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98 Raising Exceptions
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Servant methods raise IDL exceptions to indicate unexpected errors For example, the set_nominal operation of the Thermostat interface from our example CCS module can raise the BadTemp exception:
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#pragma prefix "acmecom" module CCS { typedef short TempType; interface Thermometer {/**/}; interface Thermostat : Thermometer { struct BtData { TempType TempType TempType string requested; min_permitted; max_permitted; error_msg;
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}; exception BadTemp { BtData details; }; TempType get_nominal(); TempType set_nominal(in TempType new_temp) raises(BadTemp);
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The corresponding method in the skeleton class has the following signature:
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namespace POA_CCS { class Thermostat : public virtual Thermometer { public: // virtual CCS::TempType set_nominal(CCS::TempType new_temp) throw(CORBA::SystemException, CCS::BadTemp) = 0; // }; }
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The exception specification for set_nominal allows the user-defined CCS::BadTemp exception to be thrown, and, as with all servant methods, set_nominal may also throw CORBA system exceptions 981 Exception Throwing Details An implementation of set_nominal in a derived servant class might be written as follows:
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Thermostat_impl:: set_nominal( CCS::TempType new_temp ) throw(CORBA::SystemException, CCS::BadTemp) { const CCS::TempType MIN_TEMP = 50, MAX_TEMP = 90; if (new_temp < MIN_TEMP || new_temp > MAX_TEMP) { BtData bt; btrequested = new_temp; btmin_permitted = MIN_TEMP; btmax_permitted = MAX_TEMP; bterror_msg = CORBA::string_dup("temperature out of range"); throw CCS::BadTemp(bt); } //
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Our example first checks the requested temperature setting to ensure that it is within the permitted range If the temperature is outside the permitted range, we create an instance of the BtData structure and initialize it with information about the error We then use the structure instance to construct and throw an instance of CCS::BadTemp to inform the client of the error The C++ mapping allows you to throw exceptions by value and catch them by reference Each exception class supplies a constructor that takes an initialization parameter for each exception member This allows you to create and throw exception instances all in the same statement, as shown in the preceding example The alternative of allocating exceptions on the heap and throwing them by pointer would merely add memory management responsibilities for clients, requiring them to remember to delete thrown exceptions Keep in mind that a servant method is allowed to throw only the exceptions explicitly listed in its exception specification This includes all CORBA system exceptions because of the appearance of the CORBA::SystemException base class in all servant exception specifications The C++ run time will prevent a servant method from throwing any exception not listed in its exception specification even if that exception is thrown by a function called directly or indirectly by the servant method Unfortunately, ORB implementations cannot count on C++ exception specifications to prevent servants from throwing illegal exceptions Some C++ compilers that do not fully support standard C++ supply no error or warning messages if a servant method has a less restrictive exception specification than the skeleton method it is overriding For example, with some C++ compilers we can rewrite our Thermostat_impl::set_nominal signature without any exception specification, and the C++ compiler will not complain (assuming the method is also declared in the same way in the class definition):
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Thermostat_impl:: set_nominal( CCS::TempType new_temp ) // oops, missing exception specification! { // same code as before }
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Moreover, some widely used C++ compilers do not properly implement or support exception specifications, meaning that IDL compilers are forced to elide them when generating code for certain platforms To ensure that only allowed exceptions are thrown from a servant method, the ORB and the skeleton enclose all servant method invocations in a catch block that traps all CORBA and non-CORBA exceptions Any exceptions that should not be thrown by the servant method, including CORBA user-defined exceptions and general C++ exceptions, are caught by the ORB and turned into the CORBA::UNKNOWN system exception instead This catch block acts as a barrier that prevents exception-related application errors from entering the ORB run time, where they are unexpected and could cause the entire application to abort The catch block also prevents the client application from receiving user-defined exceptions that were not declared in the operation's raises clause 982 Throwing CORBA System Exceptions We have repeatedly pointed out that all servant methods are allowed to throw CORBA system exceptions The main reason is to allow the ORB to raise exceptions for any error conditions it encounters when performing its location, activation, and request and response delivery However, this opens the door for servant method implementations to also throw CORBA system exceptions directly Unfortunately, throwing CORBA system exceptions from within servant methods can lead to systems that are difficult to debug For example, a servant method might want to throw the CORBA::BAD_PARAM exception if one of its input parameters has an unexpected value or to throw the CORBA::NO_MEMORY exception to indicate that it could not successfully allocate a variable-length out parameter However, the ORB also uses these exceptions to indicate errors that it encounters in attempting to deliver requests or replies When a client catches a CORBA system exception under these circumstances, it does not know whether it was caused by a problem with the ORB or a problem with the servant implementation To avoid confusion between ORB problems and servant problems, you should avoid directly throwing most CORBA system exceptions Instead, you should throw userdefined exceptions to indicate application-level errors This implies that you should consider all potential errors when designing your IDL interfaces so that you declare the appropriate exceptions within each operation's raises clause Of course, not all error conditions fit clearly into one case or the other For example, you should probably throw
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