On 5 April 2012 01:02, Rush Manbert <[email protected]> wrote:
>
> On Apr 2, 2012, at 12:47 AM, Piscium wrote:
>
>> On C++ generated code, __isset is set to true in the read function
>> for the fields that have been received, however it is not looked at at
>> all by the write function which always writes all fields regardless of
>> the __isset value.
>>
>> What is the reasoning behind this asymmetry? Is __isset intended only
>> to cater for the receiving side, in particular for the use case in
>> which the receiver is of a different software version than the sender?
> I don't think this is quite true. All required fields are always written,
> regardless of the state of their __isset. Optional fields are only written if
> their __isset is true. The read side always sets the __isset for everything
> it receives, but it doesn't receive the fields that are optional and didn't
> have __isset true when written.
Hi Rush,
I described what I found out in my tests. I only tested with Thrift v.
0.8.0, and only with C++ (things may be different with other Thrift
versions or with Java or Python, I wouldn't know).
Anyway please see this minimal example (with comments):
================
namespace cpp test
struct Test {
1: i32 npotatoes, // optional field
}
================
This is the generated file (with some extra comments added by myself):
uint32_t Test::write(::apache::thrift::protocol::TProtocol* oprot) const {
uint32_t xfer = 0;
xfer += oprot->writeStructBegin("Test");
xfer += oprot->writeFieldBegin("npotatoes",
::apache::thrift::protocol::T_I32, 1);
xfer += oprot->writeI32(this->npotatoes); // <<<<<<<< This optional
field is always written, regardless of __isset
xfer += oprot->writeFieldEnd();
xfer += oprot->writeFieldStop();
xfer += oprot->writeStructEnd();
return xfer;
=====
uint32_t Test::read(::apache::thrift::protocol::TProtocol* iprot) {
//snip
switch (fid)
{
case 1:
if (ftype == ::apache::thrift::protocol::T_I32) {
xfer += iprot->readI32(this->npotatoes); // <<<<<<<< This
optional field is always read, regardless of __isset
// As __isset is not sent over the wire, it makes sense for
the read function to set it when received
this->__isset.npotatos = true;
} else {
xfer += iprot->skip(ftype);
}
break;
default:
xfer += iprot->skip(ftype);
break;
}
//snip
