Direct variable modification

ID:2110988
 
Jul 4 2016, 2:02 pm
Applies to:DM Language
Status: Open
Issue hasn't been assigned a status value.
I'm wondering if it would be possible to add the ability to directly modify proc arguments, sorta like pointers in C++, so that we can avoid having to create/copy a list and return said list every time to preserve variable changes in a proc chain.

Not sure on the details of how it would work, but if deemed feasible, I'm sure there are some people here who can offer some ideas.

Thanks.
Jul 4 2016, 2:11 pm
Lists kind of are pointers right now.

mob/verb/Test()
    var/list/my_list = list("One","Two")
    ModifyList(my_list)
    src << my_list.len
    for(var/item in my_list)
        src << item
    
proc/ModifyList(list/some_list)
    if(some_list) some_list += "Three"


No copies needed, you can actually do this with the 'vars' list too, it's sneaky stuff.
Jul 4 2016, 2:22 pm
In response to Nadrew
True. Having to use a list seems very bulky in some cases though.

Here's the case I was considering when I posted this:

// attempting to receive damage from attacker.
proc/on_damage(combatant/attacker, stam_dmg=0, dmg_source, dmg_source_dir, params[])
    if(stam_dmg <= 0 || action == global.actions[ACTION_DEATH])
        return 0

    var/damage_params[] = args.Copy()
    if(has_event(EVENT_ON_DAMAGE) && !can_trigger_event(EVENT_ON_DAMAGE, src, damage_params))
        return 0
    . = round(damage_params[2])
    // continues from here...


In this case, I'm copying the args list because I need stam_dmg preserve changes through the event chain. Granted, I could leave out the information I don't need by doing damage_params = list(stam_dmg), but I'd much rather avoid having to use lists this way if possible. And so this topic came to mind.
Jul 4 2016, 2:41 pm
This feature request is for the ability to pass variables by reference. Normally, you can't actually pass a variable to a proc; when you write SomeProc(some_variable), some_variable is evaluated and the result -- the value contained in some_variable -- is passed to the proc. But what if you want to pass in a variable to be modified from inside the called proc?

C# does it with the "ref" or "out" keyword:
class RefExample
{
    static void Method(ref int i)
    {
        i = i + 44;
    }

    static void Main()
    {
        int val = 1;
        Method(ref val);
        Console.WriteLine(val);

        // Output: 45
    }
}

In DM, you currently have to "box" the value to a reference type, such as a list or a datum:
mob
    proc/Method(BoxedNumber[])
        BoxedNumber[1] += 44

    Login()
        var boxed_number[] = list(1)
        Method(boxed_number)
        world << boxed_number[1]

// or
boxed_value
    var value
    New(Value) value = Value

mob
    proc/Method(boxed_value/N)
        N.value += 44

    Login()
        var boxed_value/n = new (1)
        Method(N)
        world << N.value

A common usage for this in Unity is when you want to test for raycasts. If you cast a ray, the function returns true if the ray hits something, and the given "out" variable is assigned information about the hit.
RaycastHit hit; // contains information about a hit, if any

if (Physics.Raycast(transform.position, -Vector3.up, out hit, 100.0f))
    print("Found an object - distance: " + hit.distance);
Jul 4 2016, 2:46 pm
In response to Kaiochao
Exactly this post.