The colon is for implementing methods that pass self
as the first parameter. So x:bar(3,4)
should be the same as x.bar(x,3,4)
.
For definition it is exactly the same as specifying self manually - it will even produce same bytecode on compilation. I.e. function object:method(arg1, arg2)
is same as function object.method(self, arg1, arg2)
.
On use :
is almost the same as .
- a special kind of call will be used internally to make sure object
and any possible side-effects of calculations/access are calculated only once. Calling object:method(arg1, arg2)
is otherwise same as object.method(object, arg1, arg2)
.
To be completely precise, obj:method(1, 2, 3)
is the same as
do
local _obj = obj
_obj.method(_obj, 1, 2, 3)
end
Why the local variable? Because, as many have pointed out, obj:method()
only indexes _ENV
once to get obj
. This normally just important when considering speed, but consider this situation:
local tab do
local obj_local = { method = function(self, n) print n end }
tab = setmetatable({}, {__index = function(idx)
print "Accessing "..idx
if idx=="obj" then return obj_local end
end})
end
tab.obj.method(tab.obj, 20)
--> Accessing obj
--> Accessing obj
--> 20
tab.obj:method(10)
--> Accessing obj
--> 10
Now imagine the __index
metamethod did more than just printing something. Imagine it increased a counter, logged something to a file or deleted a random user from your database. There's a big difference between doing that twice or only once. In this case, there's a clear difference between obj.method(obj, etc)
and obj:method(etc)
.
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