creation_of_struct_or_union

Subclasses of 'Struct' and 'Union' are backed by native memory, and can't be instantiated by a generative constructor.

The analyzer produces this diagnostic when a subclass of either Struct or Union is instantiated using a generative constructor.

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

The following code produces this diagnostic because the class C is being instantiated using a generative constructor:

import 'dart:ffi';

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

void f() {
  C();
}

If you need to allocate the structure described by the class, then use the ffi package to do so:

import 'dart:ffi';
import 'package:ffi/ffi.dart';

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

void f() {
  final pointer = calloc.allocate<C>(4);
  final c = pointer.ref;
  print(c);
  calloc.free(pointer);
}

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.