ch4part1

1

Chapter 4:

CONSTRUCTORS, DESTRUCTORS, FRIENDS

(part 1)

DCS5088 :: Chapter 4 (part 1) 2

Objectives

At the end of this lecture, students shouldbe able to :• Explain what are constructors and destructors• Implement the various types of constructors

default, parameterized, overloaded, initialization list, copy constructors

• Implement destructors


4.1 Constructor

• DefinitionA constructor is a member (method) that is called

automatically each time when an object is created.The constructor allocates sufficient memory space

for the object.C++ will automatically supply a constructor if it is not

supplied in the program.


4.1.1 Characteristics of a Constructor

1. Constructor has the same name as the class.

class Staff{ public: Staff ( ); // constructor declaration

};


4.1.1 Characteristics of a Constructor (cont…)

2. Constructor’s purpose is to allocate memory for object creation.

3. It is called automatically when objects are created, this means… we don’t have to invoke it like functions.

4. A constructor does not return a value.5. An argument can be sent to the constructor.


Example (constructor)#include<iostream>using namespace std;class Numbers { private: int n1, n2;

public:

Numbers() { cout<<"This is the constructor"<<endl; n1 = n2 = 0; }

} ;

Constructor

Continue….


Example (constructor)- (cont…)

void main(){ Numbers A, B

}Numbers() { cout<<"This is the constructor"<<endl; n1 = n2 = 0; }

First call by First object (A)

Second call by Second object (B)


Initialization of Object’s values

n1

n2

0

0

n1

n2

0

0

A B


Output:Numbers A, B Call constructor twice

in the order of object creation.

This is the constructor This is the constructor

Output:


4.2 Default constructor

• A constructor with empty arguments.• Whenever a new object is created, the

default constructor is invoked.• The previous example uses this.


4.3 Parameterized Constructor

• A constructor can have many parameters.

• If the constructor receives a parameter when the object is created, the parameter must be included with the declaration of the object.


Example (Parameterized Constructor)

#include<iostream>using namespace std;class Numbers { private: int n1, n2;

public:

Numbers(int a, int b) { n1=a; n2=b;

cout<<"This is the parameterized constructor"<<endl; }

} ; Continue….


Example (Parameterized Constructor) – (cont…)

void main(){ Numbers Ob(10, 18);

}

Numbers(int a, int b) { n1=a; n2=b; cout<<"This is the parameterized constructor"<<endl; }

10 is copied into a,18 is copied into b



n1

n2

10

18

Ob


Output:Numbers Ob(10, 18);

Call parameterized constructor

This is the parameterized constructor

Output:


4.4 Constructors with initialization list

• Applies to default constructors and also parameterized constructor

• The value is entered after an argument list for function, starting with single point symbol ( : ).


Example (Parameterized Constructor with Initialization list)


public:

Numbers(int a, int b) : n1(a), n2(b) { cout<<"This is the parameterized constructor"<<endl;

}} ;

Similar to earlier parameterized constructor


Exercise 1• Given a class below, create a parameterized

constructor with initialization list

class DoubleNum{ private: double a, b;

};


4.5 Overloaded Constructors

• Constructors can also be overloaded.• You only have to list down all the

different version of constructor needed to be used in the class.

• The main purpose of overloading a constructor is to provide options or give the object initial value or not.


Example (Overloaded Constructors)


public: Numbers(int a, int b) : n1(a), n2(b) { cout<<"This is the parameterized constructor"<<endl;

} Numbers( ) : n1(0), n2(0) {cout<<"This is the default constructor"<<endl; } } ; Continue….


Example (Overloaded Constructors) – (cont…)

void main(){ Numbers Ab, Cd(3,5);

}Default constructor will

be called first

Parameterized constructor will be called next



n1

n2

0

0

n1

n2

3

5

Ab Cd


Output:Numbers Ab, Cd(3,5);

Default constructor called first followed by parameterized constructor

This is the default constructor This is the parameterized constructor

Output:


4.6 Copy Constructors• Copy constructors are a specialized form of a

constructor used to construct a copy of an object from an object of the same type.

• A copy constructor is supposed to construct a new object which is initialized to a copy of an existent object.

• The copied constructor takes a reference to a const parameter.

• It is const to guarantee that the copy constructor doesn't change it.


4.6 Copy Constructors (cont..)

• If you don't provide your own copy constructor, C++ generates one for you.

• The copy constructor provided by C++ performs a member-by-member copy of each data member.


#include<iostream>using namespace std;class base{ private: int val; public: base(int x=0) { val=x; cout<<endl<<"Constructor; value:"<<val; }

~base( ) { cout<<endl<<"Destructor; value:"<<val; }

base(const base &rc) { cout<<endl<<"Copy constructor"; val=rc.val + 10; }};

Continue….

Copy Constructor


int main( ){ base b1; //creates a new object newobj1 based on an existing class b1 base newobj1 = b1; //calls copy constructor

//another way to create a new object newobj2 based on existing

//object b1 base newobj2 (b1); //calls copy constructor return 0;}


Output


Exercise 2

• Create a class called BoxDeclare a private data member: int lengthDefine a mutator and accessor functionCreate overloaded constructors (default and

parameterized). In the default constructor, initialize the length with 10 using initialization list

Create a copy constructor


Exercise 3• Given a class (in the next slide) and

sample output (in the next two slides), in the main() :Declare an array of 3 objectsSet the objects to values 20.0, 30.0, 40.0 by

using the appropriate function.Display the values of all the objects using

the appropriate function


#include<iostream>#include<string>#include<iomanip>using namespace std;class Marks{private: double point;public: Marks() { point = 0.00;}

Marks(double p) { point = p;}

void setPoint(double p) { point = p; } double getPoint() { return point; }};


Sample Output


//Answer:int main( ){ Marks m1[3] ; double val=20;

for(int i=0; i<3; i++) { m1[i].setPoint(val+(i*10)); cout<<fixed<<showpoint<<setprecision(2)<<

m1[i].getPoint()<<endl;

} return 0;}


Exercise 4• Using the class at Exercise 3, identify the output if you

have the following:

int main(){ Marks m, n1[3] = { 10.5, 20.5, 59.1 }; cout<<fixed<<showpoint<<setprecision(2); cout<<m.getPoint()<<endl; cout<<n1[2].getPoint()*2<<endl; return 0;

}


4.7 Destructors

• DefinitionThe destructor is a function that is called

automatically when an object is destroyed or goes out of scope.

The destructor reclaims the memory that is allocated to the object.


4.7.1 Characteristics of destructor

• A destructor has the same name as the class, but it has a tilde (~ ) in front of it as shown in the following example.Does not return a value and therefore it does not

have a type specifier. Only one destructor in a class allowed.


Example (Destructor)#include<iostream>using namespace std;class Numbers { private: int n1, n2;

public: Numbers(int a, int b) : n1(a), n2(b) { cout<<"This is the parameterized constructor"<<endl;

} Numbers() : n1(0), n2(0) {cout<<"This is the default constructor"<<endl; } Continue….


Example (Destructor) – (cont…)

~Numbers() { cout<<"Object Destructed"<<endl;

cout<<n1<<" "<<n2<<endl; }} ; //end of class

Destructor

Continue….




} Recall:1) Ab calls default constructor2) Cd calls parameterized constructor



n1

n2

0

0

n1

n2

3

5

Ab

Cd


Recall Output:Numbers Ab, Cd(3,5);

Default constructor called first. Next, parameterized constructor

This is the default constructor This is the parameterized constructor

Output:




} At this point object goes out of scope, therefore they are deallocated by destructorDestructors are called twice (2 objects) in reverse order of object creation.


Order of Destruction

n1

n2

0

0

n1

n2

3

5

Ab

Cd

Second, Destructor

called for Ab

First, Destructor

called for Cd


Extra Output due to Destructor

This is the default constructor This is the parameterized constructorObject Destructed3 5

Output:

~Numbers() { cout<<"Object Destructed"<<endl; cout<<n1<<" "<<n2<<endl; }

Cd destructed


Extra Output due to Destructor

This is the default constructor This is the parameterized constructorObject Destructed3 5Object Destructed0 0

Output:

~Numbers() { cout<<"Object Destructed"<<endl; cout<<n1<<" "<<n2<<endl; }

Ab destructed


Exercise 5

• What is the output of the following program (slide 47, 48, 49)?


#include<iostream>using namespace std;class base{ private: int val; public: base(int x=0) { val=x; cout<<endl<<"Constructor; value:"<<val; }

~base( ) { cout<<endl<<"Destructor; value:"<<val; }

base(const base &rc) { cout<<endl<<"Copy constructor"; val = rc.val + 10; }

Continue….


void setVal(int n) { val = n; }

void display( ) { cout<<“\n val = “<<val<<endl; }

}; //end of class

void abc(base obj){ cout<<"obj in function abc ::"; obj.display(); obj.setVal(10); cout<<"modified obj in function abc ::"; obj.display();}

Note: You should realize that abc(…)is NOT a member function.

The obj is a new object of class base. When this function is called, obj will have a copy of object from the calling function..

When this function ends, the memory for obj will be deallocated. Thus, destructor will be called once due to obj goes out of scope


int main( ){ base b1,b2(105);

abc(b2); cout<<"b2 :"; b2.display(); cout<<"b1 :"; b1.display();

}


Answer (output)

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