C++ Classes

C++ ClassesDefinition, Constructors, Methods, Access

Modifiers, Static/Instance members,

Learning & Development Teamhttp://academy.telerik.com

Telerik Software Academy

Table of Contents1. Classes and Objects Concept2. Defining and Instantiating Classes3. Constructors, Initialization & this

keyword Destructors

4. Methods5. Operator Overloading6. Static and Instance Members7. Classes and const8. Pointers to classes

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Classes and Objects in Programming

Modelling the Real World in Code

Classes and Objects Concept

Classes model real-world objects and define Attributes (state, properties, fields) Behavior (methods, operations)

Classes describe the structure of objects Objects describe particular instance

of a class Properties hold information about

the modeled object relevant to the problem

Operations implement object behavior

Classes and Objects in C++

Classes in C++ can have members: Fields, constants, methods,

operators, constructors, destructors, …

Inner types (inner classes, structures)

Access modifiers define the scope of sets of members (scope) public, private, protected

Members can be static (class) or specific for a given

object

Class Definition Example

Defining a simple class to represent a Jediclass Jedi{public: string name; int forceMastery;

Jedi(string name, int forceMastery) { this->name = name; this->forceMastery = forceMastery; }

string getName() { return this->name; } //example continues

Class Definition Example

void Speak() { string sentence = "Greetings, I am "; switch(this->forceMastery) { case 0: sentence+="padawan"; break; case 1: sentence+="jedi-knight"; break; case 2: sentence+="master"; } sentence += " "; cout<<sentence + this->name<<endl; }};

Simple Class DefinitionLive Demo

Defining Classes in C++

Syntax, Keywords, Basic Members

Defining Classes in C++ Classes are either defined with the class or struct keyword Both syntaxes are almost the same

Class definition is in the form:class ClassName : InheritedClass1, InheritedClass2, ...{access-modifier : //e.g. public: members //e.g. int a; or int answer(){return 42;}access-modifier : members...}; Don't forget the

semicolon after definition

Defining Classes in C++ A class has a name A class can "inherit" other classes

i.e. reuse their logic and members A class declares accessibility of its members Which members are visible from

outside Which members are visible from

inheritors Which members are only visible

from inside These are known as "access

modifiers" or "scopes"

Defining Classes in C++ Access modifiers in classes

public: accessible from anywhere outside or inside the class and its inheritors

protected: accessible from inside the class and from its inheritors

private: accessible only from inside the class

If not specified, members of a class are private If not specified, members of a struct

are public

An access modifier affects all members, declared after it up to the next modifier (or up to the end of the class definition)

Defining Classes in C++ Fields are the simplest members of classes Fields are variables inside the class Can be of any type, including other

classes, or even of the same class The latter case is usually called a

recursive class Can be initialized on declaration (C+

+11) Keep in mind they can be changed by

the constructor or other methods

class Person {public: string name; int age = 0;};

Defining Classes in C++ Creating objects of classes

Usually to use a class, you instantiate an object of that class E.g. you instantiate a individual of

type Person

Note: "instantiate", i.e. we create an instance. We haven't "initialized" it with values yet

Accessing object members Members are accessed through the

"." operator

Person somebody;

somebody.name = "Waspy";cout<<somebody.name;

Creating Classes & Instantiating Objects

Live Demo

ConstructorsInitializing Objects, Calling

Constructors

Constructors Objects need to be initialized before usage If not, we could get undetermined

results Just like with uninitialized variables

Objects usually can't be initialized by literal: E.g. can't initialize

a Person with just a name, or just a number, it needs both to set its age and name Some classes need even more values Some classes need complex

initialization logic

Person somebody = "Tony";Person somebody = 5;//both of the above are wrong

Constructors Constructors are special functions, responsible for initializing objects Declared inside the class Have same name as the class Accept parameters like normal

functions Can be overloaded like normal

functions Have direct access to the class

members Don't have a return type Execute after inline initialization

i.e. if a field has a value at declaration, the constructor can change it

Constructors Constructor for the Person class

class Person {public: string name; int age = 0; Person(string nameParameter, int ageParameter) { name = nameParameter; age = ageParameter; //changes the 0 value of age }};

Constructors Constructors can be called in several waysa) Parenthesis after identifier, at

declaration

b) Class name, followed by parenthesis

i.e. create a temporary & assign it to an instance

c) Same as 2), but with "new" Allocates dynamic memory for

objects Returns a pointer to the memory

Person p("Tony", 22);

Person p = Person("Tony", 22);

Person *p = new Person("Tony", 22);

Basic ConstructorsLive Demo

Constructors Mistaken constructor (ambiguous identifiers)

name and age here hide the class fields Function variables and parameters

are "more local" than global or class variables

class Person {public: string name; int age = 0; Person(string name, int age) { name = name; age = age; }};

These assignments do nothing – they set the

parameters to their own values

Constructors & this The this keyword

Explicitly refers to the current class instance

Used to explicitly point to a instance member Even if it is hidden by local variables

class Person {public: string name; int age = 0; Person(string name, int age) { this->name = name; this->age = age; }};

Constructors & this More info on the this keyword

Typed pointer to current instance E.g. for a Person instance named p,

this could be expressed as: Person* this = &p;

Can be used in any constructor or function (method) inside the class

Recommended way of accessing instance members

don't try to compile

that

Using "this" in ConstructorsLive Demo

Overloading Constructors

Constructors can be overloadedclass Person {public: string name; int age = 0;

Person(string name, int age) { this->name = name; this->age = age; }

Person(string personInfo) //e.g. format: "022Tony"-> Tony, age 22 { this->age = 100*(personInfo[0] - '0') + 10*(personInfo[1] - '0') + personInfo[2] - '0'; this->name = personInfo.substr(3); }};

Overloading Constructors

Constructor parameters can have default values, just like functionsclass Person {public: string name; int age; Person(string name = "Anonymous", int age = 0) { this->name = name; this->age = age; }};

Constructor OverloadingLive Demo

Destructors Destructors are special functions, called when an object is freed from memory

A destructor is usually responsible for: Cleaning up allocated dynamic

memory by the instance Resetting any changes a

constructor could have made outside the instance

Syntax: same as constructor, with a ~ prefix and no parameters

~Person(){ ... }

DestructorsLive Demo

MethodsFunctions in Classes

Methods Methods are functions, belonging to a class

Methods have all the properties of functions Can accept parameters and be

overloaded Can have default values for

parameters Have return values

Methods have access to other class members Recommended to use the this

pointer Methods can be accessed through any instance through the "." operator

Methodsclass Person{public: string name; int age = 0;

Person(string name, int age) { this->name = name; this->age = age; }

string GetInfo() { stringstream infoStream; infoStream<<this->name<<", age: "<<this->age<<endl; return infoStream.str(); }};

MethodsLive Demo

Operator OverloadingRedefining basic operations for

complex classes

Operator Overloading Classes define new types in C++

Types interact with assignments, function calls and operators

Instances of a new class can also use operators By defining how operators work on

its instances This is called operator overloading

Syntaxtype operator sign (parameters) { /*... body ...*/ } 

Operator Overloading Example overloading + for 2D vectorsclass Vector2D{public: double x; double y;

Vector2D(double x = 0, double y = 0) { this->x = x; this->y = y; }

Vector2D operator + (const Vector2D &other) { return Vector2D(this->x + other.x, this->y + other.y); }};

Basic Operator OverloadingLive Demo

Operator Overloading Overloaded operators are just special functions Using operators is just calling those

functions Operators can be overloaded both as members and as non-members Members can access the calling

object through the this pointer Non-members take an additional

parameter to refer to the object, calling the operator

Operator Overloading Form of common overloaded operators:Expression Operator Member function Non-member

function

@a + - * & ! ~ ++ -- A::operator@() operator@(A)

a@ ++ -- A::operator@(int) operator@(A,int)

a@b+ - * / % ^ & | < > == != <= >= << >> && || ,

A::operator@(B) operator@(A,B)

a@b= += -= *= /= %= ^= &= |= <<= >>= []

A::operator@(B) -

a(b,c...) () A::operator()(B,C...) -

a->b -> A::operator->() -(TYPE) a TYPE A::operator TYPE() -

Overloading Operators as

Members an Non-MembersLive Demo

Operator Overloading Calling operators can be done in two ways As normal operators in expressions By their function name i.e.:

prefixed with operator keyword, followed by the actual operator and its parameters in paranthesesc = a + b;

c = a.operator+ (b);

Static MembersMembers Common for all Instances

Static Members There is data (and behavior) which can be the same for all instances of a class E.g. average person age – doesn't

need to be specific for each instance

Static members Single common variables for objects

of a class Marked by the static keyword Must be initialized from outside the

class To avoid reinitialization

Static Members Example: Person static members (1) class Person{private: static int personCount; static int totalPersonAge;public: string name; int age;

Person(string name = "", int age = 0) { this->name = name; this->age = age;

Person::totalPersonAge += this->age; Person::personCount++; } //example continues

Static Members Example: Person static members (2)

Two ways of accessing static members: Through class name, followed by :: Through instance, like any other

member

~Person() { Person::totalPersonAge -= this->age; Person::personCount--; }

static int getAveragePersonAge() { return Person::totalPersonAge / Person::personCount; }};

Static MembersLive Demo

Classes and constRestricting Modifications Compile-Time

Classes and const Class members can be defined const Fields follow the same rules as

const variables Methods and operators cannot

modify the instance they are called on Syntax: place const after method

parentheses

class Person { ... string getInfo() const { stringstream infoStream; infoStream<<this->name<<", age: "<<this->age<<endl; return infoStream.str(); }}

Classes and const const methods are frequently used

Mark a non-modifying method Hence, required by many standard

library methods, accepting references

A const reference to an object can only call const methods

An object can be const like any variable Only consturctor & const methods

can be called

const Person p = Person("Tony", 20);cout<<p.getPersonInfo()<<endl;

Classes and constLive Demo

Pointers to ClassesInstances in Dynamic Memory

Pointers to Classes As mentioned, creating an instance with the new keyword returns a pointer

A pointer can also be obtained by using the reference operator &

All typical pointer operations are valid

Access to members is done through the -> operator

Person *samPtr = new Person("Sam", 18);Person frodo = Person("Frodo", 18);Person *frodoPtr = &frodo;

cout<<samPtr->getPersonInfo()<<endl;

Pointers to ClassesLive Demo

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C++ Classes

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Exercises1. Define a class that holds information

about a mobile phone device: model, manufacturer, price, owner, battery characteristics (model, hours idle and hours talk) and display characteristics (size and number of colors). Define 3 separate classes (class GSM holding instances of the classes Battery and Display).

2. Define several constructors for the defined classes that take different sets of arguments (the full information for the class or part of it). Assume that model and manufacturer are mandatory (the others are optional). All unknown data fill with null.

3. Add an enumeration BatteryType (Li-Ion, NiMH, NiCd, …) and use it as a new field for the batteries.

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Exercises (2)4. Add a method in the GSM class for

displaying all information about it. 5. Use methods to encapsulate the data

fields inside the GSM, Battery and Display classes. Ensure all fields hold correct data at any given time.

6. Add a static field IPhone4S in the GSM class to hold the information about iPhone 4S.

7. Write a class GSMTest to test the GSM class:

Create an array of few instances of the GSM class.

Display the information about the GSMs in the array.

Display the information about the static field IPhone4S.

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Exercises (3)8. Create a class Call to hold a call

performed through a GSM. It should contain date, time, dialed phone number and duration (in seconds).

9. Add a property CallHistory in the GSM class to hold a list of the performed calls. Try to use the class vector<Call>.

10.Add methods in the GSM class for adding and deleting calls from the calls history. Add a method to clear the call history.

11.Add a method that calculates the total price of the calls in the call history. Assume the price per minute is fixed and is provided as a parameter.

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Exercises (4)12. Write a class GSMCallHistoryTest to

test the call history functionality of the GSM class.

Create an instance of the GSM class. Add a few calls. Display the information about the

calls. Assuming that the price per minute is

0.37 calculate and print the total price of the calls in the history.

Remove the longest call from the history and calculate the total price again.

Finally clear the call history and print it.

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