How do I execute a function reference saves in a variable?

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1

I'm trying to have an object save a function to a variable, that it should call when it's done processing something. Does anybody know how to call a reference to another function you saved to a variable?

2

If you're talking about Lua, my memory is rusty but I seem to recall functions can be passed around willy-nilly like in JavaScript and called as any method of an object normally would be.

If you're talking about C, the way this is done is something called a function pointer. A function pointer has the following syntax:

returnType (*)(argType1, argType2, ...)

For example, a function pointer for a two-operand operation on integers might look like this:

int (*)(int, int)

Unintuitively, if a name is being associated with the type, as in a typedef or variable declaration, the name appears next to the asterisk:

int (*funcVar)(int, int); // Declares a variable named funcVar

The function being associated with that type needs to have the same prototype, or else gloom and doom. Pointers are obtained with the & operator as usual:

int Multiply(int a, int b) {
     return a * b;
}

void main() {
    int (*funcVar)(int, int) = &Multiply;
    int c = funcVar(5, 3); // 15
}

For brevity, function pointers can also be used in typedef statements:

typedef int (*TwoOp)(int, int);

TwoOp funcVar = &Multiply;

Lastly, to use a function pointer in a cast:

void *funcVar = &Multiply;
TwoOp cast1 = (TwoOp) funcVar; // From a typedef
int (*cast2)(int, int) = (int (*)(int, int)) funcVar; // With the naked type
int c = cast1(5, 3); // 15
int d = cast2(2, 7); // 14
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3

Hi,

I'm talking about Lua.

local toCallWhenDone = someOtherFunction
-- how do I execute the stored reference toCallWhenDone?
4

Just call the variable like a function.

In Lua, functions don't actually have names. Every "function" is actually a reference variable to an anonymous function. The name in a function declaration is the name of the variable you're storing the initial reference into.

So in your example, you're copying the reference from the variable someOtherFunction into the variable toCallWhenDone.

5

This may be of help

Lua - Function Calls
https://www.lua.org/manual/5.4/manual.html#3.4.10

6

Can somebody please write a 2-line example? I really don't understand the above explanations.

7 
local toCallWhenDone = someOtherFunction
toCallWhenDone()

Though I don't quite get what you're trying to do, or why you'd do this.

8

Thank you very much Matt!

I was trying to create a callback from an object (a video player) who can inform the main application when a video is ready. As far as I understand there's no built-in Event model in Lua, so the main app can't add an event listener to the videoPlayer to listen for a "videoReady" event.

(I made complex games in AS3 long ago, where most communications between objects were handled by weak event listeners to avoid storing hard references so the garbage collection would be easier)

1 Like
9

Understood! Keep going :slight_smile:

Also check this out A list of helpful libraries and code - #3 by dustin and this A list of helpful libraries and code - #4 by dustin