References to native types

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References to native types

Michael Steinberg
Hi there,
I was playing around with the rvalue reference stuff whereby I broke a
test case, which happened to declare a function to lua with the
signature "int(int&)".
Now you could make that work by registering "int" as a class and when
passing in a value by reference, it would wrap that one up to a userdata
object. However when you do that, you basically disable the ability to
call functions on the lua side that accept numbers. My question is,
should registering functions taking references to native types even be
allowed?

I think I will have to somewhate differ in all calls between native&user
defined types. Because for example when you call into lua with a rvalue
ref to a user defined type, it could move it to the internal pointer
holder (and on the other end it could move it out, if there was a flag
indicating rvalue reference), however for native types they must be
treated as per-value under all circumstances (probably the passing is
broken in current head because of that, call_function took a const& of
all parameters and the default_converter was somehow "optimized" to
catch const int& (but not int&!) and treat it by value. This stuff is
pretty fragile and I intend to replace it by something that is
reasonably understandable).

Michael

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Re: References to native types

Michael Steinberg
Hello again,
digging through the list I found this issue to pop up again and again.
People were trying to use refs to native types, or there were
complaining why luabind wouldn't support it etc.. Maybe it would be wise
to add a paragraph to the documentation stating that is just not
possible to implement that, since lua does not know about
references/pointers?
The only thing that I can imagine one could do would be to provide an
opt-in type boxing support. So you could inject boxed native types into
lua that could be unboxed into references on the c++ side again (I think
this is quite similar to what Java has?). And some way to box native
types on the lua side to, to simulate references. One would then have to
differ between lua and c++ boxes, because c++ boxes would contain
pointers to a c++ value, whereas boxes created in lua would have a true
datamember...
The unboxing converter would then have to differ between c++ boxes and
lua boxes, because the latter would have to write back to the table,
whereas the former supplied the true pointer and wouldn't have to do
anything.
But any way, we'd have to manually box and unbox the types in native and
in lua code. So I really doubt that it's worth the great effort that is
necessary to implement that.

Michael

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