Why can't I loop through struct fields with runtime loop?

Explain
,
1

I’m able to loop through struct fields only by inline for. Why is that?

const Point = struct {
    x: u32,
    y: u32,
};

pub fn main() !void {
    const points_fields = std.meta.fields(Point);
    for (points_fields) |f| {
        std.debug.print(" - {s}\n", .{f.name});
    }
}

const std = @import("std");

This is the error message I’m getting

error: values of type '[]const builtin.Type.StructField' must be comptime-known, but index value is runtime-known
2

The struct in question contains compile-time only types that cannot be represented at runtime:

    pub const StructField = struct {
        name: [:0]const u8,
        type: type, // ← types cannot be represented at runtime
        default_value_ptr: ?*const anyopaque,
        is_comptime: bool,
        alignment: comptime_int, // ← comptime integers, as the name suggests, cannot be represented at runtime
4 Likes
3

ahh it makes sense now. thank you

4

You can use inline for to generate the loop body at comptime.

5

If you really want to do it at runtime, you could cast your structure to an array of u32, but this only works if all your fields are u32.

6

While this works now, I don’t think the language intends to guarantee field layout unless the struct is marked extern or packed in the future. Happy to be corrected on that.

7

To address the question in the title directly: how would your program know the offsets of the fields at runtime?

2 Likes