理解Lua中的OOP

Question

I do most of my programming in Python, and I use OOP practices for most of my projects. I recently started taking a look at the Love2D game library and engine. I managed to get some things configured, and then thought about making a GameObject class. But, what's this? Lua doesn't have classes! It has tables, and metatables, and other such things. I'm having a lot of trouble making heads or tails of this even after reading the documentation several times over.

Consider the following example:

catClass = {}
catClass.__index = catClass
catClass.type = "Cat"

function catClass.create(name)
    local obj = setmetatable({}, catClass)
    obj.name = name
    return obj
end

cat1 = catClass.create("Fluffy")
print(cat1.type)

cat2 = catClass.create("Meowth")

cat1.type = "Dog"

print(cat1.type)
print(cat2.type)
print(catClass.type)

The output of this is as follows:

Cat
Dog
Cat
Cat

What I do not understand is why changing cat1.type to "Dog" does not cause identical changes in cat2 and catClass. Does setting a metatable create a copy of the table? Google did not provide useful results (there are very few good Lua explanations).

Answer 1

When you index a table and a key does not exist then Lua will look to see if a metatable exists for the table. If one does then it will use the __index key of that metamethod to re-index your key.

When you created cat1 it inherited the catClass metatable. Then when you indexed type it will see that cat1 does not have a table entry called type and thus looks to the metatable to find it.

Then you set type on cat1 to Dog , which only sets the table key of cat1 itself, not the metatable. That is why when you index cat1 again for type you get Dog and not Cat .

If you go to http://www.lua.org/ there is documentation and some older copies of the Programming in Lua, written by the authors of Lua itself.

Answer 2

See the setmetatable documentation - the table returned is the table specified in the first argument.

This is a different table for each invocation of create (thanks to {} ) and each new table is also distinct from the metatable used. No copy was made, but rather a new table was created which is then "linked" ¹ to the metatable.

Thus there are three different tables in the above - cat1 (with mt catClass), cat2 (also with mt catClass) and catClass itself. Altering cat1 , as done, therefor has no effect on the other two tables.

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¹ See Lua Metatables Tutorial ; the use of the __index presented in the metatable effectively emulates JavaScript's [prototype] resolution.

When you look up a table with a key , regardless of what the key is, and a value hasn't been assigned for that key .. Lua will look for an __index key in the table's metatable .. If __index contains a table, Lua will look up the key originally used in the table belonging to __index .

However, __index has no affect on assigning a new index to one of the tables - the particular table is just modified as normal. (The tutorial also goes on to explain __newindex , if such write-through behavior is desired.)