Constructors in the Base and Derived Classes
It is important to note that the constructors are not inherited in the inheritance process, unlike other methods that are inherited. This fact puts the burden on the programmer to provide a compulsorily special constructor in the derived class.
When you declare an object of a derived class, the compiler executes the constructor function of the base class followed by the constructor function of the derived class.
The parameter list for the derived class's constructor function could be different from that of the base class's constructor function. Therefore, the constructor function for the derived class must tell the compiler what values to use as arguments to the constructor function for the base class.
The derived class's constructor function specifies the arguments to the base class's constructor function.
D ( int a1, int a2, float b1, float b2, int d1 ) : A ( a1, a2 ), B ( b1, b2 )
{
d = d1 ;
}
The colon ( : ) operator after the derived constructor's parameter list specifies in this case that an argument list for a base class's constructor follows. The argument list is in parentheses and follows the name of the base class.
When a base class has more than one constructor function, the compiler decides which one to call based on the types of the arguments in the base constructor argument list as specified by the derived class constructor function.
One important thing to note here is that, 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 have a constructor and pass the arguments to the base class constructors.
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.
In case of multiple inheritance, the base classes are constructed in the order in which they appear in the declaration of the derived class.