] args) {
try {
startApplication();
} catch (Throwable t) {
informUser("unexpected error! quitting application");
log("fatal application error", t);
}
But if you've set your other chokepoints up wisely, no exceptions will ever bubble up this far. If you want to make your general error handling complete, you can also create and assign an UncaughtExceptionHandler to important threads, including your main thread or the AWT thread if you are using Swing.
TL;DR; Don't believe the dogma that you should always catch exceptions as specifically as possible. There are times when you want to catch and handle a specific exception, and other times when you want to use chokepoints to catch and deal with "anything else that might go wrong".