What is metatable?
Deleted User
Also I saw a lot of tutorials and examples, but still can't understand (even on Russian).
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What is metatable?
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Deleted User
Also I saw a lot of tutorials and examples, but still can't understand (even on Russian).
OfflineYou can change the behaviour of a table, simply said. It's all explained there and it's quite easy to understand.
Although I wouldn't have any use for it myself.
It could be used for a backup, but you really wouldn't need it.
For example, let's say I've got a list of the users' names who posted in this thread:
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t = {"Factis","Bolt","Yates","Flacko"}
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t[1,3]
This is when Lua metatables come to save the day. We still can't get the subset in the way I showed before, however, we could do it in a similar fashion:
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t(1,3)
A metatable is just a regular table, it has no special traits or whatever.
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subset_meta = {}
Of course, this isn't a metatable strictly speaking, as what makes a table a metatable is it's relation with another table (although a table can be metatable'd to itself).
Let's do it then:
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setmetatable(t,subset_meta)
We will define a metamethod in our metatable. What is a metamethod? Just like a metatable is just a table, a metamethod is just a function. To make a metamethod we have to define a function inside our metatable with one of the following names:
Quote
__unm --unary minus (-t)
__len --length (#t)
__sub --substraction (t-x)
__add --addition (t+x)
__mult --multiplication (t*x)
__div --division (t/x)
__pow --power (t^x)
__mod --modulo (t%x)
__concat --concatenation (t..x)
__eq --equality (t==x)
__lt --less than (t<x)
__le --less-equal (t<=x)
__index --index (t[x])
__newindex --new index (t[x] = y)
__call --calling (t(x,y,...))
__len --length (#t)
__sub --substraction (t-x)
__add --addition (t+x)
__mult --multiplication (t*x)
__div --division (t/x)
__pow --power (t^x)
__mod --modulo (t%x)
__concat --concatenation (t..x)
__eq --equality (t==x)
__lt --less than (t<x)
__le --less-equal (t<=x)
__index --index (t[x])
__newindex --new index (t[x] = y)
__call --calling (t(x,y,...))
There are some gotchas for some metamethods, for example, the comparison metamethods (__eq,__le,__lt) only work if both elements have the same metatable. Meaning that if t's metatable is subset_meta then x's metatable must be subset_meta too.
The __index and __newindex metamethods only work if the table index you're trying to access is not defined. Meaning that if t[x] isn't nil, the metamethods won't be called.
Let's get back to our example and define our __call metamethod once and for all:
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subset_meta.__call = function(tbl,start,e)
local r = {}
for i=start,e do
r[#r+1] = tbl[i]
end
return r
end
So let's test our work:
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for k,v in pairs(t(1,3)) do print(v) end
Note that all this work could be packed into a separate function without using metatables:
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function getsubset(tbl,start,e) --same code as in subset_meta.__call end for k,v in pairs(getsubset(t,1,3)) print(v) end
edited 1×, last 23.07.12 11:39:58 am
Flacko, you saved the day. Now I understand it better; I gonna to learn it until I understand it perfectly. Re: What is metatable?
Deleted User 1