Default constructor in derived classes

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

Sorry, i was supposed to add definitions to the constructors given and when they gave me Rectangle(): , I did not know what to fill in after the colon. — ricefieldboy, May 16 '15 at 18:57
Note that c++ a class declaration definition has to be followed by a semi-colon. — Nipun Talukdar, May 16 '15 at 19:05

Vlad from Moscow · Answer 1 · 2015-05-16T19:20:18.470

5

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.:)

edited May 16 '15 at 19:20

answered May 16 '15 at 19:00

Vlad from Moscow

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Second solution is only for c++11 isn't it? – jpo38 May 16 '15 at 19:06
1

Thank you sir, i would upvote, but i apparently i do not have enough rep – ricefieldboy May 16 '15 at 19:06
1

@jpo38 Yes, it is. And what year is now?:) – Vlad from Moscow May 16 '15 at 19:07
@ricefieldboy I have enough reputation so I will upvote your question.:) – Vlad from Moscow May 16 '15 at 19:08
One addition: third approach is _not_ to have a default constructor in `Rectangle`. Indeed, if you do not allow a shape to be constructed without parameters, why at all a `Rectangle` can be constructed without parameters? – Petr May 16 '15 at 19:10
@Petr It is a question to the professor who wrote this assignment.:) – Vlad from Moscow May 16 '15 at 19:12

score 1 · Answer 2 · answered May 16 '15 at 19:39

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

score 0 · Answer 3 · answered May 16 '15 at 19:00

0

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

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

answered May 16 '15 at 19:00

Christophe

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Default constructor in derived classes

3 Answers3