派生类中的默认构造函数

Question

i am going over my old exams to study for finals and have noticed some stuff I still do not understand.

class Shape
{
  private:
  int center_x, int center_y;
  public:
  Shape (int x, int y) : center_x(x), center_y(y); {} //constructor initializer
}
class Rectangle : public Shape
{
  private:
  int length, width;
  public:
  Rectangle(): //this is where i have trouble, I am supposed to fill in missing code here
  //but shape does not have a default constructor, what am i supposed to fill in here?
  Rectangle (int x, int y, int l, int w) : Shape(x,y);{length = l; width = w;}
}

Thanks

Answer 1

There are two approaches. Either you call the base class construcor in the mem-initializer list of the default constructor with some default values as for example (I use zeroes as the default values):

Rectangle() : Shape( 0, 0 ), length( 0 ), width( 0 ) {}

Or you can delegate all the work from the default constructor to the constructor with parameters.

For example

Rectangle() : Rectangle( 0, 0, 0, 0 ) {}

Take into account that class definitions shall be ended with semicolon.:)

Answer 2

You're asking the wrong question. You should be asking

What should a default constructed Rectangle be?

Once you answer this question, one of the following will happen:

• It will become clear how to initialize the Shape base
• You will realize that Rectangle should not have a default constructor
• You will realize that something needs to be redesigned

Answer 3

You can assume that no coordinates are defined for your default rectangle. So it would be:

Rectangle(): Shape(x,y) , length(0), width(0) { }