Question

I have two subclasess inheriting from the same abstract superclass. There is a common operation to all subclasses which depends on several attributes. Let me explain with an example:

Say this is superclass A (A is abstract):

class superClass
{
  int valueA;
  int valueB;
  float* array;

   public superClass(){
      array[valueA + valueB]
   }

   virtual foo(){
   }

}

And these are my subclasses:

class firstSubClass: superClass
{

   public firstSubClass():superClass(), valueA(100),valueB(2){
   }

   foo(){
   }

}

class secondSubClass: superClass
{

   public secondSubClass():superClass(), valueA(50),valueB(3){
   }

   foo(){
   }

}

Will the array be properly initialized? Which translates to, is the subclass constructor called before the superClass one, or is it the other way around?

Is there a way to make the initialization behavior common to both subclasses by putting it into the superClass?

Thanks in advance.

Answer 1

The superClass constructor is called first.

The superClass constructor should be responsible for initializing the superClass's data members
and firstSubClass should initialize its own data members and call the superClass's constructor;

More info on the constructor’s calling order here and you might also want to read about constructor initializer lists here

So, I would define superClass , firstSubClass and secondSubClass as :

class superClass
{
  int valueA;
  int valueB;
  float* array;

  public:

   superClass( int a , int b ): valueA(a),valueB(b) {

      array = new float[valueA + valueB];
   }
   virtual foo(){
   }

}



class firstSubClass: public superClass
{

   public:

   firstSubClass():superClass(100,2){ //calls superclass's constructor
   }

   foo(){
   }

}

class secondSubClass: public superClass
{

   public :

   secondSubClass():superClass(50 , 3){
   }

   foo(){
   }

}