When do we need to have a default constructor?

Question

My question is simple. When do we need to have a default constructor? Please refer to the code below:

class Shape {     int k;  public:     Shape(int n) : k(n) {}     ~Shape() {} };  class Rect : public Shape {     int l;  public:     Rect(int n): l(n)     {}      //error C2512: 'Shape' : no appropriate default constructor available      ~Rect() {} }; 

1. Why is the compiler not generating the zero argument default constructor implicitly in the class Rect?

2. As per my knowledge, if a class (Rect) is derived from another class (Shape) that has default constructor (either implicitly generated or explicitly provided), the default constructor should be generated by the compiler.

Answer 1

A default constructor is not synthesised if you created your own constructor with arguments. Since you gave Shape a constructor of your own, you'd have to explicitly write out a default Shape constructor now:

class Shape {       int k;    public:       Shape() : k(0) {}       Shape(int n) : k(n) {}       ~Shape() {} }; 

(You can leave out the empty ~Rect() {} definitions, as these will be synthesised.)

However, it looks to me like you don't want a default constructor for Shape here. Have Rect construct the Shape base properly:

class Shape {       int area; // I've had to guess at what this member means. What is "k"?!    public:       Shape(const int area)          : area(area)       {} };  class Rect : public Shape {      int l;      int w;    public:      Rect(const int l, const int w)         : Shape(l*w)         , l(l)         , w(w)      {} }; 

Also note that this example is oft cited as an abuse of OO. Consider whether you really need inheritance here.