How to use "self" properly?

[sololove2d]

When I create a table for a character, I can do it this way:

Code: Select all

h01 = {}
h01.body = love.physics.newBody(runWorld, 350, 100, "dynamic")
h01.shape = love.physics.newRectangleShape(66, 92)
h01.fixture = love.physics.newFixture(h01.body, h01.shape, density)

Note h01.fixture are using previously defined properties as parameters for newFixture.

However, if I want to use metatable, it does not like "self":

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h01 = {}

function h01:Create()
    local this =
    {

        body = love.physics.newBody(runWorld, 350, 100, "dynamic"),
        shape = love.physics.newRectangleShape(66, 92),
        fixture = love.physics.newFixture(self.body, self.shape, density)


    }
    setmetatable(this, h01)
    return this
end

I got error:

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bad argument #1 to 'newFixture' (Body expected, got nil)

How should I refer to these 2 previously defined properties in fixture in this case?

[duaner]

At the point you're referencing body in the table, self doesn't yet have the meaning you want. You're probably going to have to do it the way you did in the first example. At least, that's what I've been doing.

Code: Select all

function h01:Create()
    local this =
    {

        body = love.physics.newBody(runWorld, 350, 100, "dynamic"),
        shape = love.physics.newRectangleShape(66, 92),

    }

    this.fixture = love.physics.newFixture(this.body, this.shape, density)

    setmetatable(this, h01)
    return this
end

[4vZEROv]

Code: Select all

function h01:Create()
    local tbl = {}
        tbl.body = love.physics.newBody(runWorld, 350, 100, "dynamic"),
        tbl.shape = love.physics.newRectangleShape(66, 92),
        tbl.fixture = love.physics.newFixture(tbl.body, tbl.shape, density)
    return setmetatable(tbl , h01)
end

"self" is just a hidden parameter passed to the function corresponding to the table calling the function.

[sololove2d]

Thanks. I thought I could use "self" within the function when defining it.

[MrFariator]

Consider the following for illustration purposes:

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local myTable = {}
function myTable.myFunc ( self ) -- note the use of ".", note that in a situation like this 'self' is just a convention
  print ( self == myTable )      -- prints true
end
-- is equivalent to
local myTable = {}
function myTable:myFunc ( ) -- note the use of ":", "self" is received implicitly
  print ( self == myTable ) -- prints true
end

-- these two are equivalent
myTable.myFunc(myTable)
myTable:myFunc()

When you use a colon (":") when calling a function contained within a table, the table itself is automatically passed to the function as its first parameter. And when you define a function inside a table using a colon, the table ("self") is received implicitly under the hood. It's all syntax sugar.

In your use case specifically, you were trying to use h01 itself as the "self", even though what you wanted to use was the newly created object (this, or tbl in zero's fix).

[4vZEROv]

Also in your code you tried to do some calculation on a field inside the same table declaration :

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-- error
local myTable = {
  a = 5,
  b = a * 2
}

You can't do that because a is not yet declared, if you want to do that:

Code: Select all

local myTable = {
  a = 5
}
myTable.b = myTable.a * 2
-- or
local myTable = {}
myTable.a = 5
myTable.b = myTable.a * 2

[sololove2d]

Thanks a lot. After played around for a while, I found out the way duaner mentioned works best for me.