object - C++ calling base class constructors

Question

#include <iostream>
#include <stdio.h> 
using namespace std;

// Base class
class Shape 
{
   public:
      void setWidth(int w)
      {
         width = w;
      }
      void setHeight(int h)
      {
         height = h;
      }
      Shape()
      {
    printf("creating shape 
");
      }
      Shape(int h,int w)
      {
     height = h;
         width = w;
         printf("creatig shape with attributes
");
      } 
   protected:
      int width;
      int height;
};

// Derived class
class Rectangle: public Shape
{
   public:
      int getArea()
      { 
         return (width * height); 
      }
      Rectangle()
      {
     printf("creating rectangle 
");
      }
      Rectangle(int h,int w)
      {
     printf("creating rectangle with attributes 
");
     height = h;
         width = w;
      }
};

int main(void)
{
   Rectangle Rect;

   Rect.setWidth(5);
   Rect.setHeight(7);

   Rectangle *square = new Rectangle(5,5);
   // Print the area of the object.
   cout << "Total area: " << Rect.getArea() << endl;

   return 0;
}

The output of the program is given below

creating shape 
creating rectangle 
creating shape 
creating rectangle with attributes 
Total area: 35

When constructing both the derived class objects I see that it is always the default constructor of the base class that is called first. Is there a reason for this? Is this the reason why languages like python insist on explicit calls of base class constructors rather than implicit calls like C++?

Answer 1

The short answer for this is, "because that's what the C++ standard specifies".

Note that you can always specify a constructor that's different from the default, like so:

class Shape  {

  Shape()  {...} //default constructor
  Shape(int h, int w) {....} //some custom constructor


};

class Rectangle : public Shape {
  Rectangle(int h, int w) : Shape(h, w) {...} //you can specify which base class constructor to call

}

The default constructor of the base class is called only if you don't specify which one to call.