How constructors are called in c++ (inheritance)

struggling with constructors in c++.. why in a code below base class default constructor is called despite of constructor with parameter being already executed? In D class constructor with parameter D::D(int n) : B(n) constructor is called explicitly first but after that default constructor B() is still executed before anything else and I couldnt figure out why? Thank you for any help.

Code:

---

class B
  {
      public:
        B();
        B(int n);
  };

  B::B()
  {
      cout << "B::B()\n";
  }

  B::B(int n)
  {
      cout << "B::B(" << n << ")\n";
  }


  class D : public B
  {
      public:
        D();
        D(int n);
      private:
        B b;
  };

  D::D()
  {
      cout << "D::D()\n";
  }

  D::D(int n) : B(n)
  {
      b = B(-n);
      cout << "D::D(" << n << ")\n";
  }

  int main()
  {
      D d(3);
      return 0;
  }

---

output is:

B::B(3)
B::B()
B::B(-3)
D::D(3)

sorry about smiley above, should be :: there :)

>> but after that default constructor B() is still executed before anything else and I couldnt figure out why?

You have a member variable of type B inside class D. That will be constructed when a variable of type D is created. That construction happens in the initialization list. If you don't explicitly construct b inside the initialization list, the default constructor will be called before the constructor's code is executed. That means that this line:

Code:

---

b = B(-n);

---

comes after b was already default constructed. It then creates a temporary B object with -n and assigns that temporary to b.

If you created an assignment operator and added a cout output to it, you would see that that code is assigning to b, not constructing it.

It's called because you don't initialize b to anything. Therefore, it gets initialized to the default value: a default constructor.

To see the output you were expecting, use this:

Code:

---

  D::D(int n) : B(n), b(-n)
  {
      //b = B(-n);
      cout << "D::D(" << n << ")\n";
  }

---

See, there's a difference between assignment and initialization.

BTW, you can turn off smileys for the entire post, or put code in noparse tags like so: [b][/b][noparse]:)[/noparse] :)

Thank you for such a quick help. I cant believe I missed private b variable, now it makes perfect sense to me. Thank you again for your help.