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Optional Fields in toStruct do not get null value #171
Description
Summary
When a struct contains an optional field without a default value, an arbitrary value is assign (I suspect it is the uninitialized memory).
Possible solution
I noticed that if I change
Lines 4856 to 4862 in 6889b2d
to
if (lua_field_type == .nil) {
if (field.defaultValue()) |default| {
@field(result, field.name) = default;
} else if (field_type_info != .optional) {
return error.LuaTableMissingValue;
} else {
@field(result, field.name) = null;
}
} else { resolves the issue, but I'm not sure this is the right approach (hence issue and not PR)
Reproducing
Code
const zlua = @import("zlua");
const std = @import("std");
const Lua = zlua.Lua;
const A = struct {
a: ?u64,
};
pub fn main() !void {
var gpa = std.heap.GeneralPurposeAllocator(.{}){};
const allocator = gpa.allocator();
defer _ = gpa.deinit();
var lua = try Lua.init(allocator);
defer lua.deinit();
try lua.doString("return {a=4};");
std.debug.print("{}\n", .{try lua.toStruct(A, null, false, 1)});
lua.pop(1);
try lua.doString("return {a=nil};");
std.debug.print("{}\n", .{try lua.toStruct(A, null, false, 1)});
lua.pop(1);
}Expected
main.A{ .a = 4 }
main.A{ .a = null }
Result
main.A{ .a = 4 }
main.A{ .a = 12297829382473034410 }
Workaround
Defining a default value of null circumvent this issue. In the example above:
const A = struct {
a: ?u64 = null,
};Metadata
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[-]Optional Fields in toStruct[/-][+]Optional Fields in toStruct do not get null value[/+]robbielyman commentedon Aug 11, 2025
I'd accept a PR with the Possible Solution fix. I'm not sure the API as-written is in a state that I'm completely happy with, given that nullability and gracefully handling type mismatches are fundamental differences between Zig and Lua, but it seems clear that
nullis intended in this situation and should not require a default value.tmrlvi commentedon Aug 17, 2025
That's a good point - on the
luaside all the following are equivalent:So actually defaulting optionals to
nullmight be a footgun that will cover issues in the lua code. However, this is an issue even when default value exists, and I think people usually expect implicitnullwhen using optionals.I come to think the solution above is the best option. I'll probably submit the PR later this week