hi, I am a develop from LeTV China.

I use mongo cxx client ver. legacy, and have a question with the oid of the insert operation.

In the github wiki, I have read below:

Use the GENOID helper to add an object id to your object. The server will add an _id automatically if it is not included explicitly.

BSONObj p = BSON( GENOID << "name" << "Joe" << "age" << 33 );
// result is:

Unknown macro: { _id }

BSONObj p = BSONObjBuilder().genOID().append("name","Joe").append("age",33).obj();

It means that mongo client will not generate the oid for me unless I included it explicitly in the BSONbuilder.
But in `insert_write_operation.cpp`, I found that mongo client will gerate oid for me automatically.

    BSONObj InsertWriteOperation::_ensureId(const BSONObj& doc) {
        BSONElement id = doc.getField("_id");
        if (!id.eoo()) {
            uassert(0,
                    "value of _id element cannot contain any fields starting with $",
                    !id.isABSONObj() || id.Obj().okForStorage());
            return doc;
        }   

        BSONObjBuilder bob;
        bob.append("_id", OID::gen());
        bob.appendElements(doc);
        return bob.obj();
    }

So, will the mongo client give the choice to user to select who will genrate the oid, from the mongo client or the mongo server.