How to initialize generic fields

[deadsoul44]

Since type parameter cannot be accessed from static void _initializeBuilder method, it is not possible to initialize generic fields. Is there any workaround? I want to initialize based on all possible types of the generic. For example if T is int, initial value will be 0, if T is String, it will be ''.

[davidmorgan]

It should work to pass the builder to a generic method:

static void _initializeBuilder(FooBuilder b) => _initializeBuilderG(b);
static void _initializeBuilderG<T>(FooBuilder<T> b) {
  if (T == int) ...
}

it would make sense to have built_value support making _initializeBuilder itself generic, I'll treat this issue as a feature request.

Thanks.

[deadsoul44]

I have to add type cast because analyzer is complaining about it.

  static void _initializeBuilder(FooBuilder b) => _initializeBuilderGeneric(b);
  static void _initializeBuilderGeneric<T>(FooBuilder<T> builder) {
    if (T == int) {
      builder
        ..field1 = (-1) as T?
        ..field2 = (-1) as T?;
    }
  }

[deadsoul44]

This doesn't seem to work.

  static void _initializeBuilder(FooBuilder b) => _initializeBuilderGeneric(b);
  static void _initializeBuilderGeneric<T>(FooBuilder<T> builder) {
      builder
        ..field1 = ListBuilder<T>();
  }

I get this:

---------- ERROR ----------
Unsupported operation: explicit element type required, for example "new ListBuilder<int>"

------- STACK TRACE -------
#0      ListBuilder._checkGenericTypeParameter (package:built_collection/src/list/list_builder.dart:289:7)
#1      new ListBuilder._uninitialized (package:built_collection/src/list/list_builder.dart:267:5)
#2      new ListBuilder (package:built_collection/src/list/list_builder.dart:28:12)

[deadsoul44]

The first example does not work either.

[davidmorgan]

Sorry, yes, that doesn't work. You'll need to use 'is', like this

  static void _initializeBuilder(InitializeGenericValueBuilder b) {
    if (b is InitializeGenericValueBuilder<int>) {
      b.value = 3;
    }
  }

actually it looks like it does work to get the generic type parameter, although I did not design this :) e.g.

  static void _initializeBuilder<T>(InitializeGenericValueBuilder<T> b) {
    if (T == int) {
      b.value = 3 as T;
    }
  }

you still to use is somehow to avoid casting to T, though, so it doesn't actually get you much.

I'll add some test coverage for this.

[davidmorgan]

3d971e4