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If a particular context variable is not set by the client, its value is (silently) not transmitted to the server This means that the server cannot rely on the value of a particular context variable being available even though it appears in the context clause Context variables are untyped For the preceding example, the server may expect to find a numerical user ID in the USER variable However, the client may have placed the user name into the variable This illustrates that context clauses provide no guarantees to the server implementation A context variable may not be set at all, and, even if it is set, it may contain a string that does not correctly decode to the expected type This is a recipe for disaster because it shoots a big hole through the IDL type system CORBA implements strict type checking for operations, and that makes it impossible for a client to forget to supply a parameter or to supply a parameter of the wrong type[3] In contrast, context variables provide no such guarantees
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[3] It is possible to violate the type system by using "sledgehammer" C++ casts However, if you insist on using casts, you deserve what you get It is also possible to violate the type system by using the DII incorrectly, but that is the price of its flexibility
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Because IDL contexts are unsafe, we recommend that you avoid using them It is also possible that contexts may be removed from CORBA, so the future of this (mis)feature is uncertain anyway
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414 Attributes
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An attribute definition can be used to create something akin to a C++ public member variable:
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interface Thermostat { readonly attribute short attribute short }; temperature; nominal_temp; // Probably bad // Probably bad
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The attribute keyword may be used only inside an interface definition Attributes can be of any type (including user-defined complex types) An attribute defines a pair of operations the client can call to send and receive a value A readonly attribute defines a single operation the client can call to receive a value Attributes look like C++ public member variables, but in fact they do not define storage or state For example, the following interface is semantically equivalent to the preceding one:
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interface Thermostat {
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IT-SC book: Advanced CORBA Programming with C++
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short short void
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get_temperature(); get_nominal_temp(); set_nominal_temp(in short t);
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Even though attribute definitions look like variables, in reality they are just a shorthand for defining a pair of operations (or a single operation for readonly attributes) There simply is no semantic difference between the preceding two interfaces In both cases, attribute access is implemented by remote procedure calls There is a problem relating to attributes, though: an attribute definition cannot contain a raises expression The following is illegal:
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interface Thermostat { exception TooHot {}; exception TooCold {}; readonly attribute short attribute short
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temperature; nominal_temp raises( // Illegal TooHot, TooCold );
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Attributes cannot raise user exceptions (system exceptions are possible) This makes attributes second-class citizens, because error reporting is quite limited For example, setting the temperature of a thermostat should raise an out-of-range exception if an attempt is made to set the nominal temperature too high or too low However, attributes limit you to error reporting via system exceptions This means that you must resort to a system exception (for example, CORBA::BAD_PARAM) when an illegal temperature is requested This exception is less informative than TooHot and TooCold user exceptions You cannot safely use the minor member in a system exception to encode the "too hot" and "too cold" conditions This is because the specification gives no guarantee that an ORB will preserve the minor value of a system exception Most ORBs will preserve it, but, if you rely on this behavior, you are, strictly speaking, outside the CORBA specification (And, as we point out in Section 411, you should not use system exceptions for application-level error conditions anyway) The implementation of attributes by the ORB run time is identical to using operations (attributes are implemented as a pair of operations) This means that there is no difference in performance between attribute accesses and operation invocations Because attributes offer no performance advantage but suffer from limited error reporting, some organizations have banned attributes in their style guides You may want to consider doing the same
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