Re: Object (de)serialization
On Mon, 25 Jan 2010 21:31:55 +0100, Thomas J. Gritzan wrote:
At this point I haven't even managed to get an example implementation
of the C++FAQ deserialiser working -- the static ctors aren't being
called, so the std::map doesn't contain anything, thus the code bombs
(current version throws an exception, the one I posted segfaults)...
The map isn't filled because you don't create triangle, so the line
creationMap["triangle"] = new Triangle();
isn't executed. You have to move this line somewhere else so that it's
invoked before you use creationMap, like a registerShape function
that'll be called from main.
I've actually shuffled it into a "TriangleInitialiser" class --
static class TriangleInitialiser {
public:
TriangleInitialiser() {
cerr<<"ctor: TriangleInitialiser\n";
if (Shape::creationMap.count("triangle") == 0) {
Shape::creationMap["triangle"] = new
Triangle();
}
}
} _x_Initialiser_Triangle;
(Obviously this is a test, and any real code would be hiding the map
behind a couple of functions -- RegisterPrototype and FreePrototypes)
This stays in Triangle.cpp and isn't referenced by (or even accessible
by, thanks to the static prefix). That leaves the problem of deallocating
the memory (admittedly only a few bytes, but it's still more fluff to
wade through in the Valgrind log). Adding a destructor to Shape deals
with that:
~Shape() {
// dealloc the prototypes
while (!creationMap.empty()) {
std::map<std::string, Shape *>::iterator
i = creationMap.begin();
Shape *x = (*i).second;
creationMap.erase(i);
delete x;
}
};
The catch being that the "delete x" invokes ~Shape again, thus (AIUI) it
will consume one stack level for each prototype in the map... I've had a
quick play, but it doesn't seem to be possible to specify that a
destructor applies to the base class, but not any derived classes.
I didn't use an iterator loop because AIUI calling erase() on a container
or map invalidates any iterators active against it. The "Shape *x" is
there for a similar reason.
But instead using this prototype based meachanism, I suggest using a
factory functor and storing a boost::function in creationMap, if you
have access to Boost (std::tr1::function is the same). Example:
(snip code)
That looks better than my solution, but I'm not keen on adding Boost to
my application's build dependencies. As nice as it is, it's an utter pig
to build on Win32 (IIRC last time I did it, I had to build Cmake, which
was great fun). Dead easy on *nix, but unfortunately this code has to
work in the Evil Empire too...
The only other thing I'm not keen on is having to hack around with main()
to add new chunks, although that's probably solvable by putting the
registration stuff in a static class's ctor or a global
RegisterChunkDeserialisers() function.
Still not as nice as just being able to create a module with two classes,
and have that module auto-initialise and register on startup (see
TriangleInitialiser above). But that said, I'm still concerned about what
happens if the TriangleInitialiser object gets initialised before
Shape... unless the compiler is clever enough to figure out that Shape
needs setting up first (probably not, even though it is gcc).
Thanks,
Phil.