Union in thrift shows all values setted in c++

Question

I made a simple union of three fields

union example{
    1:string STRING,
    2:i64 INT64,
    3:double DOUBLE
}

And I instantiate the exam

Answer 1

uint32_t type_ex::write(::apache::thrift::protocol::TProtocol* oprot) const {
  uint32_t xfer = 0;
  apache::thrift::protocol::TOutputRecursionTracker tracker(*oprot);
  xfer += oprot->writeStructBegin("type_ex");

  xfer += oprot->writeFieldBegin("STRING", ::apache::thrift::protocol::T_STRING, 1);
  xfer += oprot->writeString(this->STRING);
  xfer += oprot->writeFieldEnd();

  xfer += oprot->writeFieldBegin("INT64", ::apache::thrift::protocol::T_I64, 2);
  xfer += oprot->writeI64(this->INT64);
  xfer += oprot->writeFieldEnd();

  xfer += oprot->writeFieldBegin("DOUBLE", ::apache::thrift::protocol::T_DOUBLE, 3);
  xfer += oprot->writeDouble(this->DOUBLE);
  xfer += oprot->writeFieldEnd();

  xfer += oprot->writeFieldStop();
  xfer += oprot->writeStructEnd();
  return xfer;
}

Looks like a bug with unions. The values are written unconditionally, whcih is right for struct but not for union. Consequently, the workaround would be to add an explicit optional:

union type_ex
{
    1 : optional string STRING,
    2 : optional i64 INT64,
    3 : optional double DOUBLE
}

Which gives us:

uint32_t type_ex::write(::apache::thrift::protocol::TProtocol* oprot) const {
  uint32_t xfer = 0;
  apache::thrift::protocol::TOutputRecursionTracker tracker(*oprot);
  xfer += oprot->writeStructBegin("type_ex");

  if (this->__isset.STRING) {
    xfer += oprot->writeFieldBegin("STRING", ::apache::thrift::protocol::T_STRING, 1);
    xfer += oprot->writeString(this->STRING);
    xfer += oprot->writeFieldEnd();
  }
  if (this->__isset.INT64) {
    xfer += oprot->writeFieldBegin("INT64", ::apache::thrift::protocol::T_I64, 2);
    xfer += oprot->writeI64(this->INT64);
    xfer += oprot->writeFieldEnd();
  }
  if (this->__isset.DOUBLE) {
    xfer += oprot->writeFieldBegin("DOUBLE", ::apache::thrift::protocol::T_DOUBLE, 3);
    xfer += oprot->writeDouble(this->DOUBLE);
    xfer += oprot->writeFieldEnd();
  }
  xfer += oprot->writeFieldStop();
  xfer += oprot->writeStructEnd();
  return xfer;
}

Bug report filed.