Unfortunately, single inheritance is not expressive enough to capture this lattice of relationships, so we must turn to multiple inheritance. Objects contribute to the behavior of a system by collaborating with one another. It is good engineering practice to encapsulate the state of an object rather than expose it.

In this manner, runtime searching is eliminated: Referring to a virtual member function of an object is just an indirect reference through the appropriate pointer, which immediately invokes the correct code without searching.

In the context of Figure 35, FlowController acts as a controller object, DisplayPanel acts as a server object, and Valve acts as a proxy. In particular, C++ allows a developer to make explicit distinctions among all four of these different parts. For this subclass, the redefined operation draw draws a circle of the given radius, centered on theCenter. Whereas links denote peer-to-peer or client/supplier relationships, aggregation denotes a whole/part hierarchy, with the ability to navigate from the whole (also called the aggregate) to its parts. Second, the language semantics might regard the same name introduced by different classes as referring to the same attribute. The structure and behavior of similar objects are defined in their common class. Friendships break a classs encapsulation and so, as in life, must be chosen carefully. In C++, one may declare functions having the same names, as long as their invocations can be distinguished by their signatures, consisting of the number and types of their arguments. Designing a suitable class structure involving inheritance, and especially involving multiple inheritance, is a difficult task. For example, attributes such as beauty or color are not objects, nor are emotions such as love and anger. Figure 311 illustrates such a class structure. Only item2 and item3 actually point to distinct DisplayItem objects (because in their declarations we allocated a new DisplayItem object); item4 designates no such object. Unfortunately, we have introduced a memory leak: The object originally designated by item2 can no longer be named, either directly or indirectly, and so its identity is lost. In the class DisplayItem, suppose that we define the instance variable theCenter (denoting the coordinates for the center of the displayed item), along with the following operations as in our earlier example: location Return the location of the item. On the other hand, the relationship between a shareholder and his or her shares is an aggregation relationship that does not require physical containment.

Hence, the lifetimes of these two objects are not so tightly coupled as before: We may create and destroy instances of each class independently. How can one know if a given class or object is well designed? In languages such as Smalltalk and Java, such objects will be garbage-collected and their storage reclaimed automatically, but in languages such as C++, their storage will not be reclaimed until the program that created them finishes. Stocks and bonds are managed quite differently than bank accounts, so we might classify stocks, bonds, mutual funds, and the like as kinds of securities, which in turn are also kinds of assets. We call this concept overloading. For this subclass, the operation draw draws a rectangle with the given height and width, again centered on theCenter. In the case of the class TemperatureController, we have aggregation as containment by value, a kind of physical containment meaning that the Heater object does not exist independently of its enclosing Temperature Controller instance. Failure to recognize the side effects of operating on an object through aliases often leads to memory leaks, memory access violations, and, even worse, unexpected state changes. First, the language semantics might regard such a clash as illegal and reject the compilation of the class. Third, we can treat multiple references to the same class as denoting the same class. Second, we can permit duplication of superclasses but require the use of fully qualified names to refer to members of a specific copy. Specifically, if the method is declared as virtual, then late binding is employed, and the function is considered to be polymorphic. state and behavior of an object collectively define the roles that an object may play in the world. Of these different kinds of class relationships, associations are the most general but also the most semantically weak. For example, we might say that a man (an object) loves his wife (another object), or that a particular cat (yet another object) is gray. We suggest five meaningful metrics: define coupling as the measure of the strength of association established by a connection from one module to another. The implementation of a class primarily consists of the implementation of all of the operations defined in the interface of the class. The class Circle must include the instance variable theRadius and appropriate operations to set and retrieve its value. The identification of associations among classes is often an activity of analysis and early design, For a vehicle, two of our key abstractions include the vehicle and wheels. Thus, a petal is not a kind of a flower; it is a part of a flower. If you allow multiple inheritance into a language, then sooner or later someone is going to write a class D with two parents B and C, each of which has a class A as a parentor some other situation in which D inherits twice (or more) from A. First, we can treat occurrences of repeated inheritance as illegal. an object is a concrete entity that exists in time and space, a class repre- sents only an abstraction A class represents a set of objects that share a common structure and a common behavior.A single object is simply an instance of a class. The invocation of draw demands polymorphic behavior.

Specifically, the manner in which we instantiate these classes sets aside four locations in memory whose names are item1,item2, item3, and item4, respectively. Coupling with regard to modules still applies to object-oriented analysis and design, but coupling with regard to classes and objects is equally important. Consider also Figure 34c, which illustrates the results of modifying the value of the item2 pointer to point to item1. The C++ friendship mechanism permits a class to distinguish certain privileged classes that are given the rights to see the classs protected and private parts. It can be uniquely identified by its name and it defines a state which is represented by the values of its attributes at a particular time.

The state of an object must have some representation in its corresponding class and so is typically expressed as constant and variable declarations placed in the protected or private part of a classs interface. Now item2 designates the same object as item1. For example, as shown in Figure 36, the object Temperature-Controller has a link to the object TemperatureRamp as well as to Heater. If, for example, the class DisplayItem included the virtual function Rotate, which was not redefined in the class Rectangle, then the vtable entry for Rotate would point to the implementation of Rotate in the class DisplayItem. As we show in Figure 312, the class TemperatureController denotes the whole, and the class Heater is one of its parts. Consider an abstraction of an employee record. The shareholder uniquely owns shares, but the shares are by no means a physical part of the shareholder. In this case, the class Temperature Controller still denotes the whole, and an instance of the class Heater is still one of its parts, although that part must now be accessed indirectly. Furthermore, the names of the objects pointed to by item2 and item3 are anonymous: We can refer to these distinct objects only indirectly, via their pointer value. In this manner, the representation common to all instances of a class is encapsulated, and changes to this representation do not functionally affect any outside clients. Rather, the lifetimes of these two objects are intimately connected: When we create an instance of Temperature Controller, we also create an instance of the class Heater. - A rose is a (different) kind of flower. Rather, this whole/part relationship is more conceptual and therefore less direct than the physical aggregation of the parts that form an airplane. Given an instance of Wheel, we should be able to locate the object denoting its Vehicle, and given an instance of Vehicle, we should be able to locate all the wheels, Here we show a one-to-many association: Each instance of Wheel relates to one Vehicle, and each instance of Vehicle may have many Wheels. This table typically consists of a list of pointers to virtual functions.

A proxy is usually created to represent a real-world object in the domain of the application. As an example, suppose that we define the (ill-conceived) MutualFund class as a subclass of the classes Stock and Bond. g%^.|U> "B}2E S/NWf;41!QdgDr*}Xqf5^&L#"/6jVy%[5L"o`Lz/%^O1vM]4MZ] j.~:Dtgfx P1H PRQpLxB6cs73m\ x!( 6?dtRJ1RjuOtEE2WxSI ng/gO'K[Oglt`>fTWhbwVaGfr"434Cd+J,{\H,Fgb9qC6?F%. As a participant in a link, an object may play one of three roles. most programming and database languages use variable names to distinguish temporary objects, mixing addressability and identity. Also, note that we have changed the state of the object designated by item3 by operating on it through its new indirect name, item4. Java simply performs late binding without the need for an explicit declaration such as virtual. - Ladybugs eat certain pests such as aphids, which may be infesting certain kinds of flowers. For example, suppose that the classes InsurableItem and Asset both have attributes named presentValue, denoting the present value of the item. The object TemperatureController is thus the whole, and Heater is one of its parts. When we destroy our TemperatureController object, by implication we also destroy the corresponding Heater object. Java does not have friendship. The state of an object encompasses all of the (usually static) properties of the object plus the current (usually dynamic) values of each of these properties.The fact that every object has state implies that every object takes up some amount of space, be it in the physical world or in computer memory. There are various approaches to dealing with the problem of repeated inheritance. There are three basic approaches to resolving this kind of clash. Stated another way, a link denotes the specific association through which one object (the client) applies the services of another object (the supplier), or through which one object may navigate to another. In some contexts, the terms active object and controller are interchangeable. For instance, a rose is a kind of flower, meaning that a rose is a specialized subclass of the more general class, flower. In this sense, aggregation is a specialized kind of association. Each part of this abstraction denotes a particular property of our abstraction of an employee. - Red roses and yellow roses are both kinds of roses. - A petal is a part of both kinds of flowers. Since the class RealEstate inherits from both of these classes, what does it mean to inherit two operations with the same name?