From C ++ to objective-C: Exception Handling and Multithreading

Http://www.devbean.info/2011/04/from_cpp_to_objc_17/

Exception Handling

Compared with C ++, exception handling in objective-C is more like Java, mainly because objective-C has a @ finally keyword. A similar finally keyword exists in Java, but not in C ++. Finally is try ()... An optional additional block of the catch () block.CodeIt must be executed, whether or not exceptions are caught. This design can easily write brief and clean code, such as releasing resources. In addition, @ try… in objective-C... @ Catch... @ Finally is a classic design with no difference from most languages. However, unlike C ++, objective-C can only be discarded by objects.

Without finally	With finally
BOOL problem = YES; @try {      dangerousAction ();      problem = NO; } @catch (MyException * e) {      doSomething ();      cleanup (); } @catch (NSException * e) {      doSomethingElse ();      cleanup ();      // rethrow exception      @throw } if (! problem)      cleanup ();	@try {      dangerousAction (); } @catch (MyException * e) {      doSomething (); } @catch (NSException * e) {      doSomethingElse ();      @throw // rethrow exception } @finally {      cleanup (); }

Strictly speaking, @ finally is not necessary, but it is indeed a powerful tool to handle exceptions. As shown in the previous example, we can also throw an exception in @ catch. In fact, @ finally is executed only after @ try is completed. We will explain this below.

int f (void)
{
     printf ("f: 1-you see me \ n");
     // Pay attention to the output string and experience the exception handling process
     @throw [NSException exceptionWithName: @ "panic"
                                    reason: @ "you do n’t really want to known"
                                  userInfo: nil];
     printf ("f: 2-you never see me \ n");
}
 
int g (void)
{
     printf ("g: 1-you see me \ n");
     @try {
         f ();
         printf ("g: 2-you do not see me (in this example) \ n");
     } @catch (NSException * e) {
         printf ("g: 3-you see me \ n");
         @throw;
         printf ("g: 4-you never see me \ n");
     } @finally {
         printf ("g: 5-you see me \ n");
     }
     printf ("g: 6-you do not see me (in this example) \ n");
} 

Finally, catch (…) of C ++ (...) Any value can be captured, but not in objective-C. In fact, only objects can be thrown, that is, we can always use ID to capture exceptions.

Note that there is an nsexception class in cocoa. We recommend that you use this class as the parent class for all Exception classes. Therefore, catch (nsexception * E) is equivalent to catch (…) of C ++ (...).

Multithreading

Thread Security

In objective-C, POSIX APIs 2 can be clearly used to implement multithreading. Cocoa provides its own multi-thread class management. One thing to note is that when multiple threads access the same memory area at the same time, unexpected results may occur. Both POSIX APIs and cocoa provide lock and mutex objects. Objective-C provides a keyword @ synchronized, which is the same as the keyword of the same name in Java.

@ Synchronized

By @ synchronized (...) The surrounding block is automatically locked to ensure that only one thread is used at a time. This is not the best solution for processing concurrency, but it is the simplest, most lightweight, and most convenient solution for most key blocks. @ Synchonized requires that an object be used as a parameter (can be any object, such as Self) and used as a lock.

  @ implementation @implementation MyClass
 
-(void) criticalMethod: (id) anObject {
     @synchronized (self) {
         // This code is mutually exclusive to other @synchronized (self)
         // self is the same object
     }
     @synchronized (anObject) {
         // This code is mutually exclusive to other @synchronized (anObject)
         // anObject is the same object
     }
}
@end