Re: Proposal: a new __key metamethods for tables and userdata(s)

[Carsten Fuchs <carsten.fuchs@...>]

Hi David,

Am 04.11.2013 21:56, schrieb David Demelier:
> I often use the C++11 class std::shared_ptr<> template class to push
> to Lua objects and it's great because it handles reference counting,
> that is if the host does not own the ressource, Lua will delete it
> later with the __gc. But the drawback, is that you usually push a new
> shared_ptr<> instance with the same underlying object, thus the
> pointer of the userdata is different but not its content.

We too use a shared pointer implementation as the instances that are 
bound to userdata. As you said, they beautifully decouple both the C++ 
and the Lua code from concerns about the lifetimes of the object pointed to.

However, even with shared pointers, you *still* have to check the 
identity of the object (by looking at the raw pointer value of the 
shared pointer) when it is (re-)bound to Lua, and use that with a table 
with light userdata, for mapping to the Lua instance of the full 
userdata (the shared pointer), just as described in chapter 29.5, page 
301 of the PiL3 book.
In other words: Only a single shared pointer instance is ever needed on 
the Lua side (Lua then copies references to that userdata, not the 
userdata (shared pointer) itself), whereas on C++, you can have 
arbitrarily many copies.

(For completeness, using shared pointers is not the perfect solution as 
well, see http://thread.gmane.org/gmane.comp.lang.lua.general/92550, for 
which I wished there was a more straightforward solution...)

Best regards,
Carsten


--
Dipl.-Inf. Carsten Fuchs

Carsten Fuchs Software
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