throwing an exception in objective-c cocoa

Question

What s the best way to throw an exception in objective-c cocoa

User · Answer

I believe you should never use Exceptions to control normal program flow. But exceptions should be thrown whenever some value doesn't match a desired value.

For example if some function accepts a value, and that value is never allowed to be nil, then it's fine to trow an exception rather then trying to do something 'smart'...

Ries

User · Answer

I believe you should never use Exceptions to control normal program flow. But exceptions should be thrown whenever some value doesn't match a desired value.

For example if some function accepts a value, and that value is never allowed to be nil, then it's fine to trow an exception rather then trying to do something 'smart'...

Ries

User · Answer

Sample code for case   throw  NSException exceptionWithName      -  void parseError  NSError   error        completionBlock  void     NSString  error  completionBlock         NSString  resultString    NSString new         try        NSData  errorData    NSData dataWithData error userInfo   SomeKeyForData          if  errorData bytes              throw  NSException exceptionWithName    lt Set Yours exc  name   gt  Test Exc  reason    lt Describe reason   gt  Doesn t contain data  userInfo nil                NSDictionary  dictFromData    NSJSONSerialization JSONObjectWithData errorData                                                                  options NSJSONReadingAllowFragments                                                                    error  amp error        resultString   dictFromData   someKey                 catch  NSException  exception             NSLog    Caught Exception Name       exception name           NSLog    Caught Exception Reason       exception reason         resultString   exception reason      finally        completionBlock resultString          Using    self parseError error completionBlock   NSString  error                NSLog        error                 Another more advanced use-case   -  void parseError  NSError   error completionBlock  void     NSString  error  completionBlock    NSString  resultString    NSString new    NSException  customNilException    NSException exceptionWithName   NilException                                                            reason   object is nil                                                          userInfo nil    NSException  customNotNumberException    NSException exceptionWithName   NotNumberException                                                                  reason   object is not a NSNumber                                                                userInfo nil     try        NSData  errorData    NSData dataWithData error userInfo   SomeKeyForData          if  errorData bytes              throw  NSException exceptionWithName    lt Set Yours exc  name   gt  Test Exc  reason    lt Describe reason   gt  Doesn t contain data  userInfo nil                NSDictionary  dictFromData    NSJSONSerialization JSONObjectWithData errorData                                                                  options NSJSONReadingAllowFragments                                                                    error  amp error        NSArray   array   dictFromData   someArrayKey         for  NSInteger i 0  i  lt  array count  i               id resultString   array i            if    resultString isKindOfClass NSNumber class                   customNotNumberException raise       lt        HERE is just the same as   throw customNotNumberException               break             else if   resultString                 throw customNilException             lt                     break                       catch  SomeCustomException   sce           most specific type        handle exception ce              catch  CustomException   ce           most specific type        handle exception ce              catch  NSException  exception           less specific type         do whatever recovery is necessary at his level                  rethrow the exception so it s handled at a higher level       throw  SomeCustomException   customException       finally          perform tasks necessary whether exception occurred or not

User · Answer

throw  NSException exceptionWith        Xcode recognizes  throw statements as function exit points  like return statements  Using the  throw syntax avoids erroneous  Control may reach end of non-void function  warnings that you may get from  NSException raise        Also   throw can be used to throw objects that are not of class NSException

User · Answer

Regarding  NSException raise format    For those coming from a Java background  you will recall that Java distinguishes between Exception and RuntimeException  Exception is a checked exception  and RuntimeException is unchecked  In particular  Java suggests using checked exceptions for  normal error conditions  and unchecked exceptions for  runtime errors caused by a programmer error   It seems that Objective-C exceptions should be used in the same places you would use an unchecked exception  and error code return values or NSError values are preferred in places where you would use a checked exception

User · Answer

I think to be consistant it s nicer to use  throw with your own class that extends NSException  Then you use the same notations for try catch finally    try            catch         finally          Apple explains here how to throw and handle exceptions  Catching Exceptions Throwing Exceptions

User · Answer

I think to be consistant it s nicer to use  throw with your own class that extends NSException  Then you use the same notations for try catch finally    try            catch         finally          Apple explains here how to throw and handle exceptions  Catching Exceptions Throwing Exceptions

User · Answer

You should only throw exceptions if you find yourself in a situation that indicates a programming error  and want to stop the application from running  Therefore  the best way to throw exceptions is using the NSAssert and NSParameterAssert macros  and making sure that NS BLOCK ASSERTIONS is not defined

User · Answer

There is no reason not to use exceptions normally in objective C even to signify business rule exceptions   Apple can say use NSError who cares  Obj C has been around a long time and at one time ALL C   documentation said the same thing   The reason it doesnt matter how expensive throwing and catching an exception is  is the lifetime of an exception is exceedingly short and   its an EXCEPTION to the normal flow  I have never heard anyone say ever in my life  man that exception took a long time to be thrown and caught    Also  there are people that think that objective C itself is too expensive and code in C or C   instead   So saying always use NSError is ill-informed and paranoid   But the question of this thread hasnt yet been answered whats the BEST way to throw an exception   The ways to return NSError are obvious   So is it   NSException raise        throw   NSException alloc  initWithName     or  throw   MyCustomException       I use the checked unchecked rule here slightly differently than above   The real difference between the  using the java metaphor here  checked unchecked is important --  whether you can recover from the exception   And by recover I mean not just NOT crash    So I use custom exception classes with  throw for recoverable exceptions  because its likely I will have some app method looking for certain types of failures in multiple  catch blocks   For example if my app is an ATM machine  I would have a  catch block for the  WithdrawalRequestExceedsBalanceException    I use NSException raise for runtime exceptions since I have no way to recover from the exception  except to catch it at a higher level and log it   And theres no point in creating a custom class for that   Anyway thats what I do  but if there s a better  similarly expressive way I would like to know as well  In my own code  since I stopped coding C a hella long time ago I never return an NSError even if I am passed one by an API

User · Answer

Since ObjC 2 0  Objective-C exceptions are no longer a wrapper for C s setjmp   longjmp    and are compatible with C   exception  the  try is  free of charge   but throwing and catching exceptions is way more expensive   Anyway  assertions  using NSAssert and NSCAssert macro family  throw NSException  and that sane to use them as Ries states

User · Answer

Use NSError to communicate failures rather than exceptions   Quick points about NSError    NSError allows for C style error codes  integers  to clearly identify the root cause and hopefully allow the error handler to overcome the error   You can wrap error codes from C libraries like SQLite in NSError instances very easily  NSError also has the benefit of being an object and offers a way to describe the error in more detail with its userInfo dictionary member  But best of all  NSError CANNOT be thrown so it encourages a more proactive approach to error handling  in contrast to other languages which simply throw the hot potato further and further up the call stack at which point it can only be reported to the user and not handled in any meaningful way  not if you believe in following OOP s biggest tenet of information hiding that is     Reference Link   Reference

User · Answer

This is how I learned it from  The Big Nerd Ranch Guide  4th edition      throw  NSException exceptionWithName   Something is not right exception                                 reason   Can t perform this operation because of this or that                               userInfo nil

User · Answer

A word of caution here  In Objective-C  unlike many similar languages  you generally should try to avoid using exceptions for common error situations that may occur in normal operation   Apple s documentation for Obj-C 2 0 states the following   Important  Exceptions are resource-intensive in Objective-C  You should not use exceptions for general flow-control  or simply to signify errors  such as a file not being accessible    Apple s conceptual Exception handling documentation explains the same  but with more words   Important  You should reserve the use of exceptions for programming or unexpected runtime errors such as out-of-bounds collection access  attempts to mutate immutable objects  sending an invalid message  and losing the connection to the window server  You usually take care of these sorts of errors with exceptions when an application is being created rather than at runtime          Instead of exceptions  error objects  NSError  and the Cocoa error-delivery mechanism are the recommended way to communicate expected errors in Cocoa applications    The reasons for this is partly to adhere to programming idioms in Objective-C  using return values in simple cases and by-reference parameters  often the NSError class  in more complex cases   partly that throwing and catching exceptions is much more expensive and finally  and perpaps most importantly  that Objective-C exceptions are a thin wrapper around C s setjmp   and longjmp   functions  essentially messing up your careful memory handling  see this explanation

User · Answer

You can use two methods for raising exception in the try catch block    throw NSException exceptionWithName     or the second method   NSException e   e raise

User · Answer

Regarding  NSException raise format    For those coming from a Java background  you will recall that Java distinguishes between Exception and RuntimeException  Exception is a checked exception  and RuntimeException is unchecked  In particular  Java suggests using checked exceptions for  normal error conditions  and unchecked exceptions for  runtime errors caused by a programmer error   It seems that Objective-C exceptions should be used in the same places you would use an unchecked exception  and error code return values or NSError values are preferred in places where you would use a checked exception

User · Answer

Use NSError to communicate failures rather than exceptions   Quick points about NSError    NSError allows for C style error codes  integers  to clearly identify the root cause and hopefully allow the error handler to overcome the error   You can wrap error codes from C libraries like SQLite in NSError instances very easily  NSError also has the benefit of being an object and offers a way to describe the error in more detail with its userInfo dictionary member  But best of all  NSError CANNOT be thrown so it encourages a more proactive approach to error handling  in contrast to other languages which simply throw the hot potato further and further up the call stack at which point it can only be reported to the user and not handled in any meaningful way  not if you believe in following OOP s biggest tenet of information hiding that is     Reference Link   Reference

User · Answer

A word of caution here  In Objective-C  unlike many similar languages  you generally should try to avoid using exceptions for common error situations that may occur in normal operation   Apple s documentation for Obj-C 2 0 states the following   Important  Exceptions are resource-intensive in Objective-C  You should not use exceptions for general flow-control  or simply to signify errors  such as a file not being accessible    Apple s conceptual Exception handling documentation explains the same  but with more words   Important  You should reserve the use of exceptions for programming or unexpected runtime errors such as out-of-bounds collection access  attempts to mutate immutable objects  sending an invalid message  and losing the connection to the window server  You usually take care of these sorts of errors with exceptions when an application is being created rather than at runtime          Instead of exceptions  error objects  NSError  and the Cocoa error-delivery mechanism are the recommended way to communicate expected errors in Cocoa applications    The reasons for this is partly to adhere to programming idioms in Objective-C  using return values in simple cases and by-reference parameters  often the NSError class  in more complex cases   partly that throwing and catching exceptions is much more expensive and finally  and perpaps most importantly  that Objective-C exceptions are a thin wrapper around C s setjmp   and longjmp   functions  essentially messing up your careful memory handling  see this explanation

User · Answer

Since ObjC 2 0  Objective-C exceptions are no longer a wrapper for C s setjmp   longjmp    and are compatible with C   exception  the  try is  free of charge   but throwing and catching exceptions is way more expensive   Anyway  assertions  using NSAssert and NSCAssert macro family  throw NSException  and that sane to use them as Ries states

User · Answer

I think to be consistant it s nicer to use  throw with your own class that extends NSException  Then you use the same notations for try catch finally    try            catch         finally          Apple explains here how to throw and handle exceptions  Catching Exceptions Throwing Exceptions

User · Answer

This is how I learned it from  The Big Nerd Ranch Guide  4th edition      throw  NSException exceptionWithName   Something is not right exception                                 reason   Can t perform this operation because of this or that                               userInfo nil

User · Answer

You should only throw exceptions if you find yourself in a situation that indicates a programming error  and want to stop the application from running  Therefore  the best way to throw exceptions is using the NSAssert and NSParameterAssert macros  and making sure that NS BLOCK ASSERTIONS is not defined

User · Answer

A word of caution here  In Objective-C  unlike many similar languages  you generally should try to avoid using exceptions for common error situations that may occur in normal operation   Apple s documentation for Obj-C 2 0 states the following   Important  Exceptions are resource-intensive in Objective-C  You should not use exceptions for general flow-control  or simply to signify errors  such as a file not being accessible    Apple s conceptual Exception handling documentation explains the same  but with more words   Important  You should reserve the use of exceptions for programming or unexpected runtime errors such as out-of-bounds collection access  attempts to mutate immutable objects  sending an invalid message  and losing the connection to the window server  You usually take care of these sorts of errors with exceptions when an application is being created rather than at runtime          Instead of exceptions  error objects  NSError  and the Cocoa error-delivery mechanism are the recommended way to communicate expected errors in Cocoa applications    The reasons for this is partly to adhere to programming idioms in Objective-C  using return values in simple cases and by-reference parameters  often the NSError class  in more complex cases   partly that throwing and catching exceptions is much more expensive and finally  and perpaps most importantly  that Objective-C exceptions are a thin wrapper around C s setjmp   and longjmp   functions  essentially messing up your careful memory handling  see this explanation

User · Answer

I use  NSException raise format   as follows    NSException raise   Invalid foo value  format   foo of  d is invalid   foo

User · Answer

You can use two methods for raising exception in the try catch block    throw NSException exceptionWithName     or the second method   NSException e   e raise

User · Answer

throw  NSException exceptionWith        Xcode recognizes  throw statements as function exit points  like return statements  Using the  throw syntax avoids erroneous  Control may reach end of non-void function  warnings that you may get from  NSException raise        Also   throw can be used to throw objects that are not of class NSException

User · Answer

I think to be consistant it s nicer to use  throw with your own class that extends NSException  Then you use the same notations for try catch finally    try            catch         finally          Apple explains here how to throw and handle exceptions  Catching Exceptions Throwing Exceptions

User · Answer

I use  NSException raise format   as follows    NSException raise   Invalid foo value  format   foo of  d is invalid   foo

User · Answer

I use  NSException raise format   as follows    NSException raise   Invalid foo value  format   foo of  d is invalid   foo

User · Answer

A word of caution here  In Objective-C  unlike many similar languages  you generally should try to avoid using exceptions for common error situations that may occur in normal operation   Apple s documentation for Obj-C 2 0 states the following   Important  Exceptions are resource-intensive in Objective-C  You should not use exceptions for general flow-control  or simply to signify errors  such as a file not being accessible    Apple s conceptual Exception handling documentation explains the same  but with more words   Important  You should reserve the use of exceptions for programming or unexpected runtime errors such as out-of-bounds collection access  attempts to mutate immutable objects  sending an invalid message  and losing the connection to the window server  You usually take care of these sorts of errors with exceptions when an application is being created rather than at runtime          Instead of exceptions  error objects  NSError  and the Cocoa error-delivery mechanism are the recommended way to communicate expected errors in Cocoa applications    The reasons for this is partly to adhere to programming idioms in Objective-C  using return values in simple cases and by-reference parameters  often the NSError class  in more complex cases   partly that throwing and catching exceptions is much more expensive and finally  and perpaps most importantly  that Objective-C exceptions are a thin wrapper around C s setjmp   and longjmp   functions  essentially messing up your careful memory handling  see this explanation

User · Answer

throw  NSException exceptionWith        Xcode recognizes  throw statements as function exit points  like return statements  Using the  throw syntax avoids erroneous  Control may reach end of non-void function  warnings that you may get from  NSException raise        Also   throw can be used to throw objects that are not of class NSException

User · Answer

I use  NSException raise format   as follows    NSException raise   Invalid foo value  format   foo of  d is invalid   foo

User · Answer

Sample code for case   throw  NSException exceptionWithName      -  void parseError  NSError   error        completionBlock  void     NSString  error  completionBlock         NSString  resultString    NSString new         try        NSData  errorData    NSData dataWithData error userInfo   SomeKeyForData          if  errorData bytes              throw  NSException exceptionWithName    lt Set Yours exc  name   gt  Test Exc  reason    lt Describe reason   gt  Doesn t contain data  userInfo nil                NSDictionary  dictFromData    NSJSONSerialization JSONObjectWithData errorData                                                                  options NSJSONReadingAllowFragments                                                                    error  amp error        resultString   dictFromData   someKey                 catch  NSException  exception             NSLog    Caught Exception Name       exception name           NSLog    Caught Exception Reason       exception reason         resultString   exception reason      finally        completionBlock resultString          Using    self parseError error completionBlock   NSString  error                NSLog        error                 Another more advanced use-case   -  void parseError  NSError   error completionBlock  void     NSString  error  completionBlock    NSString  resultString    NSString new    NSException  customNilException    NSException exceptionWithName   NilException                                                            reason   object is nil                                                          userInfo nil    NSException  customNotNumberException    NSException exceptionWithName   NotNumberException                                                                  reason   object is not a NSNumber                                                                userInfo nil     try        NSData  errorData    NSData dataWithData error userInfo   SomeKeyForData          if  errorData bytes              throw  NSException exceptionWithName    lt Set Yours exc  name   gt  Test Exc  reason    lt Describe reason   gt  Doesn t contain data  userInfo nil                NSDictionary  dictFromData    NSJSONSerialization JSONObjectWithData errorData                                                                  options NSJSONReadingAllowFragments                                                                    error  amp error        NSArray   array   dictFromData   someArrayKey         for  NSInteger i 0  i  lt  array count  i               id resultString   array i            if    resultString isKindOfClass NSNumber class                   customNotNumberException raise       lt        HERE is just the same as   throw customNotNumberException               break             else if   resultString                 throw customNilException             lt                     break                       catch  SomeCustomException   sce           most specific type        handle exception ce              catch  CustomException   ce           most specific type        handle exception ce              catch  NSException  exception           less specific type         do whatever recovery is necessary at his level                  rethrow the exception so it s handled at a higher level       throw  SomeCustomException   customException       finally          perform tasks necessary whether exception occurred or not

User · Answer

throw  NSException exceptionWith        Xcode recognizes  throw statements as function exit points  like return statements  Using the  throw syntax avoids erroneous  Control may reach end of non-void function  warnings that you may get from  NSException raise        Also   throw can be used to throw objects that are not of class NSException

User · Answer

There is no reason not to use exceptions normally in objective C even to signify business rule exceptions   Apple can say use NSError who cares  Obj C has been around a long time and at one time ALL C   documentation said the same thing   The reason it doesnt matter how expensive throwing and catching an exception is  is the lifetime of an exception is exceedingly short and   its an EXCEPTION to the normal flow  I have never heard anyone say ever in my life  man that exception took a long time to be thrown and caught    Also  there are people that think that objective C itself is too expensive and code in C or C   instead   So saying always use NSError is ill-informed and paranoid   But the question of this thread hasnt yet been answered whats the BEST way to throw an exception   The ways to return NSError are obvious   So is it   NSException raise        throw   NSException alloc  initWithName     or  throw   MyCustomException       I use the checked unchecked rule here slightly differently than above   The real difference between the  using the java metaphor here  checked unchecked is important --  whether you can recover from the exception   And by recover I mean not just NOT crash    So I use custom exception classes with  throw for recoverable exceptions  because its likely I will have some app method looking for certain types of failures in multiple  catch blocks   For example if my app is an ATM machine  I would have a  catch block for the  WithdrawalRequestExceedsBalanceException    I use NSException raise for runtime exceptions since I have no way to recover from the exception  except to catch it at a higher level and log it   And theres no point in creating a custom class for that   Anyway thats what I do  but if there s a better  similarly expressive way I would like to know as well  In my own code  since I stopped coding C a hella long time ago I never return an NSError even if I am passed one by an API

[objective-c] throwing an exception in objective-c/cocoa

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