Re: Best way to have table-like fields on userdata
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- Subject: Re: Best way to have table-like fields on userdata
- From: Mark Hamburg <mhamburg@...>
- Date: 2006年11月28日 11:07:06 -0800
Implementing both methods and properties gets convoluted and results in
slower code for both the userdata case and the table case. We did it, but
I'm not sure it was a great choice in retrospect because of the performance
issues.
The problem is that __index can't just be a table but instead needs to be a
function which then figures out whether it's dealing with a property that
needs to be computed from the object somehow or a method which gets shared.
One writes something like:
__index = function( t, k )
local method = methods[ k ]
if method then
return method
end
return properties[ k ]( t, k )
end
Where methods and properties are tables of appropriate functions.
What would mitigate this would be support for accessing the base table in a
chain of __index lookups. That way one could set the methods table as the
__index table for the object and have a properties function as the __index
metamethod for that table:
setmetatable( methods, { __index = function( t, k, base )
return properties[ k ]( base, k )
end }
obj_mt = { __index = methods }
This way, method dispatch never leaves the optimized path through the Lua VM
for dealing with __index table chaining and properties are probably a bit
faster as well since again the Lua VM does most of the recognition about
which case applies.
Mark