best way to make an id to class instances.

Programming Questions

v.k.

best way to make an id to class instances.

in Programming Questions • 1 year ago

How to make an incrementing id to every class instance without a global variable nor passing it as a parameter?

I'd like to not depend on me to keep tracking of how many instance has been created or deleted. And I think "id" should be a class var to avoid changing it after creation (private?)

I was reading about C++, and it seams that the way to do it in C++ would be "static int id". But static gives an error:

"The field id cannot be declared static; static fields can only be declared in static or top levels types"

Well... This does not help me...

Below the two ways i figured and don't want to use...

int _id = 100; // using a global var
Foo[] test = new Foo[10];
void setup()
{
for (int i = 0;i< test.length; i++ )
{
test[i]=new Foo(i); // passing it
}
}
void draw() {
}
class Foo
{
int id;
int name;
// int innerId; // i'd like it here..
Foo(int _name)
{
name = _name;
id = ++_id;
println("this instance name is "+name+" with the id "+id);
}
}

Replies(5)

benja

Re: best way to make an id to class instances.

1 year ago

The instance of a class should not know anything about other instances, so i don't see a way to create a unique id from inside a class. I think you have to pass it and i don't see the downside of handling the ids yourself.
In most cases even an array-index is enough for me. But i am curious if you could describe a case where these ways are not sufficient.

v.k.

Re: best way to make an id to class instances.

1 year ago

Thanks benja.

Well you might be right this is probably not a downside, and both ways i did in the code above works fine.

It's more a pursuit for more reusable and "modular" code. And as i have seen this in C++ I was wondering how to make it in java/processing. Here is simple page of a a C++ tutorial explaining that in case you want to look. It is more a class variable that is independent of class instances than one instance knowing about another. Which turns out to be almost the same as a global var anyway.

For your curiosity sake. It crossed my mind when I was doing sliders. They have different ranges, so I pass the range to the constructor. So i made 3 of them that have the same range using a for() loop to initialize them passing the iterator as id as well. Than i need more three, each one with different ranges, so I initialize them one by one. Well occurred to me that if later i add one more slider in the for() loop (increasing the array size), if I forget to change the ones initialized after them I would end with duplicates ids. For instance:

Slider[] same = new Slider[3];

Slider[] diff = new Slider[3];

for( int i =0; i < same. length; i++)

{

same[i] = new Slider (range1, i) // i as id

}

diff[0] = new Slider(range2, 3); // manually minding the id

diff[1] = new Slider(range3, 4); // manually minding the id

diff[2] = new Slider(range4, 5); // manually minding the id if same[] length changes it is broken

So i prefer making a global id and incrementing it at the constructor... Maybe i should not initalize with the range and pass range in a setRange() method...Hummm maybe i should make a class like SlidersLaunch to handle all of them, cause i'm also struggling to keep track of mouseover and clicks. Let's see... I'll post it later cause i'll need some other help with this sketch. :-) But I'm not ready to post yet. Still needing little cleaning, a little more effort to make the question clear to me...

:-)

benja

Re: best way to make an id to class instances.

1 year ago

Ah, ok it's more clear to me now.
Actually there are static variables and functions in java and you can do the same thing as in the c++ example you mentioned. But because normal processing-classes are "inner" classes, i think you have to create a seperate java file for that class.
Save the following code as a text-file with the ending .java in the same folder as your .pde-file:

class User
{
public int id;
static int next_id;

public User()
{
id = this.next_id++;
}
};

When you reopen your sketch, you will see it as a new tab in the ide. Here is a small test, that shows the behaviour you described::

User[] users = new User[5];

for(int i =0; i<5; i++){
users[i] = new User();
println(users[i].id);
}

quarks

Re: best way to make an id to class instances.

1 year ago

As you have discovered you can only have static variables in top-level classes. In Processing there are two ways ofhaving top-level classes.

Method 1

Create the Foo class in a seperate tab called Foo.java do not call it Foo.pde because it will still be an inner class.

Method 2

Declare the inner class as static. The code below is a modification of your first post that does this.

Foo[] test = new Foo[10];
int y;
void setup(){
for (int i = 0;i< test.length; i++ ){
test[i]=new Foo(i); // passing it
}
}
void draw() {
}
static class Foo
{
static int _id = 0;
int id;
int name; // int innerId; // i'd like it here..
Foo(int _name){
name = _name;
id = ++_id;
println("this instance name is "+name+" with the id "+id);
}
}

Once a class is top-level it will not have direct access to global variables declared in the main sketch!

v.k.

Re: best way to make an id to class instances.

1 year ago

Thanks a lot quarks and benja. I'll look trough this and come back if i still in trouble :-).

Top Reply