11 constructors in derived classes

09/04/131 VIT - SCSE
1. The derived class need not have a constructor as long as
the base class has a no-argument constructor.
2. However, if the base class has constructors with
arguments (one or more), then it is mandatory for the
derived class to have a constructor and pass the
arguments to the base class constructor.
3. When an object of a derived class is created, the
constructor of the base class is executed first and later the
constructor of the derived class.
Constructors in Derived Classes

09/04/132 VIT - SCSE
1. No constructors in the base class and derived class
2. Constructor only in the base class
class A
{
public:
A()
{
cout<<”No-argument constructor of the base class B is executed”;
}
};
class B:public A
{
public:
};
void main()
{
B ob; //accesses base constructor
}
Constructors in Derived Classes

09/04/133 VIT - SCSE
3. Constructor only in the derived class
class A
{
public:
};
class B:public A
{
public:
B()
{
cout<<”No-argument constructor of the base class B is executed”;
}
};
void main()
{
B ob; //accesses derived constructor
}

09/04/134 VIT - SCSE
4. Constructors in both base and derived classes
class A
{
public:
A()
{
cout<<”No-argument constructor of the base class A executed first”;
}
};
class B:public A
{
public:
B()
{
cout<<”No-argument constructor of the base class B is executed next”;
}
};
void main()
{
B ob; //accesses both constructor
}

09/04/135 VIT - SCSE
class A
{
public:
A()
{
cout<<”No argument constructor of
base class A”;
}
A(int a)
{
cout<<”One argument constructor of
the base class A”;
}
};
class B:public A
{
public:
B(int a)
{
cout<<”One argument
constructor of the
derived class B”;
}
};
void main()
{
B ob(3);
}
5. Multiple constructors in base class and a single
constructor in derived class

09/04/136 VIT - SCSE
Output:
No argument constructor of base class A
One argument constructor of the derived class B

09/04/137 VIT - SCSE
class A
{
public:
A(int a)
{
cout<<”One argument constructor of the base
class A”;
} };
class B:public A
{
public:
B(int a)
{
cout<<”One argument constructor of the
derived class B”;
} };
void main()
{
B ob(3);
}
Output:
Error
6. Constructor in base and derived classes without default
constructor

09/04/138 VIT - SCSE
class A
{
public:
A(int a)
{
the base class A”;
}};
class B:public A
{
public:
B(int a):A(a)
{
the derived class B”;
}};
void main()
{
B ob(3);
}
Output:
One argument
constructor of the
base class A
One argument
constructor of the
derived class B
7. Explicit invocation in the absence of default constructor

09/04/139 VIT - SCSE
class A1
{
public:
A1()
{
cout<<”No-argument constructor of the base
class A1”;
}};
class A2
{
public:
A2()
{
cout<<”No argument constructor of the base
class A2”;
}};
class B:public
A2,public A1
{
public:
B()
{
cout<<”No argument
constructor of the
derived class B”;
}};
void main()
{
B ob;
}
8. Constructor in a multiple inherited class with default
invocation

09/04/1310 VIT - SCSE
Output:
No argument constructor of the base class A2
No-argument constructor of the base class A1
No argument constructor of the derived class B

09/04/1311 VIT - SCSE
class A1
{
public:
A1()
{
class A1”;
}};
class A2
{
public:
A2()
{
class A2”;
}};
class B:public
A2,public A1
{
public:
B():A2(),A1()
{
constructor of the
derived class B”;
}};
void main()
{
B ob;
}
9. Constructor in a multiple inherited class with explicit
invocation

09/04/1312 VIT - SCSE
Output:
No argument constructor of the base class A2
No-argument constructor of the base class A1
No argument constructor of the derived class B

09/04/1313 VIT - SCSE
class A
{
public:
A1()
{
class A”;
}
};
class B:public A
{
public:
B()
{
class B”;
}
};
class C:public B
{
public:
C()
{
constructor of the
derived class C”;
}
};
void main()
{
C ob;
}
10. Constructor in multilevel inheritance

09/04/1314 VIT - SCSE
Output:
No argument constructor of the base class A
No-argument constructor of the base class B
No argument constructor of the derived class C

09/04/1315 VIT - SCSE
Destructors in derived classes
1.Unlike constructors, destructors in the class hierarchy
(parent and child class) are invoked in the reverse order of
the constructor invocation.
2.The destructor of that class whose constructor was
executed last, while building object of the derived class, will
be executed first whenever the object goes out of scope.

09/04/1316 VIT - SCSE
class B1
{
public:
B1()
{
class B1”;
}
~B1()
{
cout<<”Destructor in the base class B1”;
}};
class B2
{
public:
B2()
{
class B2”;
}
~B2()
{
cout<<”Destructor in the
base class B2”;
}
};
class D:public B1,public
B2
{
public:
D()
{
constructor of the
derived class D”;
}

09/04/1317 VIT - SCSE
~D()
{
cout<<”Destructor in the
derived class D”;
}
};
void main()
{
D ob;
}
Output:
No argument constructor of the base
class B1
No argument constructor of the base
class B2
No argument constructor of the derived
class D
Destructor in the derived class D
Destructor in the base class B2
Destructor in the base class B1

11 constructors in derived classes

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