Const in struct: driving me nuts

Help
1

I thought I was getting it, too. So much for that.

OK, I have a Sprite structure, its init takes a SpritePt, which represents a fixed f32 or a Tween, that’s all good and dandy.

I have a struct representing the ‘Title screen’, it has init(), step() and draw() for Raylib to make it move. My issue is const regarding the Sprite, defined as a file container i.e. not a ‘struct’ but then imported as and where need with @import.

Now, the ‘level’ CONTAINS the Sprite, and a few more fields:

game_state: *g.Gamestate,
font: r.Font,
mp3: r.Music,
sprites: Spritesheet,
star_map: Starmap,
sprite1: Sprite,

The init() creates the sprite, given the base Sprite sheet, which just wraps the Raylib texture for now, nothing complex for now.

So, I have a struct that has a Sprite in it, that I created in the init():

pub fn init(game_state: *g.Gamestate, font: r.Font, mp3: r.Music, star_map: Starmap, sprite_sheet: Spritesheet) !Level_00_Title {
    const sp1 = try Sprite.init(
        Spritesheet.Type.redShip1,
        150.0,
        250.0,
        sprite_sheet,
    );
    return Level_00_Title{
        .game_state = game_state,
        .font = font,
        .mp3 = mp3,
        .sprites = sprite_sheet,
        .star_map = star_map,
        .sprite1 = sp1,
    };
}

And when I come to step update the sprite I get this error:

src/level_00_title.zig:79:17: error: expected type '*sprite', found '*const sprite'
    self.sprite1.step(dT);

Where the hell did *const come from, that is not how I declared it?!

79: self.sprite1.step(dT);

The step method has *Sprite to get the free level of dereference too:

pub fn step(self: *Sprite, dT: f32) void {
}

For reference the level step() is:

pub fn step(self: *Level_00_Title, dT: f32) void {
    r.ClearBackground(r.BLACK);
    r.UpdateMusicStream(self.mp3);
    self.star_map.draw();
    self.star_map.step(dT);
    self.sprite1.step(dT);
    self.sprite1.draw();

    if (r.IsKeyReleased(r.KEY_SPACE)) {
        r.StopMusicStream(self.mp3);
        self.game_state.* = .level_1;
    }
}

Argh. It’s driving me nuts. I have music playing, text, a three coloured vertical scrolling star-field and yet this f* Sprite is getting up my conk I can tell you. It’s obv. I have not yet understood ‘const’ ness WRT to Zig so, I have read and watched a lot but yeah, here we are.

2

I don’t know the answer from the code snippets you provided, but at least this error message looks strange:

error: expected type '*sprite', found '*const sprite'

I would expect that the type is named Sprite, not sprite.

…also, does it compile if you remove the self.sprite1.step(dT0); call, and if yes what’s the signature of the self.sprite1.draw() function - e.g. is that any different than the .step() function signature?

1 Like
3

Making the type readable instead of inferring the type can help find the problem too.

const sp1: *const Sprite = try Sprite.init(...)

I always do this. Inferring is for lazy people I think :slight_smile:

4

Look at the full stacktrace. And also make use of @compileLog().
But hard to say without seeing the full error message.

Likely that you aren’t using a ptr somewhere/passing it as value or defining some instance as a const instead of a var.

5

So I am lazy?

Writing the full type is great if you understand exactly what it should be, I don’t have that knowledge down pat yet.

6

Yes, I find that confusing too as the type IS Sprite …when I solve this one I look forward to the brief feeling of satisfaction!

7

i was just joking. hence the smiley

8

Looks like you typo’ed sprite somewhere, but that’s not the issue.

Without looking at the actual code that’s causing the problem, I’m gonna go out on a limb here and assume you have this somewhere:

fn doSomething(self: Level_00_Title) void{
  self.sprite1.step(dT);
}

Parameters in Zig are const. Therefore, self is const, and the sprite in it is also const. When a method is called on a value, Zig automatically coerces it to a pointer, if necessary to match the signature. Here, since the sprite is const, it got coerced into a const pointer, hence the error.

4 Likes
9

Sorry eric, my bad. Somedays I just totally misjudge things. My limited exposure to ziggit so far has been nothing but pleasant.

4 Likes
10

Ok, after something of a break, I fixed it… it was the variable capture !!!

pub fn step(self: *Sprite, dT: f32) void {
    if (self.tween_x) |*tx| {    // I had |tx| here !
        tx.step(dT);
        self.pos.x = tx.val;
    }
    if (self.tween_y) |*ty| {    // I had |ty| here !
        ty.step(dT);
        self.pos.y = ty.val;
    }
}

Along the way I got some great advice, found some good stuff to read and learned a lot more about zig; my space ship now tweens perfectly, thanks all.

Now I am working on a TweenStream: how to trigger one tween at some point during another and call a function, I’ve played with calling functions as higher order things etc… what I want is to build a simple, ‘engine’ if you will, that lets me pre-script a whole bunch of actions for the title screen such that it “runs itself”.

This will also get massively reused elsewhere in the game.

Ta!