CHOOSING CLASS, METHOD AND PARAMETER NAMES in Java

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CHOOSING CLASS, METHOD AND PARAMETER NAMES
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it s easier when looking back at the code to understand what it means For example, take the following function declaration in class CGpSurgery:
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class CGpSurgery : public CBase { public: void MakeHospitalAppointment(TBool aExistingPatient, TBool aUrgency, TBool aTestData); // Other functions omitted for clarity };
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Without looking at the declaration of MakeHospitalAppointment(), it s not immediately clear how to call the method, for example to make an urgent hospital appointment for a new patient who already has some test data available Unless you name your parameters carefully, your client may well have to consult your documentation too, to nd out the appropriate boolean value for each variable
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MakeHospitalAppointment(EFalse, ETrue, ETrue);
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A far clearer approach is to use enumeration values whose name clearly indicates their purpose However, if you de ne them in global scope, you or your clients may well nd that a name clash arises with code in other header les, particularly if you choose short, simple names such as TSize or TColor Even if your code doesn t get a clash, this (ab)use of the global scope means that code which includes your header may get a con ict later down the line It s preferable to de ne them inside the class to which they apply The caller then uses the class scope to identify them This is a useful approach for class-speci c enumerations, typedefs and constants If you prefer to keep them out of your class scope, you could alternatively use a C++ namespace to prevent spilling your de nitions into the global scope
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class CGpSurgery : public CBase { public: enum TPatientStatus { EExistingPatient, ENewPatient }; enum TAppointmentUrgency { EUrgent, ERoutine }; enum TTestData { ETestResultsPending, ETestResultsAvailable }; public: void MakeHospitalAppointment(TPatientStatus aExistingRecords, TAppointmentUrgency aUrgency, TTestData aTestData); };
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Looking at the function call, it is now signi cantly clearer what each parameter refers to; in effect, the code has documented itself
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MakeHospitalAppointment(CGpSurgery::ENewPatient, CGpSurgery::EUrgent, CGpSurgery::ETestResultsAvailable);
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The use of enumerations rather than boolean values also provides extensibility in the future; in the case of the CGpSurgery class, additional levels of appointment urgency, patient status or test data availability can be introduced without the need to change the signature of MakeHospitalAppointment() This avoids breaking compatibility which is the subject of 18
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I ve discussed some of the things to consider when de ning your class, but before concluding this chapter, it s worth brie y describing the functions that the compiler generates for you if you don t add them yourself If you have not declared a copy constructor, assignment operator or destructor, the compiler generates them implicitly for a class in case they need to be invoked If there are no constructors declared, it declares a default constructor too The implicitly-generated functions are public; the constructor and destructor are simply placeholders for the compiler to add the code required to create and destroy an object of the class (for example, to set up or destroy the virtual function table) The destructor does not perform any cleanup code The compiler-generated copy constructor and assignment operator perform a copy or assignment on each member of your class, invoking the copy constructor or assignment operator if there is one de ned, or applying the rule for the members of the encapsulated object if not Built-in types, pointers and references are copied or assigned using a shallow bitwise copy This is problematic for pointers to objects on the heap, since a bitwise copy is rarely desirable, opening up opportunities for dangling pointers; these can result in a memory leak or multiple deletion through the same pointer, which raises a panic (USER 44) This is particularly true for CBase-derived classes which should be constructed through the NewL() and NewLC() functions, which are guaranteed to initialize the object correctly using two-phase construction (as described in 4) They should not be copy constructed or assigned to, because this bypasses the two-phase construction and makes shallow copies of any pointers to dynamic memory To prevent accidental copies, CBase declares a private copy constructor and assignment operator (as you ll see from the class de nition in e32baseh) Declaring, but not de ning, a private copy constructor and assignment operator prevents calling code from performing invalid copy operations using compiler-generated code If your C class does need a copy constructor, you must explicitly declare and de ne one publicly or provide a CloneL() or CopyL() method, which allows you to make leaving calls, which are not possible in a copy constructor
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