I’m very new to Zig – I’m using it (for the first time) to work through the Advent Of Code problems.

As part of testing my solution to the the linked problem, I’m trying to create a struct containing an array created from an literal, but I’m getting an error that I can’t resolve:

const std = @import("std");
const print = std.debug.print;

const Update = struct { values: []u32 };

test "Create and operate on an Update" {
    const update = Update{ .values = [3]u32{ 1, 2, 3 } };
    print("DEBUG - update is {}n", .{update});
}

$ zig test scratch.zig
scratch.zig:7:40: error: array literal requires address-of operator (&) to coerce to slice type '[]u32'
    const update = Update{ .values = [3]u32{ 1, 2, 3 } };
                           ~~~~~~~~~~~~^~~~~~~~~~~~~~~

That seems clear enough – except that when I follow the advice, I get a type-mismatch:

const update = Update{ .values = &[3]u32{ 1, 2, 3 } };

$ zig test scratch.zig
scratch.zig:7:29: error: expected type '[]u32', found '*const [3]u32'
    const update = Update{ .values = &[3]u32{ 1, 2, 3 } };
                           ~^~~~~~~~~~~~~~~~~~~~~~~~~~~

That makes sense – so far as I can understand, & converts a variable to a pointer, so it’s correct for the compiler to complain about being passed a *[]u32 when it expects a []u32. So, both options give me an error – how can I pass a []u32 to the Update constructor?

EDIT – after reading this article, I think I have a better idea of what’s going on:

• []u32 is a slice of u32s, not an array thereof.
  • Which is correct for what I want – because an array must have a comptime-defined size, which would imply that all Updates much have values of the same size.
• So I need to instantiate an Update like so:

const Update = struct { values: []u32 };

test "Create and operate on an Update" {
    var gpa = std.heap.GeneralPurposeAllocator(.{}){};
    defer _ = gpa.deinit();
    const allocator = gpa.allocator();

    var values = try allocator.alloc(u32, 3);
    defer allocator.free(values);
    values[0] = 1;
    values[1] = 2;
    values[2] = 3;
    const update = Update{ .values = values };
    print("DEBUG - update is {}n", .{update});
}

Which works as-expected. Is this the idiomatic way to do this? I’m surprised there’s no built-in way to define a “slice-literal” as there is for arrays – 5+n lines to define an n-length slice (or 3+n lines if you already have an allocator available) seems verbose, to me!