Fabulous Adventures In Coding
Eric Lippert is a principal developer on the C# compiler team. Learn more about Eric.
Congdon's blog (http://www.bobcongdon.net/blog/)
points out that in the dark days before exception handling you could always use setjmp/longjmp to
do non-local gotos.
fact, the script engines are compiled in C++ with exception handling turned off (for
performance reasons), and the mainline loop of the bytecode interpreter uses setjmp-longjmp exception
handling to implement error handling. When
you have a script that calls an object that returns an error, we longjmp back to the
start of the interpreter loop and then figure out what to do next.
VBScript of course it depends on whether On
Error Resume Next is
on or not, and in JScript we construct an exception object and start propagating it
back up the stack until we find an interpreter frame that has a catch block. (If
there are multiple script engines on the stack then things get extremely complicated,
so I won't even go there.)
a long jump does not call any destructors, it was very important that we design our
interpreter loop to not put anything on the system stack that required destructing. Fortunately,
since we were designing the interpreter to be an interpreter for a garbage-collected
language, it was pretty easy. Everything
that the interpreter does that requires memory either takes the memory out of the
area reserved for the script's stack (which will be cleaned up when the frame goes
away) or heap-allocates it and adds the memory to the garbage collector.
everyone has the luxury of having a longjmp-safe
garbage collector already implemented, so kids, don't try this at home! If
you must use exception handling in C++, take my advice and use real C++ exception