Friday, 5 November 2010

boost serialize - not such a good idea!

I'm a big fan of the boost libraries for C++. Mostly, they are a huge productivity gain - things like regex, function and bind can reduce the work involved in a complex program by half or more. So when I needed to pickle a very large (gigabytes) and complex data structure between restarts, boost was the obvious place to look.

Boost serialize certainly looked like the answer. Just add a few lines to each class saying what you what to save and restore, then with a single function call, you can pickle the whole structure and later reload it. It takes care of loops, diamonds and all the other things that happen in real-life data structures. And if you don't like the default way of saving something, it's easy to write your own. How good can it get!?

Well, that's the theory, and it could have been the practice too. But it isn't. For some reason known only to themselves, serialize's authors decided that the C++ inheritance mechanism wasn't for them. They invented their own completely parallel mechanism for dealing with polymorphism and subclasses. The effect is that trying to save my structure results in an exception thrown from somewhere in an enormous depth of function calls. I persevered, and tracked down what was happening. For some reason - and it's just impossible to plough through all the code and figure out the details - it silently ignores some of the calls to the subclass registration process. After several days of trying to figure out the details - on a live application because that's the only place the data can be collected - I have finally given up. I'll live without this capability in my program.

Unfortunately this is a common problem with boost. It seems to be de rigeur to invent new ways of doing things even though they are more complex and don't work especially well. The build system is another case in point - instead of using make, known and hated by generations of programmers, they invented their own, bjam, which is unknown and incomprehensible. If it works for you, great. If not, forget it.

I'll carry on using boost, but you do have to be selective. Unfortunately.

More thoughts on Boost here.

No comments: