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Following is an example of an IDL interface showing the legal nested definitions that can occur (We have not yet discussed all the features shown in the example, which are covered over the next few pages)
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interface Haystack { exception NotFound { unsigned long num_straws_searched; }; const unsigned long MAX_LENGTH = 10; // Max len of a needle // Stack size
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readonly attribute unsigned long num_straws; typedef long typedef string Needle; // ID type for needles Straw; // ID type for straws
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void add(in Straw s); // Grow stack boolean remove(in Straw s); // Shrink stack void find(in Needle n) raises(NotFound); // Find needle
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The scope resolution rules of IDL are the same as for C++ In the preceding example, the type Needle is used in the definition of the find operation Because both the type and the operation definition are in the same scope, no qualification is needed Because the nested definitions are not hidden, you can use types defined in a different scope by using the :: scope resolution operator to qualify a name:
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interface FeedShed { typedef sequence<Haystack> StackList; StackList feed_on_hand(); // Return all stacks in shed void add(in Haystack s); // Add another haystack void eat(in Haystack s); // Cows need to be fed // Look for needle in all haystacks boolean find(in Haystack::Needle n) raises(Haystack::NotFound); // Hide a needle void hide(in Haystack s, in Haystack::Needle n); };
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Note that this definition uses the qualified type names Haystack::Needle and Haystack::NotFound As with C++, these names also could have been written as ::Haystack::Needle and ::Haystack::NotFound (a leading :: indicates the global scope) 482 Interface Semantics and Object References The haystack example illustrates a central IDL feature Note that a feed shed is a collection manager for haystacks, which in turn are collection managers for straws You add a haystack to the shed by passing a parameter of type Haystack to the add operation This illustrates two things
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IT-SC book: Advanced CORBA Programming with C++
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Interface names become type names in their own right Interface instances can be passed as parameters Conceptually, a client invoking the add operation passes a particular haystack to be added to the feed shed The semantics are as if the haystack object itself were passed However, what really happens is that the client passes an object reference to the add operation, and the implementation of add appends that object reference to its list of haystacks in the shed In other words, an object reference acts as a pointer and can be stored in a collection The semantics of object references are very much like those of a C++ class instance pointer except that an object reference can point at an object outside the caller's address space It follows that if each of two clients holds an object reference to the same object, any changes made by one client will be visible to the other client If a client does not want to share state changes, it must make an explicit copy of the object We discuss how to do this in 12 Like C++ pointers, object references are strongly typed The FeedShed::add operation expects a parameter of type Haystack You cannot pass some other interface to the operation unless that interface is derived from Haystack For the C++ mapping, the type safety of object references is enforced at compile time, in keeping with the strong typing model of C++ Conversely, for dynamically typed languages such as Smalltalk, type safety is enforced at run time instead CORBA defines a special nil object reference Like a C++ null pointer, a nil reference denotes no object (points nowhere) Nil references are useful for implementing optional or "not found" semantics The Haystack::find operation looks for a particular needle in the haystack and, if it finds the needle, removes it from the stack The FeedShed::find operation searches all the haystacks in the shed for a needle (One possible implementation is simply to iterate over the shed's list of haystacks and invoke the find operation on each haystack via its stored object reference) Of course, FeedShed and Haystack instances may be implemented in different address spaces (that is the whole point of making them IDL interfaces) When the FeedShed implementation invokes the find operation on a Haystack, it sends a remote procedure call to the object nominated by the object reference In OO terms, it sends a message to the object Because the feed shed interacts with each haystack only through a defined interface, all the sheds and haystacks can in fact be implemented on different machines The semantics of this are the same as if haystack objects were implemented in the same address space as their feed shed
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