On Sun, 08 Mar 2009 23:12:31 +1300, Daniel Keep
<[email protected]> wrote:
That's how I think: it's easy to come up with simple cases where a
design works. The trick is to come up with a design that works even for
the complex cases.
What I am suggesting has strong importance without any changes to syntax,
or any changes that may brake existing code.
It's somewhat hypocritical to say "the compiler [needing to know] every
possible call doesn't make that much sense" and then require the
compiler know of every possible subclass of a given class.
That argument has no validity as non templated functions have been working
as virtual for years.
Avoiding dynamic instantiation, you HAVE to know what template instances
are going to be needed at compile time. You can't wave your hands and
make it go away.
I'm not saying guess the template instance. I'm saying use the same exact
types for instance in class A to instantiate template in class B where the
signature is exactly the same. There is absolutely no guess work as
everything is KNOWN.
OK: an instance has a pointer to its vtable. Virtual method resolution
works by assigning each method an offset in this table at compile time.
When called, the method's slot is found and the method's address is
read out.
So in order to have virtual templated members, you have to stuff the
instantiations into the vtable, or something like the vtable, somewhere.
Don't worry the class won't feel a thing as long as the subclasses are
recompiled. This is required for non templated functions anyway but a
different subject for another day.
Let's take your example: the compiler knows all subclasses. This
requires all-at-once compiling to be mandated by the language; any given
file will depend not only on the modules it directly references, but all
modules that subclass any classes it references. It also means you
cannot ever have classes subclassed by an external library, since these
will be unavailable at compile time.
There is really no extra complexities that non template functions already
deal with.
PS: .net is in a whole different ball game and is storred in a common
intermediate language not source but that kind of a high level is still
not required to have a virtual templated function.
