struct Foo: Encodable { var optionalInt: Int? private struct FooEncode: Encodable { var optionalInt: KeyedOptional<Int> /* Foo <-> FooEncode 変換書く */ } func encode() throws -> Data { return FooEncode(self).encode() } } こげなstruct Foo: Encodable { var optionalInt: Int? private struct FooEncode: Encodable { var optionalInt: KeyedOptional<Int> /* Foo <-> FooEncode 変換書く */ } func encode() throws -> Data { return FooEncode(self).encode() } } こげな