Object model

Foundations of the Object Model
• Structured Design Methods evolved
– For Developer (who build complex systems)
– Using Procedural PL
– Using algorithms as their fundamental building blocks
(Algorithmic Abstraction)
• Object-Oriented Design Methods evolved
– For Developers (who build complex systems)
– Using object-based and object-oriented PL
– using the class and object as basic building blocks.

Object Model
• Object Model is applicable
– not just to the programming languages
– but also
– to the design of
• User Interfaces,
• Databases, and
• even Computer Architectures.
• The reason for this widespread application is that
– an object orientation helps to cope with the
complexity (inherent in many different kinds of
systems).

Object Model
• Unfortunately, most programmers are trained
only in the principles of Structured Design (SD)
– Many good engineers have developed useful
software using SD
– However, there are limits to the amount of
complexity we can handle using only algorithmic
decomposition
– Thus we must turn to object-oriented
decomposition.

Object Model
• if we try to use object based languages such as
C++ and Ada
• as if they were only traditional, algorithmically
oriented languages,
• we not only miss the power available to us,
• but we usually end up worse off than if we had
used an older language such as C or Pascal.

OOP, OOD, and OOA
• As Object Model is derived from so many
disparate sources, it includes lot of confusion
in terminology
• Some unified concepts
– the concept of an object is central to anything
object-oriented
• Where an object is a tangible entity that exhibits some
well-defined behavior

OOP, OOD, and OOA
• Some unified concepts …
– Objects are "entities that combine the properties of procedures
and data since they perform computations and save local state".
– Objects serve to unify the ideas of algorithmic and data
abstraction.
– In the object model, emphasis is placed on crisply characterizing
the components of the physical or abstract system to be
modeled by a programmed system.
– Objects have a certain 'integrity' which should not (in fact,
cannot) be violated.

OOP, OOD, and OOA
• An object can only change state, behave, be manipulated,
or stand in relation to other objects in ways appropriate to
that object.
• Stated differently, there exist invariant properties that
characterize an object and its behavior.
• An elevator, for example, is characterized by invariant
properties including [that] it only travels up and down
inside its shaft.
• Any elevator simulation must incorporate these invariants,
for they are integral to the notion of an elevator

Object-Oriented Programming (OOP)
Definition
• Object-oriented programming is a method of
implementation in which programs are
organized as cooperative collections of
objects, each of which represents an instance
of some class, and whose classes are all
members of a hierarchy of classes united via
inheritance relationships.

OOP
• Three important parts of the definition
• object-oriented programming
– (1) uses objects (not algorithms) as its
fundamental logical building blocks (the “part of”
hierarchy)
– (2) each object is an instance of some class, and
– (3) classes are related to one another via
inheritance relationships (the "is a" hierarchy)

OOP
• if any of these elements is missing, it is not an
object-oriented program
• Specifically, programming without inheritance
is distinctly (definitely) not object-oriented
– we call it programming with abstract data types

OOP
• OO PL
– Well support for the object-oriented style of
programming is must for OOPL
– That is, it should be convenient to use OO style in
writing program in OOPL
– No exceptional effort or skill should be required

OOP
• Theoretically OOP can be done using non-object
oriented PL like Pascal and COBOL
• But, it is badly awkward to do so.
•
• A language is object-oriented if and only if it satisfies
the following requirements:
– It supports objects that are data abstractions with an
interface of named operations and a hidden local state.
– Objects have an associated type [class].
– Types [classes] may inherit attributes from super types
[super classes]

OOP
• We say that a PL supports inheritance
– If it is possible to express "is a" relationships among types
– for example, a red rose is a kind of flower, and a flower is a kind of
plant.
• If a language does not provide direct support for inheritance, then it
is not object-oriented. We distinguish such languages by calling
them object-based rather than object-oriented.
• Under this definition, for example
– Smalltalk, Object Pascal, and C++ are object-oriented, and
– Ada is object-based.
•
• However, since objects and classes are elements of both kinds PL
– object-oriented design methods can be used with both

Object-Oriented Design (OOD)
• The emphasis in programming methods is
primarily on the proper and effective use of
particular language mechanisms.
• By contrast, design methods emphasize the
proper and effective structuring of a complex
system.
• What then is object-oriented design?

OOD
Definition
• Object-oriented design is a method of design
encompassing the process of object-oriented
decomposition and a notation for depicting
[both] logical and physical as well as static
and dynamic models of the system under
design.

OOD
• Two important parts of the definition
• object-oriented design
– (1) leads to an object oriented decomposition and
– (2) uses different notations to express different
models of the logical (class and object structure)
and physical (module and process architecture)
design of a system, in addition to the static and
dynamic aspects of the system.

OOD
• OOD
– support for object-oriented decomposition
– uses class and object abstractions to logically
structure systems
• Structured Design
– uses algorithmic abstractions

Object-Oriented Analysis (OOA)
• Object Model has influenced even earlier phases
of SE
• Structured Analysis
– focuses upon the flow of data within a system.
• OOA
– emphasizes the building of real-world models, using
an object-oriented view of the world.

OOA
Definition
• Object-oriented analysis is a method of
analysis that examines requirements from the
perspective of the classes and objects found in
the vocabulary of the problem domain.

OOA, OOD, OOP
• How are OOA, OOD, and OOP related?
• OOD uses outcome of OOA
• OOP uses outcome of OOD
• That is, OOA  OOD  OOP

Elements of the Object Model
Kinds of Programming Pattern
• Most Programmers work in one PL and use
only one programming style
– Unable to think Alternate ways (more appropriate
style) to solve problem
• programming style is
– a way of organizing programs on the basis of some
conceptual model of programming and an
appropriate language to make programs written in
the style clear

• Five main types of Programming Style and
respective Abstraction used
– Procedure-oriented Algorithms
– Object-oriented Classes and objects
– Logic-oriented Goals (Predicate Calculus)
– Rule-oriented If-then rules
– Constraint-oriented Invariant relationships

• There is no single programming style that is best for all kinds of
applications.
– For example, rule-oriented programming would be best for the design
of a knowledge base, and procedure-oriented programming would be
best suited for the design of computation-intense operations.
• The object-oriented style is best suited to the broadest set of
applications
– indeed, this programming paradigm often serves as the architectural
framework in which we employ other paradigms.
• Each of these styles of programming is based upon its own
conceptual framework. Each requires a different mindset, a
different way of thinking about the problem.
• For all things object-oriented, the conceptual framework is the
Object Model.

Object Model (OM)
• There are four major elements of OM
– Abstraction
– Encapsulation
– Modularity
– Hierarchy
• By major, we mean that a model without any
one of these elements is not object-oriented.

Object Model
• There are three minor elements of the OM
– Typing
– Concurrency
– Persistence
• By minor, we mean that each of these
elements is a useful, but not essential, part of
the object model.

Object Model
• Without this conceptual framework, you may be
programming in a OOPL (such as C++, Java, or C#).
• But, your design is going to smell like a procedural PL
(such as C, FORTRAN, or Pascal)
• You will miss expressive power of the object-oriented
language
• More importantly, it is likely that you have not
mastered the complexity of the problem at hand.

1. Abstraction
• Abstraction is one of the fundamental ways to cope with complexity
• Abstraction arises from a recognition of similarities between certain
objects, situations, or processes in the real world, and the decision to
concentrate upon these similarities and to ignore for the time being the
differences
• Abstraction is a simplified description, or specification, of a system that
emphasizes some of the system's details or properties while suppressing
others. A good abstraction is one that emphasizes details that are
significant to the reader or user and suppresses details that are, at least
for the moment, immaterial or diversionary
• A concept qualifies as an Abstraction only if it can be described,
understood, and analyzed independently of the mechanism that will
eventually be used to realize it

1. Abstraction
• Combining these different viewpoints, we
define an abstraction as follows:
• An abstraction denotes the essential
characteristics of an object that distinguish it
from all other kinds of objects and thus
provide crisply defined conceptual boundaries,
relative to the perspective of the viewer.

1. Abstraction
• An abstraction focuses on the outside view of an object, and so
serves to separate an object's essential behavior from its
implementation
• This behavior/implementation division is called abstraction barrier
achieved by applying the principle of least commitment, through
which the interface of an object provides its essential behavior, and
nothing more
• We like to use an additional principle that we call the principle of
least astonishment, through which an abstraction captures the
entire behavior of some object, no more and no less, and offers no
surprises or side effects that beyond the scope of the abstraction.

1. Abstraction
• Spectrum/Kinds of Abstractions (most to least
useful)
– Entity Abstraction
– Action Abstraction
– Virtual Machine Abstraction
– Coincidental Abstraction

1. Abstraction
• Entity Abstraction
– An object that represents a useful model of a
problem domain or solution-domain entity
– Most Useful
– We should strive to build Entity Abstractions, as it
closely models the problem domain

1. Abstraction
• Action Abstraction
– An object that provides a generalized set of
operations, all of which perform the same kind of
function

1. Abstraction
• Virtual Machine Abstraction
– An object that groups together operations that are
all used by some superior level of control, or
operations that all use some junior-level set of
operations

1. Abstraction
• Coincidental Abstraction
– An object that packages a set of operations that
have no relation to each other
– Least useful (Not useful)

1. Abstraction
• Client Object uses resources of Server Object
• Contract Model of Programming
– the outside view of each object defines a contract
upon which other objects may depend, and which in
turn must be carried out by the inside view of the
object itself (often in collaboration with other objects)
– this contract encompasses (include) the
responsibilities of an object, namely, the behavior for
which it is held accountable

1. Abstraction
• Individually, each operation that contributes
to this contract has a unique signature
comprising all of its formal arguments and
return type.
• We call the entire set of operations that a
client may perform upon an object, together
with the legal orderings in which they may be
invoked, its protocol.

1. Abstraction
• An invariant is some Boolean (true or false)
condition whose truth must be preserved.
• For each operation associated with an object,
we may define
– preconditions (invariants assumed by the
operation) as well as
– post conditions (invariants satisfied by the
operation).

1. Abstraction
• Violating an invariant breaks the contract
associated with an abstraction.
• If a precondition is violated, this means that a
Client has not satisfied its part, and hence the
server cannot proceed reliably.
• Similarly, if a post-condition is violated, this
means that a server has not carried out its part of
the contract

1. Abstraction
• An exception is an indication that some invariant has
not been or cannot be satisfied.
• Certain languages permit objects to throw exceptions
so as to abandon processing and alert other object to
(about) the problem, who in turn may catch the
exception and handle the problem.
• The terms operation, method, and member function
evolved from three different PL (Ada, Smalltalk, and
C++, respectively). They all mean virtually the same
thing can be used interchangeably.

1. Abstraction
• All abstractions have static as well as dynamic properties.
• For example, a file object
– amount of space
– Name
– content
• These are all static properties
• The value of each of these properties is dynamic, relative to
the lifetime of the object
– a file object may grow or shrink in size
– its name may change
– Its contents may change.

1. Abstraction
• All abstractions have static as well as dynamic
properties.
– Car
• Static Property: Registration No., Company Name
• Dynamic Property: Current Speed, Owner Name
– Person
• Name, Date of Birth, SSN
• Cell Phone, Address, Hobby, Salary

1. Abstraction
• In an object-oriented style of programming, things
happen whenever we operate upon an object
• (i.e., when we send a message to an object).
• Thus, invoking an operation upon an object elicits
(reveals) some reaction from the object.
• What operations we can meaningfully perform upon an
object and how that object reacts constitute the entire
behavior of the object.

1. Abstraction
• Examples of Abstraction (Assume)
• Sensors (Air, Water, Humidity, Light, pH, etc.)
• Temperature Sensor
– Along with Location Info.
• Responsibility?
– Reporting the current Temp. in F

1. Abstraction
typedef float Temperature; //Temperature in degrees Fahrenheit
typedef unsigned int Location; // Number uniquely denoting the location of a sensor
class TemperatureSensor
{
Public:
TemperatureSensor(Location);
~TemperatureSensor() ;
void calibrate(Temperature actualTemperature);
Temperature currentTemperature() const;
private:
…
};

1. Abstraction
• Private Parts
– Its representation (Implementation) is Hidden in
Private parts of object
• Instance (Object that can be operated upon)
Temperature temperature;
//Creates two different objects (instances) of the class “TemperatureSensor)
TemperatureSensor greenhouselSensor(l);
TemperatureSensor greenhouse2Sensor(2);
temperature = greenhouselSensor.currentTemperature();

1. Abstraction
• Invariants (associated with the operation
currentTemperature)
– Pre-condition
• Object will be created with valid location
– Post-condition
• Returns Temperature Value in Fahrenheit

1. Abstraction
• Passive VS Active Object
– Passive abstraction is one in which some client
object must operate on abstracted entity to get
service (e.g., to get current Temperature)
– In Active abstraction, abstracted entity acts upon
(rather than being acted upon by) other objects
whenever some condition arise (e.g., I need SMS,
when temperature is not in a given range)

• Responsibility Changes in Active abstraction
– Now, Sensor is responsible to report current
temperature
– Earlier, client was responsible to demand temp.
• This scenario can be implemented with the
“callback” feature of PL

• When client creates an Object
– An additional information, particularly, CallBack
function also needs to be passed. This callback
function will be called on detecting predetermine
condition
• Server Object calls “CallBack” function when
some condition arises

1. Abstraction
class ActiveTemperatureSensor {
public:
ActiveTemperatureSensor(Location, void (*f)(Location, Temperature));
~ActiveTemperatureSensor();
void calibrate(Temperature actualTemperature);
void establishSetpoint(Temperature setpoint, Temperature delta);
Temperature currentTemperature() const; //Client can still inquire
private:
…
};

1. Abstraction
• Object may collaborate with other objects to achieve
some behavior
– Plan Condition & Growing Plan
// Structure denoting relevant plan conditions
struct Condition //Struct Vs. Class
{
Temperature temperature;
Lights lighting;
pH acidity;
Concentration concentration;
};

1. Abstraction
class GrowingPlan {
public:
GrowingPlan(char* name);
virtual ~GrowingPlan();
void clear();
virtual void establish(Day, Hour, const Condition&);
const char* name() const;
const Condition& desiredConditions(Day, Hour) const;
protected:
…
};

1. Abstraction
• Constructor and Destructor (Creates/Destroys)
– GrowingPlan(), ~GrowingPlan()
– Called Meta-Operations
• Modifiers – Update state
– Clear(), Establish()
• Selectors (Returns/Selects state value)
– name(), desiredConditions()

Example
• Example in C++
– Abstraction
– Encapsulation

2. Encapsulation
• Abstraction of an object should precede the
decisions about its implementation.
• Once an implementation is selected, it should be
treated as a secret of the abstraction and hidden
from most clients.
• No part of a complex System should depend on
the internal details of any other part

2. Encapsulation
• Abstraction - helps people to think about what they- are
doing
• Encapsulation - allows program changes to be reliably
made with limited effort
• Abstraction and encapsulation are complementary
concepts: abstraction focuses upon the observable
behavior of an object, whereas encapsulation focuses upon
the implementation that gives rise to this behavior.
• Encapsulation is most often achieved through information
hiding

2. Encapsulation
• Information Hiding
– is the process of hiding all the secrets of an object that
do not contribute to its essential characteristics
• (typically, the structure of an object is hidden, as well as the
,implementation of its methods)
• Encapsulation provides explicit barriers among
different abstractions of system
– objects at one level of abstraction are shielded from
implementation details at lower levels of abstraction

2. Encapsulation
• For abstraction to work, implementations must be
encapsulated
– this means that each class must have two parts: an
interface and an implementation.
• The interface of a class captures only its outside view,
encompassing our abstraction (of the behavior
common to all instances of the class)
• The implementation of a class comprises the
representation of the abstraction as well as the
mechanisms that achieve the desired behavior.

2. Encapsulation
• The interface of a class is the one place where
we assert/state all of the assumptions that a
client may make about any instances of the
class
• the implementation encapsulates details
about which no client may make assumptions.

2. Encapsulation
• Definition
• Encapsulation is the process of
compartmentalizing the elements of an
abstraction that constitute its structure and
behavior; encapsulation serves to separate the
contractual interface of an abstraction and its
implementation.

2. Encapsulation
• Example
• Consider 3 abstractions
– Temperature Sensor
– Heater (Turn ON, Turn OFF, IsON)
– Temperature Control
• Higher-level behavior build on a top of Sensor & Heater
• Adds new semantic, prevent frequent ON, OFF of Heater;
when temperature is near boundary conditions
(hysteresis)

2. Encapsulation
• Example
enum Boolean {FALSE, TRUE}; // Boolean type
Class Heater {
public:
Heater(location);
~Heater();
void turnOn();
void turnOff();
Boolean isOn() const;
private:
…
}; // This interface represent all that a client needs to know
// In-side implementation may highly complex using
// Serial Ports [Class] to send signals to distant Heaters

2. Encapsulation
• In future, developer may choose to use Memory-
mapped I/O instead of serial communication lines
• This means Implementation is modified. But, it does
not require to change Interface
• As long as Interface (functional behavior of abstraction)
is not changed
– Client code that uses this class need not have to be
modified
– Unless a particular client depends upon the time or space
semantics of original implementation (highly undesirable)

2. Encapsulation
• The representation of an object is often
changed by developer (based on system’s
performance) so that more efficient algorithm
can be applied
• This ability to change the representation of an
abstraction without disturbing any of its
clients is essential benefits of encapsulation

2. Encapsulation
• The Encapsulation is supported by different PL
in various ways and extents
• For example, C++ offers flexible control over the
member data elements and member functions.
Member may be placed in
– Public (visible to all)
– Private or (fully encapsulated)
– Protected part (visible to class and its subclasses)

2. Encapsulation
• C++ also supports the notion of friends:
– Cooperative classes that are permitted to see/use each
other’s parts.
• Hiding is relative concept.
– The underlying representation of an object can be
revealed, if developer wish to do so.
– Encapsulation cannot stop a developer from doing stupid
things
– Further, no PL prevents a human from literally seeing the
implementation of a class (although, an OS can prevent)
– There are times when one must study the implementation
of a class to really understand its meaning

3. Modularity
• The act of partitioning a program into
individual components can reduce its
complexity to some degree
• partitioning a program creates a number of
well defined, documented boundaries within
the program.

3. Modularity
• In some languages, such as Smalltalk, there is no
concept of a module
• In many others, including Object Pascal, C++, CLOS, and
Ada, the module is a separate language construct, and
therefore warrants a separate set of design decisions.
• In these languages, classes and objects form the logical
structure of a system; we place these abstractions in
modules to produce the system's physical architecture.

3. Modularity
• Especially for larger applications, in which we
may have many hundreds of classes, the use of
modules is essential to help/manage complexity.
• modularization consists of dividing a program
into modules which can be compiled separately,
but which have connections with other modules
– The connections between modules are the
assumptions which the modules make about each
other

3. Modularity
• Module and encapsulation go hand in hand
– PL supports notion of
• Interface of module and
• its Implementation
• Again, different PL supports module in diverse way
• Module in also known as Unit, Package, Assembly, DLL,
etc.
• Deciding the right set of modules for a given system is
hard problem

3. Modularity
• Modules serve as the physical containers in which we
declare the classes and objects of our logical design
• For tiny problems, the developer might decide to
declare every class and object in the same package.
• A better solution is to group logically related classes
and objects in the same module, and expose only
those elements that other modules absolutely must
see.
• Arbitrary modularization is sometimes worse than no
modularization at all

3. Modularity
• guidelines to achieve an intelligent
modularization of classes and objects
– The overall goal of the decomposition into modules is
the reduction of software cost by allowing modules to
be designed and revised independently
– Each module's structure should be simple enough that
it can be understood fully;
– it should be possible to change the implementation of
module without knowledge of the implementation of
other modules and without affecting the behavior of
other modules

3. Modularity
– The cost of changing (and re-compilation) an
interface is very high compare to changing an
implementation
• Requires re-compilation of all module that uses services
of the module directly or indirectly.
• For this reason, For this reason, a module's interface
should be as narrow as possible
• hide as much as you can in the implementation of a
module
• Incremental addition to interface is better than
handling extra interface

3. Modularity
• The developer must balance two competing technical concerns
– the desire to encapsulate abstractions, and
– the need to make certain abstractions visible to other modules.
• Guidance
– System details that are likely to change independently should be the secrets of
separate modules
– the only assumptions that should appear between modules are those that are
considered unlikely to change.
– Every data structure is private to one module
– it may be directly accessed by one or more programs within the module but
not by programs outside the module.
– Any other program that requires information stored in a module's data
Structures must obtain it by calling module programs
• In other words, strive to build modules that are
– cohesive (by grouping logically related abstractions) and
– loosely coupled (by minimizing the dependencies among modules).

3. Modularity
• Definition
• Modularity is the property of a system that
has been decomposed into a set of cohesive
and loosely coupled modules.

3. Modularity
• Other issues that can affect modularization
decisions:
– Package classes such that Reuse is convenient
– Size of compiled unit
– Work assignment to different teams
– To save documentation & related effort
– Security (the code that need security)

3. Modularity
• Logical Design
– Identification of Classes & Objects
• Physical Design
– Identification of Modules
• Both the designs are done iteratively
– That is, No one follows the other completely

3. Modularity
• Example
• Module-X //depends on 3 modules
#include<string.h>
#include<math.h>
#include<moduleY.h>
…
Void main()
{ …}

3. Modularity
• Other way of division of Software
– 2-tier Client/Server Architecture
– 3-tier or N-tier Architecture
– MVC Architecture
– SOA

4. Hierarchy
• Encapsulation helps in managing complexity by hiding
the inside-view of abstraction
• Module helps in clustering logically related abstractions
• This is not enough. A set of abstractions often forms a
hierarchy
• identifying these hierarchies greatly simplify our
understanding of the problem.

4. Hierarchy
• Definition
– Hierarchy is a ranking or ordering of abstractions.
• The most important hierarchies in a complex
system are:
– its class structure (the "is a" hierarchy) and
– its object structure (the "part of' hierarchy).

4. Hierarchy
• Inheritance is the most important "is a” hierarchy
• Inheritance defines a relationship among classes
• one class shares the structure or behavior defined in one or
more classes (denoting single inheritance and multiple
inheritance, respectively).
• Inheritance thus represents a hierarchy of abstractions, in
which a sub-class inherits from one or more super-classes.
• Typically, a subclass augments or redefines the existing
structure and behavior of its super-classes.

4. Hierarchy
• Semantically, inheritance denotes an "is-a" relationship. For
example
• Single Inheritance
– House is a Property
– Car is a Vehicle
– Teaching Staff is a Staff
• Multiple Inheritance
• Teaching, Non-Teaching, Trainee, Permanent
– Trainee Teaching Staff is a Teaching Staff
– Trainee Teaching Staff is a Trainee Staff

4. Hierarchy
• Inheritance thus implies a generalization/
specialization hierarchy
– wherein a subclass specializes the more general
structure or behavior of its super-classes.
• Litmus test for inheritance
– if B "is not a" kind of A, then B should not inherit
from A.

4. Hierarchy
• class FruitGrowingPlan : public GrowingPlan {
– public:
• FruitGrowinjgPlan(char* name);
• virtual ~FruitGrowingPlan();
• virtual void establish(Day, Hour, Condition&); //Overridding (modified)
• void scheduleHarvest(Day, Hour); //New Methods
• Boolean isHarvested() const; //New Methods
• unsigned daysUntilHarvest() const; //New Methods
• Yield estimatedYield() const; //New Methods
– protected:
• Boolean repHarvested; //New Data Member
• Yield repYield; //New Data Member
• };

4. Hierarchy
• Inheritance
– Supports Re-Use
– Otherwise each class designer starts from scratch
– Not only it is duplication of effort, it generates
inconsistency in code also.

4. Hierarchy
• Inheritance VS. Abstraction
• Data abstraction attempts to provide an opaque
barrier behind which methods and state are
hidden
• Inheritance requires opening this interface to
some extent and may allow state as well as
methods to be accessed without abstraction

4. Hierarchy
• For a given class, there are usually two kinds
of clients:
– objects that invoke operations upon instances of
the class, and
– subclasses that inherit from the class

4. Hierarchy
• with inheritance, encapsulation can be violated in
one of three ways:
– The subclass might access an instance variable of its
super-class,
– Call a private operation of its super-class, or
– Refer directly to super-classes of its super-class
• PL may allow to control Inheritance
– E.g., in C++, one can use modifiers such as public,
private, and protected

• Example of Multiple Inheritance
class Plant {
public:
Plant (char* name, char* species);
virtual ~Plant();
void setDatePlanted(Day);
virtual establishGrowingConditions(const Condition&);
const char* name() const;
const char* species() const;
Day datePlanted() Const;
Protected:
char* repName;
char* repSpecies;
Day repPlanted;
private:
…
};

class FlowerMixin {
public:
FlowerMixin(Day timeToFlower, Day timeToSeed);
virtual ~FlowerMixin();
Day timeToFlower() const;
Day timeToSced() const;
protected:
…
};
class FruitVegetableMixin {
public:
FruitVegetablieMixin(Day timeToHarvest);
virtual ~FruitVegetableMixin();
Day timeToHarvesto const;
protected:
…
};

4. Hierarchy
• class Rose : public Plant, public FlowerMixin...
• class Carrot : public Plant, public FruitVegetableMixin
{};
• in both cases, we form the subclass by inheriting from
two super-classes.
• Instances of the subclass Rose thus include the
structure and behavior from the class Plant together
with the structure and behavior from the class
FlowerMixin

4. Hierarchy
• Multiple inheritance introduces complexities for PL
• PL must address two issues
– Clashes among names from different super-classes, and
– repeated inheritance.
• Clashes will occur when two or more super-classes provide
a field or operation with the same name or signature
– In C++, such clashes must be resolved with explicit qualification
• Repeated inheritance occurs when two or more super-
classes share a common super-class

4. Hierarchy
• Example of Hierarchy (Aggregation)
• Whereas these "is a" hierarchies denote generalization/
specialization relationships, "part of” hierarchies describe
aggregation relationships.
• For example, consider the following class:
class Garden {
public:
Garden();
virtual ~Garden();
protected:
Plant* repPlants[100];
GrowingPlan repPlan;
};

4. Hierarchy
• In terms of its "is a" hierarchy, a high-level
abstraction is generalized, and a low-level
abstraction is specialized.
– E.g., Flower class is at a higher level of abstraction
then a Plant class.
• In terms of its "part of' hierarchy, a class is at a
higher level of abstraction than any of the classes
that make up its implementation
– E.g., Garden is at a higher level of abstraction than the
type plant

4. Hierarchy
• aggregation permits the physical grouping of
logically related structures, and
• inheritance allows these common groups to
be easily reused among different abstractions.

4. Hierarchy
• Aggregation raises the issue of ownership.
• Assume that the lifetime of a garden and its
plants are independent
– Implemented by including pointers to Plant objects
rather than values.
• In contrast, we have decided that a GrowingPlan
object is inherently associated with a Garden
object, and does not exist independently of the
garden.
– For this reason, we use a value of GrowingPlan.

5. Typing

Object model

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Object model