CODEWITHSUNDEEP
functionluaself
Levy
Levy

Reputation: 231

How to properly use self to print the string in this example

In this MWE I'm trying to write a function in lua that when it is called it prints some text alongside the string that called the function.

For this to work I used self to print the string, but it actually returns a nil value. How do I correctly use the self in this example and how do I archive such task?

str = "Some text on the string"

function string.add()
    print("Hope it prints the string besides too",self)
end

str:add()

The output is the following:

Hope it prints the string besides too nil

What I would like to have:

Hope it prints the string besides too Some text on the string

Upvotes: 1

Views: 525

Answers (1)

Personage
Personage

Reputation: 484

In the case of your function, string.add(self) is equivalent to string:add(). In the latter version, which is a member function--or method--of the string class, self is the implicit first parameter. This is similar to classes in Python, where self is the first parameter of every member function.

-- Notice the self parameter.
function string.add(self)
    print("Hope it prints the string besides too", self)
    return
end

str = "Just some text on the string"
str:add()

As a side note, if you were to expose the items on the Lua stack through the C API when you call str:add(), str would be the first item on the stack, that is, the element at index 1. Items are pushed on to the stack in the order that they are passed to functions.

print("hello", "there,", "friend")

In this example, "hello" is the first argument on the stack, "there," is the second, and "friend" is the third. In the case of your add function--written as str:add() or string.add(str)--self, which refers to str, is the first item on the Lua stack. Defining member functions with the index operator, as of the form string.add, allows for flexibility, as one can use the form with the explicit self and the form with the implicit self.

Upvotes: 1

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