What are the rules for calling the superclass constructor?

Question

What are the C++ rules for calling the superclass constructor from a subclass one?

For example, I know in Java, you must do it as the first line of the subclass cons

Answer 1

If you have default parameters in your base constructor the base class will be called automatically.

using namespace std;

class Base
{
    public:
    Base(int a=1) : _a(a) {}

    protected:
    int _a;
};

class Derived : public Base
{
  public:
  Derived() {}

  void printit() { cout << _a << endl; }
};

int main()
{
   Derived d;
   d.printit();
   return 0;
}

Output is: 1