Lua: Talking about the understanding of metatable and metamethod

? Element methods related to arithmetic operations

The following table records the metamethods and their function descriptions related to arithmetic operations

? A simple example of adding two tables

local t_1 = {<!-- -->1,2,3}
local t_2 = {<!-- -->4,5,6}

local t1 = {<!-- -->}

t1.__add = function(a,b)
    local tbl = {<!-- -->}
    for k,v in pairs(a) do
        table. insert(tbl,v)
    end
    for k,v in pairs(b) do
        table. insert(tbl,v)
    end
    return tbl
end

setmetatable(t_1, t1)

local result = t_1 + t_2
print(result[1],result[2],result[3],result[4],result[5],result[6])

Let’s briefly analyze:

• We hope that the two tables can be added through the arithmetic operator +
• Add meta table t1 for table t_1
• Add a metamethod __add to the metatable
• Execute the operation of t_1 + t_2, we get a fusion table

Analysis of the problem:

Question 1: Why is metatable only set for table t_1

In fact, the execution of the metamethod of the arithmetic class does not require both at the same time
When the operation of + is performed, the __add metamethod will be retrieved from both in the order of addition
Use the first retrieved metamethod as the execution plan
Both have no __add metamethod, and an exception will be thrown

Question 2: Combine the two with specific behavioral processing

The parameters a and b of the metamethod __add are the two to be added
In the method body, we can customize the fusion logic
In the above example, we create a new table containing all subsets of the parameters, and then return

? An example of adding two sets of different data

In fact, addition is only a generalized behavior, and Lua is not limited to operations between data of the same type
With the existence of metamethods, we can perform add operations on arbitrary data
The following example we try to add table (table) and string (string)

local t_1 = {<!-- -->1,2,3}
local t_2 = "aaaaa"

local t1 = {<!-- -->}

t1.__add = function(a,b)
    local temp = ""
    for k,v in pairs(a) do
        temp = temp..v
    end
    temp = temp .. b
    return temp
end

setmetatable(t_1, t1)

print(t_1 + t_2)

We only need to process different types of data in the method body of the meta method __add
In the above example, we concatenate the subset in the table and the string into a new string as the return value

Note: Other arithmetic element methods are used in the same way as __add in the example

? Metamethods related to relational operations

There are actually only three relational operators in Lua:

• equal to (a==b)
• less than (a
• Less than or equal to (a<=b)

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