Optional fields need to be set manually after construction of IDL-generated classes, which is cumbersome. Instead, IDL could generate constructor(s) that allow one to set optional arguments, perhaps through a builder API.

e.g.

auto foo = MyIdlStruct::builder(requiredArg1, requiredArg2).setOptionalArg("bar").build(); 

This would avoid the need to construct instances of IDL-generated classes in immediately invoked lambdas or over the course of multiple setX statements.