Catch as catch can
A light-hearted look at exception handling
Stob Exception handling is a comparative newcomer to the programmer’s toolset.
for loop, the enigmatic
if statement and the cheeky little counter increment have been with us since the first automatic languages bubbled to the surface of the primordial programming bog at Manchester, more than half a century ago. But, according to my skimpy research, it wasn’t until IBM’s PL/I came along, in the mid 1970s, that exception handling appeared in a language.
PL/I’s exceptions were based on a proposal by John B. Goodenough. Goodenough was strongly motivated in his design, I believe, by the behaviour of his colleagues every time one of his programs crashed. I suggest they were in the habit of jeering — “Never mind John, that’s good enough” — and then falling about laughing. Truly is it said that ambition and bullying are the twin fuels that drive the engine of progress.
After PL/I, the next mainstream-ish language to support exceptions was Ada. Like PL/I, Ada was confidently expected to have a glorious future. At one time, the US Department of Defense mandated the use of Ada upon all its suppliers for all safety-critical work. Well, all those suppliers who carelessly failed to use the magic phrase ‘but it will be much cheaper if we do it in C’.
To the surprise of their powerful backers, neither PL/I nor Ada really “took on”. Despite guest appearances in exotica such as Clu and Modula-3, it wasn’t until C++ compilers began to support exception-handling constructs, in the early 1990s, that the idea was really adopted by the mainstream.
(Actually, I don’t believe this. I remember doing
ON ERROR GOTO stuff in Basic in the early 1980s; if that wasn’t a kind of exception handling, what the Dickens was it? Perhaps the academics take the view that, although it may have been mainstream, it was only Basic and so doesn’t count. Fair enough. Objection withdrawn, m’lud.)
Before we hit the techie nitty-gritty, a touch of etymology. The word “exception”, of course, comes from the Latin irregular verb excepto — conjugates excepto, exceptere, crashli, dumpdum — literally meaning “to encounter unexpectedly something unpleasant that you can’t prevent”. Legend has it that it was coined by a well-known peace consultant in pressing and infamous circumstances: “Et tu Brute? My, that’s an exceptionally sharp dagger.” If you picked up the teeny Carry On reference there, you may now take a biscuit.
I will discuss exceptions in the context of C++, using it as a baseline to compare with Delphi’s Object Pascal, Java and C#.
I apologise in advance to the industry’s senior commentator for not covering Scheme or other Lisp dialects and, thus, continuing the tendency to dumb down. This does not mean that I fail to recognise that Lisp is still #1 for key algorithmic techniques such as recursion and condescension. It just means that I have no idea how, or indeed if, Lisp handles exceptions.
finally is the entirely indispensable keyword that C++ doesn’t have. For the benefit of just-C++ people: a
finally clause contains all the inline tidying up that must always happen, whether or not you get an exception in the
try block. The path of execution through the code is clear… although there is always the danger that you will accidentally release a resource outside the
finally block, or forget it altogether.
C++ says you must wrap your resources up in classes, bung them on the stack, and have the destructor unpick the damage as control bubbles up through scope. C++ says that
finally leads to lazy, ad hoc coding practices.
Javaman: Oh yeah, that’s neat. So every time you want to protect some resource or teeny bit of state, you’ve got to jump out of line and knock out a whole class? Probably just to free up a couple of heap-allocated objects, I should think. Yup, that makes sense.
Mr C++: Oooh, hark at the supreme garbage collector! Just because you need a destructor (that’s like a
finalize() only it actually gets run some time soon, by the way) you don’t have to make a special class, you know. A generic is not just for a container. Whoops, hush my mouth, in your case it is.
Javaman: Here we go. Here we go. Do tell, are you planning on using
auto_ptr, and maybe munching an array that needs to be
deleteed? And after that let’s talk about Total Cost of Ownership…
Charlie C#: Hold up, Java. Too much
finally really can get up your nose. There’s a lot to be said for implementing a bit of an
IDisposable class, push a more code out of the app and into the nice, pre-tested library.
Denis Delphi: That’s right, the heterogeneous approach is best. Use
finally for ordinary objects, but do semi-hidden reference counting for the contents of strings and objects that are cast to interfaces. That’s the way to do it. Unless we are running under .NET, of course, in which case we…
Everybody: Oh shut up, you great dork.
Thus it may be seen that there are many clever and compelling arguments to be made on all sides.
Next page: When should you raise an exception?