The Object Pseudo-Interface in Software

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711 The Object Pseudo-Interface
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As you saw in Section 73, all interfaces inherit from Object, which is a pseudointerface defined in the CORBA module:
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module CORBA { // PIDL // interface Object { Object duplicate(); void release(); boolean is_nil(); boolean is_a(in string repository_id); boolean non_existent(); boolean is_equivalent(in Object other_object); unsigned long hash(in unsigned long max); // }; // };
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We have already seen the mapping for duplicate, release, and is_nil (see Table 71 for a summary) This section covers the is_a, non_existent, is_equivalent, and hash operations (Interface Object also contains other operations relating to the DII, security, and administration, but these operations are outside the scope of this book) The operations shown here map to member functions of CORBA::Object:
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class Object {
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IT-SC book: Advanced CORBA Programming with C++
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public: // Boolean Boolean Boolean ULong // };
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_is_a(const char * repository_id); _non_existent(); _is_equivalent(Object_ptr other_object); _hash(ULong max);
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Note that all four operations are mapped with a preceding underscore (is_a becomes _is_a and so on) This rule prevents clashes with user-defined IDL operations in derived interfaces For example, if you create an interface containing an is_a operation of your own, your is_a operation maps to C++ is_a, whereas the is_a inherited from Object maps to _is_a to avoid clashes All four operations are implemented as non-static member functions of class Object This means that you cannot invoke them on nil references:
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CORBA::Object_ptr p = CORBA::Object::_nil(); // Make nil ref if (p->_non_existent()) // Crash imminent!!! //
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Remember that it is illegal to invoke operations on nil references, and pseudo-references are no different The only functions that are safe for use with nil references are static member functions and functions in the CORBA namespace, such as _duplicate, release, and is_nil If you cannot be sure that a reference is not nil, you can guard the test with is_nil:
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if (CORBA::is_nil(p) || p->_non_existent()) // Objref is nil or dangles
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7111 The _is_a Operation _is_a tests whether an object reference supports the interface specified by the repository_id argument The argument must be a well-formed repository ID in one of the formats shown in Section 419 For example:
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CORBA::Object_ptr obj = ; // Get controller reference if (!CORBA::is_nil(obj)) { if (obj->_is_a("IDL:acmecom/CCS/Controller:10")) { // It's a controller } else { // It's something else } } else { // It's a nil reference
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The test returns true if the object reference supports the specified interface Note that you can use _is_a to test whether an object supports a base interface:
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CORBA::Object_ptr obj = ; // Get actual thermostat reference assert(obj->_is_a("IDL:acmecom/CCS/Thermometer:10")); assert(obj->_is_a("IDL:omgorg/CORBA/Object:10"));
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Assuming that obj really is initialized with a thermostat reference, both assertions succeed Of course, the second assertion must succeed for all references because all interfaces inherit from Object If the passed repository ID does not match the syntax for repository IDs in Section 419, _is_a throws an exception (The CORBA specification does not state which one; BAD_PARAM is a likely choice) _is_a is similar to _narrow; both functions test whether a reference supports a particular interface The difference is that _is_a does not require compile-time knowledge of the interface, whereas _narrow requires the caller to link the stubs generated by the IDL compiler _is_a is provided mainly for clients using the DII, which acquire type information at run time Note that _is_a and _narrow may send a message to the server (see Section 1341) If the server cannot be contacted, either operation will raise a system exception Whether or not _is_a and _narrow result in a remote message depends on your ORB implementation If both the type of the reference and the narrowed-to type were known at compile time, the client-side run time can determine the result statically by using the repository IDs of the reference and the narrowed-to type In that case, no remote message need be sent However, if at least one of the repository IDs was not seen at compile time, the client-side run time is forced to contact the server that implements the object to find out whether the object supports the narrowed-to type In practice, you rarely care whether _is_a or _narrow results in a remote message However, your code must be prepared to handle system exceptions from calls to these operations For example, if the server for an object cannot be reached, the client gets a TRANSIENT exception on its call to _narrow (instead of getting it when the client invokes its first operation on the object) 7112 The _non_existent Operation _non_existent tests whether a reference denotes an existing object If the reference no longer denotes an existing object (the reference dangles), _non_existent returns true
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