Lua notes-meta-tables and meta-methods

A meta-table can be used to modify the behavior of a value so that it performs a specified action when confronted with a non-predefined operation. When Lua tries to add two tables, it checks whether one of the two has a meta-table, and then checks if there is a field called __add in the original table.

LUA does not create a meta table when it creates a new table, and you can use setmetatable to set or modify any table's meta-tables.

In Lua, you can only set the table's meta-table, and to set a meta-table of other types of values, you must do so through C code. Other types do not have a meta-table by default.

1. The meta-method of arithmetic class

set = {}local mt = {}--Create a new collection based on the values in the parameter list function set.new ( l ) Local  set = {}setmetatable (SET, MT)--sets MT to the currently created table's meta-table for _, v in ipairs ( L)  do set[v] = true endreturn set endfunction set.union ( a,  b ) Local res = set.new{}for k in pairs (a)  do res[k] =  True end for k in pairs (b)  do res[k] = true end return  resendfunction set.intersection ( a, b ) local res = set.new{}for  K in pairs (a)  do res[k] = b[k] end return res endfunction  set.tostring ( set ) local l = {}for e in pairs (Set)  do  l[#l  + 1] = e end return  "{"  .. Table.concat (l,  ", ")     "}"--table.coThe NCAT function connects all the strings in the given list and returns the connection result Endfunction set.print ( s ) print (set.tostring (s)) ends1 =  set.new{1, 2, 3, 4}s2 = set.new{2, 5}--Two collections have an identical meta-table print (Getmetatable (S1) )-->table: 0x7fee11c09940print (getmetatable (S2))-->table: 0x7fee11c09940--the meta-method into the meta-table. mt.__add = set.union --when Lua tries to add two sets, it calls the Set.union function and passes two operands as arguments s3 = s1 +  s2set.print (S3)-->{1, 2, 3, 4, 5}--similar, you can also use multiplication sign to find the intersection of collections mt.__mul =  Set.intersection set.print ((S1 + S2)  * s1)-->{1, 2, 3, 4}--[[in the meta table, Each arithmetic operator has a corresponding field name. In addition to __add  and __mul, there are __sub (subtraction), __div (division), __UNM (inverse number), __mod (modulo), and __pow (UP). In addition, you can define the __concat field, which describes the behavior of the join operator. ]]

2, the meta-method of the Relationship class

Mt.__le = function (A, b)--The set contains a for k in pairs (a) doif not b[k] then return False end end return TRUEENDMT.__LT = Functio N (A, B) return a <= B and not (b <= a) Endmt.__eq = function (A, b) return a <= B and b <= a ends1 = set.new {2, 4}s2 = set.new{2, 4, 8}print (S1 <= S2)-->trueprint (S1 < S2)-->trueprint (S1 >= S2)-->falseprint (S1 &G T S2)-->falseprint (S1 = = S2 * S1)-->true

3. Meta-method for table access

--[[when accessing a field that does not exist in a table, the interpreter is prompted to find a meta-method called __index. Without this meta-method, the access result is nil, otherwise the meta-method will provide the final result. ]]window = {}--Creates a namespace-uses default values to create a prototype Window.prototype = {x=0, y=0, width=100, height=200}window.mt = {}--Create a meta-table-- The Declaration constructor function window.new (o) setmetatable (o, window.mt) return oend--defines the __index meta method window.mt.__index = function (table, Key) return Window.prototype[key]end--__index does not have to be a function, it can also be a table--window.mt.__index = window.prototype--creates a new window and queries a field it does not have w = Window.new{x = ten, y = 20}print (w.width)-->100

The __newindex meta-method is similar to the __index meta-method, except that the former is used for table updates, while the latter is used for table queries. When you assign a value to an index that does not exist in a table, the interpreter looks for the __newindex meta method. If there is a meta-method, the interpreter invokes it instead of performing the assignment.

Lua notes-meta-tables and meta-methods