Page 207 - DCAP404 _Object Oriented Programming

Object-oriented Programming

Notes 9.6 Constructors under Inheritance

As we know, the constructors play an important role in initializing objects. We did not use them
earlier in derived classes for the sake of simplicity. As long as no base class constructor takes any
arguments, the derived class need not have a constructor function. However, if any base class
contains a constructor with one or more arguments, then it is mandatory for the derived class to
pass arguments to the base class constructor. When both the derived and base classes contain
constructors, the base constructor is executed first and then the constructor in the derived class is
executed.
It case of multiple inheritance, the base classes are constructed in the order in which they appear
in the declaration of the derived class. Similarly, in a multi-level inheritance, the constructors
will be executed in the order of inheritance. Since the derived class takes the responsibility of
supplying initial values to its base classes, we supply the initial values that are required by all
the classes together when a derived class object is declared.
The constructor of the derived class receives the entire list of values as its arguments and passes
them on to the base constructors in the order in which they are declared in the derived class.

!

Caution The base constructors are called and executed before executing the statements in
the body of the derived constructor.
Let us consider an example:

Derived (int a1, int a2, float b1, float b2, int d1):
A(a1, a2), B (b1, b2)
{
d = d1;
}
In the above constructor named derived, supplies the five arguments. The A(a1, a2) invokes the
base constructor A( ) and B(b1, b2) invokes another base constructor B ( ). So the derived ( )
constructor has one argument d1 of its own the derived constructor contains two parts separated
by a colon (:). The first part provides the declaration of the arguments that are passed to the
derived constructor and the second part lists the function calls to the base constructors.
The constructor derived (.) may be invoked as follows:

……
derived obj (10, 20, 15.0, 17.5, 40);
……

The values are assigned to various parameters by the constructor derived ( ) as follows:
a1 ? 10
a2 ? 20
b1 ? 15.0
b2 ? 17.5

d1 ? 40

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