Lua meta-table
In Lua table, we can access the corresponding key to get value value, but cannot match two table to perform an operation (such as adding).
So Lua provides a Metatable that allows us to change table each behavior is associated with a corresponding meta-method
For example, using a meta-table, we can define how Lua calculates two table the addition operation of a+b .
When Lua attempts to add two tables, it first checks whether one of the two tables has a meta table, and then checks to see if there is one called \__add if it is found, the corresponding value is called \__add the corresponding value of a real-time field, such as a function or table, is a “meta-method”.
There are two important functions to deal with meta-tables:
setmetatable(table,metatable)for the specifiedtablesets the meta-table (metatable) if it exists in the meta-table (metatable)\__metatablekey valuesetmetatablewill fail.getmetatable(table)returns the meta-table (metatable) of the object.
The following example shows how to set a meta table on a specified table:
mytable = {} -- regular table
mymetatable = {} -- MetaTable
setmetatable(mytable,mymetatable) -- Set mymetatable to the metatable of mytable
The above code can also be written directly on one line:
mytable = setmetatable({},{})
The following is the meta-table of returned objects:
getmetatable(mytable) -- This will return mymetatable
\__index Meta-method
This is metatable the most commonly used key.
When you access it through the key table if the key has no value, then Lua will look for the table of metatable (assuming there is a metatable) __index key. If __index contains a table in which Lua looks for the corresponding key.
We can use it again. lua the command enters interactive mode to view:
$ lua
Lua 5.3.0 Copyright (C) 1994-2015 Lua.org, PUC-Rio
> other = { foo = 3 }
> t = setmetatable({}, { \__index = other })
> t.foo
3
> t.bar
nil
If __index include a function Lua that function will be called. table and keys are passed to the function as arguments.
The \__index meta-method checks whether the element in the table exists. If it does not, the return result is nil;. If it does, the \__index returns the result.
Example
mytable = setmetatable({key1 = "value1"}, {
\__index = function(mytable, key)
if key == "key2" then
return "metatablevalue"
else
return nil
end
end
})
print(mytable.key1,mytable.key2)
The output result of the instance is:
value1 metatablevalue
Instance resolution:
mytabletable is assigned to{key1 = "value1"}.mytablemeta-table is set, and the meta-method is\__index.In
mytablelook up in the tablekey1if found, the element is returned, and if it is not found, continue.In
mytablelook up in the tablekey2if found, returnmetatablevalueif you can’t find it, go on.Judge whether there is a meta-table.
__indexmethod, if__indexmethod is a function, the function is called.Meta method to see if the parameter of the “key2” key is passed (
mytable.key2set), return “metatablevalue” if the “key2” parameter is passed, otherwisemytablecorresponding key value.