empty_struct

The class '{0}' can't be empty because it's a subclass of '{1}'.

The analyzer produces this diagnostic when a subclass of Struct or Union doesn't have any fields. Having an empty Struct or Union isn't supported.

For more information about FFI, see C interop using dart:ffi.

The following code produces this diagnostic because the class C, which extends Struct, doesn't declare any fields:

import 'dart:ffi';

final class C extends Struct {}

If the class is intended to be a struct, then declare one or more fields:

import 'dart:ffi';

final class C extends Struct {
  @Int32()
  external int x;
}

If the class is intended to be used as a type argument to Pointer, then make it a subclass of Opaque:

import 'dart:ffi';

final class C extends Opaque {}

If the class isn't intended to be a struct, then remove or change the extends clause:

class C {}

Unless stated otherwise, the documentation on this site reflects Dart 3.7.3. Page last updated on 2025-05-08. View source or report an issue.