Order of fields in the `.Struct.fields` array

castholm · February 2, 2025, 5:30pm

I can’t find any official word about it in any official documentation, but I’m fairly confident that when you use @typeInfo to reflect on a type, the order of fields, decls and other members is guaranteed to be the same as the order they were declared in source.

At the very least, the compiler has behavior tests that assert that the order is correct:

github.com

ziglang/zig/blob/953355ebeab881abff4a2c9315daa4fbb290d733/test/behavior/type_info.zig#L283-L334


      
          test "type info: struct info" {
              if (builtin.zig_backend == .stage2_sparc64) return error.SkipZigTest; // TODO
          
              try testStruct();
              try comptime testStruct();
          }
          
          fn testStruct() !void {
              const unpacked_struct_info = @typeInfo(TestStruct);
              try expect(unpacked_struct_info.@"struct".is_tuple == false);
              try expect(unpacked_struct_info.@"struct".backing_integer == null);
              try expect(unpacked_struct_info.@"struct".fields[0].alignment == @alignOf(u32));
              try expect(unpacked_struct_info.@"struct".fields[0].defaultValue().? == 4);
              try expect(mem.eql(u8, "foobar", unpacked_struct_info.@"struct".fields[1].defaultValue().?));
          }
          
          const TestStruct = struct {
              fieldA: u32 = 4,
              fieldB: *const [6:0]u8 = "foobar",
          };

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