Note: This document covers API impact only. For more details, see the ABI compatibility page

Change a union from strict to flexible

Overview

- init step 1 step 2 step 3
fidl link link
dart link link link
go link link
hlcpp link link link
llcpp link link link
rust link link link

Initial State {#init}

FIDL {#fidl-init}

  1. strict union JsonValue {
  2.     1: int32 int_value;
  3.     2: string:MAX string_value;
  4. };

Dart {#dart-init}

  1. void useUnion(fidllib.JsonValue value) {
  2.   assert(value.$unknownData == null);
  3.   switch (value.$tag) {
  4.     case fidllib.JsonValueTag.intValue:
  5.       print('int value: ${value.intValue}');
  6.       break;
  7.     case fidllib.JsonValueTag.stringValue:
  8.       print('string value: ${value.stringValue}');
  9.       break;
  10.   }
  11. }

Go {#go-init}

  1. func useUnion(value lib.JsonValue) {
  2.     switch value.Which() {
  3.     case lib.JsonValueIntValue:
  4.         fmt.Printf("int value: %d\n", value.IntValue)
  5.     case lib.JsonValueStringValue:
  6.         fmt.Printf("string value: %s\n", value.StringValue)
  7.     default:
  8.         fmt.Println("unknown tag")
  9.     }
  10. }

HLCPP {#hlcpp-init}

  1. void use_union(fidl_test::JsonValue value) {
  2.   switch (value.Which()) {
  3.     case fidl_test::JsonValue::Tag::kIntValue:
  4.       printf("int value: %d\n", value.int_value());
  5.       break;
  6.     case fidl_test::JsonValue::Tag::kStringValue:
  7.       printf("string value: %s\n", value.string_value().c_str());
  8.       break;
  9.     case fidl_test::JsonValue::Tag::Invalid:
  10.       printf("<uninitialized union>\n");
  11.       break;
  12.   }
  13. }

LLCPP {#llcpp-init}

  1. void use_union(fidl_test::JsonValue* value) {
  2.   switch (value->which()) {
  3.     case fidl_test::JsonValue::Tag::kIntValue:
  4.       printf("int value: %d\n", value->int_value());
  5.       break;
  6.     case fidl_test::JsonValue::Tag::kStringValue:
  7.       printf("string value: %s\n", value->string_value().data());
  8.       break;
  9.   }
  10. }

Rust {#rust-init}

  1. fn use_union(value: &fidl_lib::JsonValue) {
  2.     match value {
  3.         fidl_lib::JsonValue::IntValue(n) => println!("int value: {}", n),
  4.         fidl_lib::JsonValue::StringValue(s) => println!("string: {}", s),
  5.     };
  6. }

Update Source Code {#step-1}

Dart {#dart-1}

  • Add a default case to any switch statements on the union to handle new unknown variants
  1.   void useUnion(fidllib.JsonValue value) {
  2. -   assert(value.$unknownData == null);
  3.     switch (value.$tag) {
  4.       case fidllib.JsonValueTag.intValue:
  5.         print('int value: ${value.intValue}');
  6.         break;
  7.       case fidllib.JsonValueTag.stringValue:
  8.         print('string value: ${value.stringValue}');
  9.         break;
  10. +     default:
  11. +       // Note: unknown variants will fail to decode until the union is marked flexible
  12. +       print('unknown variant: ${value.$unknownData}');
  13. +       break;
  14.     }
  15.   }

HLCPP {#hlcpp-1}

  • Add a default case to any switch statements on the union to handle new unknown variants
  1.   void use_union(fidl_test::JsonValue value) {
  2.     switch (value.Which()) {
  3.       case fidl_test::JsonValue::Tag::kIntValue:
  4.         printf("int value: %d\n", value.int_value());
  5.         break;
  6.       case fidl_test::JsonValue::Tag::kStringValue:
  7.         printf("string value: %s\n", value.string_value().c_str());
  8.         break;
  9.       case fidl_test::JsonValue::Tag::Invalid:
  10.         printf("<uninitialized union>\n");
  11.         break;
  12. +     default:
  13. +       printf("<unknown variant>\n");
  14.     }
  15.   }

LLCPP {#llcpp-1}

  • Add a default case to any switch statements on the union to handle new unknown variants
  1.   void use_union(fidl_test::JsonValue* value) {
  2.     switch (value->which()) {
  3.       case fidl_test::JsonValue::Tag::kIntValue:
  4.         printf("int value: %d\n", value->int_value());
  5.         break;
  6.       case fidl_test::JsonValue::Tag::kStringValue:
  7.         printf("string value: %s\n", value->string_value().data());
  8.         break;
  9. +     default:
  10. +       printf("<unknown variant>\n");
  11.     }
  12.   }

Rust {#rust-1}

  • Add [allow(unreachable_patterns)] and an underscore arm to match statements on the union
  1.   fn use_union(value: &fidl_lib::JsonValue) {
  2. +     #[allow(unreachable_patterns)]
  3.       match value {
  4.           fidl_lib::JsonValue::IntValue(n) => println!("int value: {}", n),
  5.           fidl_lib::JsonValue::StringValue(s) => println!("string: {}", s),
  6. +         _ => {}
  7.       };
  8.   }

Update FIDL Library {#step-2}

  • Change the union from strict to flexible
  1. - strict union JsonValue {
  2. + flexible union JsonValue {
  3.       1: int32 int_value;
  4.       2: string:MAX string_value;
  5.   };

Update Source Code {#step-3}

Dart {#dart-3}

  • Replace the default case with the unknown tag.
  1.   void useUnion(fidllib.JsonValue value) {
  2.     switch (value.$tag) {
  3.       case fidllib.JsonValueTag.intValue:
  4.         print('int value: ${value.intValue}');
  5.         break;
  6.       case fidllib.JsonValueTag.stringValue:
  7.         print('string value: ${value.stringValue}');
  8.         break;
  9. -     default:
  10. -       // Note: unknown variants will fail to decode until the union is marked flexible
  11. +     case fidllib.JsonValueTag.$unknown:
  12.         print('unknown variant: ${value.$unknownData}');
  13.         break;
  14.     }
  15.   }

Go {#go-3}

  • You may now use any flexible union specific APIs
  1.   func useUnion(value lib.JsonValue) {
  2.       switch value.Which() {
  3.       case lib.JsonValueIntValue:
  4.           fmt.Printf("int value: %d\n", value.IntValue)
  5.       case lib.JsonValueStringValue:
  6.           fmt.Printf("string value: %s\n", value.StringValue)
  7. +     case lib.JsonValue_unknownData:
  8. +         fmt.Printf("unknown data: %+v\n", value.GetUnknownData())
  9.       default:
  10.           fmt.Println("unknown tag")
  11.       }
  12.   }

HLCPP {#hlcpp-3}

  • Replace the default case with the kUnknown tag.
  • You may now use any flexible union specific APIs
  1.   void use_union(fidl_test::JsonValue value) {
  2.     switch (value.Which()) {
  3.       case fidl_test::JsonValue::Tag::kIntValue:
  4.         printf("int value: %d\n", value.int_value());
  5.         break;
  6.       case fidl_test::JsonValue::Tag::kStringValue:
  7.         printf("string value: %s\n", value.string_value().c_str());
  8.         break;
  9.       case fidl_test::JsonValue::Tag::Invalid:
  10.         printf("<uninitialized union>\n");
  11.         break;
  12. -     default:
  13. -       printf("<unknown variant>\n");
  14. +     case fidl_test::JsonValue::Tag::kUnknown:
  15. +       printf("<%lu unknown bytes>\n", value.UnknownBytes()->size());
  16. +       break;
  17.     }
  18.   }

LLCPP {#llcpp-3}

  • Replace the default case with the kUnknown tag.
  • You may now use any flexible union specific APIs
  1.   void use_union(fidl_test::JsonValue* value) {
  2.     switch (value->which()) {
  3.       case fidl_test::JsonValue::Tag::kIntValue:
  4.         printf("int value: %d\n", value->int_value());
  5.         break;
  6.       case fidl_test::JsonValue::Tag::kStringValue:
  7.         printf("string value: %s\n", value->string_value().data());
  8.         break;
  9. -     default:
  10. -       printf("<unknown variant>\n");
  11. +     case fidl_test::JsonValue::Tag::kUnknown:
  12. +       printf("<unknown data>\n");
  13. +       break;
  14.     }
  15.   }

Rust {#rust-3}

  • Remove the attribute and replace the underscore arm with the generated macro to match against unknown variants
  1.   fn use_union(value: &fidl_lib::JsonValue) {
  2. -     #[allow(unreachable_patterns)]
  3.       match value {
  4.           fidl_lib::JsonValue::IntValue(n) => println!("int value: {}", n),
  5.           fidl_lib::JsonValue::StringValue(s) => println!("string: {}", s),
  6. -         _ => {}
  7. +         fidl_lib::JsonValueUnknown!() => println!("<unknown union>"),
  8.       };
  9.   }