(language Inheritance independent)
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First view: exploit common interfaces in programming Different kinds of bank accounts Tapestry tmap, iterator, C++ function objects Implementation varies while interface stays the same Second view: share code, factor code into parent class Code in parent class shared by subclasses Subclasses can override inherited method
Can subclasses override and call?
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Polymorphism/late(runtime) binding (compare: static) Actual function called determined when program runs, not when program is compiled
2.1
CompSci 108 : Fall 2003
Inheritance guidelines in C++
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Inherit from Abstract Base Classes (ABC) one pure virtual function needed (=0)
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must have virtual destructor implemented can have pure virtual destructor implemented, but not normally needed Avoid protected data, but sometimes this isn't possible data is private, subclasses have it, can't access it keep protected data to a minimum Single inheritance, assume most functions are virtual multiple inheritance ok when using ABC, problem with data in super classes virtual: some overhead, but open/closed principle intact
2.2
Subclasses must implement, or they're abstract too
CompSci 108 : Fall 2003
Inheritance Heuristics
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A base/parent class is an interface Subclasses implement the interface
The base/parent class can supply some default behavior The base/parent class can have state
Derived classes can use, override, both Protected: inherited and directly accessible Private: inherited but not accessible directly
Behavior changes in subclasses, but there's commonality
q q
Abstract base classes are a good thing Push common behavior as high up as possible in an inheritance hierarchy If the subclasses aren't used polymorphically (e.g., through a pointer to the base class) then the inheritance hierarchy is probably flawed
2.3
CompSci 108 : Fall 2003
Inheritance Heuristics in C++
q
One pure virtual (aka abstract) function makes a class abstract Cannot be instantiated, but can be constructed (why?) Default in C++ is nonvirtual or monomorphic Must have virtual destructor, the base class destructor (and constructor) will be called
Unreasonable emphasis on efficiency, sacrifices generality If you think subclassing will occur, all methods are virtual
q
We use inheritance, public models isa relationship Private inheritance means isimplementedintermsof
Implementation technique, not design technique Derived class methods call baseclass methods, but no "usableas a" via polymorphism Access to protected methods, and can redefine virtual funcs
2.4
CompSci 108 : Fall 2003
Inheritance and Layering/Aggregation
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Layering (or aggregation) means "uses via instance variable" Use layering/attributes if differences aren't behavioral Use inheritance when differences are behavioral Consider Student class: name, age, gender, sleeping habits Which are attributes, which might be virtual methods Lots of classes can lead to lots of problems It's hard to manage lots of classes in your head Tools help, use speedbar in emacs, other class browsers in IDEs or in comments (e.g., javadoc) Inheritance hierarchies cannot be too deep (understandable?)
2.5
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CompSci 108 : Fall 2003
Inheritance guidelines (see Riel)
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Watch out for derived classes with only one instance/object For the CarMaker class is GeneralMotors a subclass or an object? Watch out for derived classes that override behavior with a noop Mammal class from which platypus derives, livebirth? Too much subclassing? Base class House Derived: ElectricallyCooledHouse, SolarHeatedHouse? What to do with a list of fruit that must support applecoring? Fr...