Question

I am using a class say baseClass, from which I derive another class derivedClass. I have a problem definition that says, apart from others:

i) A member – object initialiser should be used to initialise a data member, say var1, that is declared in the base class.

ii) i) is done inside a base class constructor. It says, this has to be invoked only via a derived class constructor.

iii) The base class is an abstract class, whose objects cannot be created. But, I have a third class, inside which, I use:

baseClass *baseObjects[5];

The compiler does not report an error.

I do not understand, what i) and ii) really mean. An explanation in simple words would be fine. Also, any assistance on iii) is welcome.

Answer 1

I think an illustration will be best.

i)

class A
{
  int i;
 public:
  A(int ii) : i(ii) {}
}

The part i(ii) is an example of a member – object initialiser. Since C++ guarantees all constructors of members will be called before the constructor body is entered, this is your only way of specifying which constructor to call for each member.

ii) In C++ there is no super keyword. You must specify the base class as such:

 class B : public A
 {
 public:
   B(int i) : A(i) {}
 }

That’s partially due to the fact C++ allows multiple inheritence.

iii) Note that you haven’t created any objects, only pointers to objects. And it’s by this method polymorphism via inheritence is acheived in C++.