The Implementation of Exceptions

This section is somewhat advanced. Unless you are planning to mix Objective-C and C++, you can skip it. Exceptions are implemented with C’s setjmp library. setjmp does not invoke any of the destructors that C++ would invoke for stack-based C++ objects when unwinding the stack. This means that all of the destructors that would have been invoked by functions between the @catch block and the @throw statement are ignored. The implementation of exceptions is straightforward. NSExceptionFrames record the location of each @try block.

typedef struct
						NSExceptionFrame
{
  jmp_buf                  state;
  struct
						NSExceptionFrame *parent;
  NSException             *exception;
} NSExceptionFrame;

static
						NSExceptionFrame* _currentExceptionFrame = nil;