lua--3. meta-table metatable

Lua's metatable is also a common table, and LUA provides metatable functionality, primarily for several functions:

1. Controlling access to a table
2. Providing support for the LUA function library

3. Overloading the behavior of arithmetic and relational operators

   1. Using MetaTable to control access to the table

   When querying a table's key, if the table's key does not have a value, then Lua looks for the __index element method in the table's metatable: If __index points to a Table,lua it finds the corresponding key in the table, and , __index can also point to a method

When assigning a key to a table, if the table exists that field is assigned a value, and if not, then Lua looks for the __newindex element method in the table's metatable: If __newindex points to a table, LUA assigns the table an operation, and if __newindex points to a method, Lua calls the method

__index meta-method in Lua
If __index contains a table, Lua looks for the corresponding key in this table.
Also, __index can point to a table or point to a method __index when it points to a table:
Before we need to know when we look for an element in a table
First look in the table, then return the corresponding value, none to see if the table has a meta-table metatable
Nil if no meta-table is returned
When there is a meta-table, LUA does not look in its meta-table, but instead finds it in the __index field of its meta-table
Therefore: only use the Setmetatable () method to set the meta-table, and cannot get the elements in the corresponding meta-table

BaseClass = {theKey1 = "the string value1"}--[[DerivedClass = {}setmetatable(DerivedClass,BaseClass)]]----注释的两句可简写为：DerivedClass = setmetatable({},BaseClass)res = DerivedClass.theKey1print(res)

After setting the __index key for the corresponding meta table:

BaseClass = {theKey1 = "the string value1"}BaseClass.__index = BaseClassDerivedClass = setmetatable({},BaseClass)res = DerivedClass.theKey1print(res)

You can also change a shorthand method:

BaseClass = {theKey1 = "the string value1"}DerivedClass = setmetatable({},{__index = BaseClass})    --即直接设  AnonymousTable = {__index = BaseClass} 为DerivedClass的元表，查找时可直接在AnonymousTable中的__index域对应的表BaseClass中查找res = DerivedClass.theKey1print(res)

Therefore, the inheritance in Lua can be expressed as:

local Car = {}Car.__index = Carfunction Car:new(o)    o = o or {}    setmetatable(o,Car)    return oendfunction Car:run()    print("Car‘s run func.")end--直接调用“父类”中的run方法Car.run()local FordCar = Car:new()--子类调用“父类”中的run方法FordCar.run()--重写fordCar中的run方法function FordCar:run()    print("FordCar‘s run func.")end--重写之后调用FordCar.run()

When __index points to a method
When __index points to a method, Lua attempts to call the __index meta method Metamethod (__index Metamethod can accept two parameters table and key) when we access the nonexistent domain of the table.

local t1 = {}t1.__index = function(table,key)    print("call the __index metamethod")    print("table"..tostring(table))    print("key"..key)    return key.." from the __index"end--set t1 as t2‘s metatablelocal t2 = setmetatable({},t1)--pass the table and the key to __indexmetamethodlocal res = t2.key1print("the result is :"..res)print("------------------")res = t2.key2print("the result is :"..res)

__newindex Meta-method
If you assign a value to a nonexistent field in the table, Lua checks for __newindex Metamethod:

1. If __newindex is a function, Lua will call the function instead of the assignment (if __newindex is a function, it can accept three parameters table key value. You can call Rawset (t, K, V) if you want to ignore the __newindex method to assign a value to a table's field
2. If __newindex is a table,lua, this table will be assigned a value

--file:newindex.lualocal mt = {    key1 = "key1‘s value"}local other = {    key1 = "the old value"}mt.__index = mt--[[mt.__newindex = function()    print "Can not set value"end--]]mt.__newindex = other local t = setmetatable({},mt)t.key1 = "the new value from the table t."print(t.key1)--对__newindex指向的table中的key1进行更新操作，但是table t 中仍然没有键key1,进行t.key1的查询时，查询的仍然是__index中对应的键key1的值print(other.key1)

2. Providing support for the LUA function library

The Lua library can define and use Metamethod to accomplish some specific operations
A typical example is the __tostring function in the Lua base library, which is called by the print function to output, and when you call print, the __tostring is checked and called Metamethod

--file:tablelib.lualocal mt = {}mt.__tostring = function(t)    return ‘{‘..table.concat(t,‘,‘)..‘}‘endlocal t = {1,2,3}print(t)print("-----------------")setmetatable(t,mt)print(t)

3. Behavior of overloaded arithmetic and relational operators

lua--3. meta-table metatable