Why do CORBA interfaces have a return type and an out keyword?

Question

I was reading about CORBA and I noticed something funny. CORBA interface methods are defined with a return type, for example string in

module HelloApp
{
  interface Hello
  {
  string sayHello();
  oneway void shutdown();
  };
};

and also has a keyword out which when passed as a parameter, means that value is returned. For example here

struct Data { ... };
typedef sequence<Data> DataSeq;
interface DataIterator {
  DataSeq next_n_items(in unsigned long how_many);
  void    destroy();
};
interface SearchEngine {
  DataSeq query(
             in  string         search_condition,
             in  unsigned long  how_many,
             out DataSeq        results,
             out DataIterator   iter);
};

Seems redundant. Why do you need both?

Answer 1

An operation can only have one return value, but can have a multiple out arguments. It is a user decision what to use, but in a lot of cases an operation returns one value and then it is easier for the user to have a return value so that he can write for example (using IDL to C++11):

int32_t my_result = foo->my_operation ();

Where with an out argument he has to write

int32_t my_result {};
foo->my_operation (my_result);

The first example is easier and also safer, no need to explicitly initialize my_result to its default value.