Can't understand casting

[Elena]

I have the following program:

#include <iostream>
using namespace std;

class B{
  public:
  int x;
  B(int i=10) { x=i;cout<<"constr B("<<i<<")"<<endl;}
  virtual B operator+(B ob) { B y(x+ob.x); cout<<"operator + from B"<<endl;return y;}
};

class D: public B{
    public:
    D(int i=10) { x=i;cout<<"constr D("<<i<<")"<<endl;}
    void operator=(B p) { x=p.x;  cout<<"operator = from D"<<endl;}
    B operator+(B ob) { B y(x+ob.x+1);  cout<<"operator + from D"<<endl; return y; }
    void pr(){ cout<<x<<endl; }
};

int main(){
D p1(-59),p2(32),*p3=new D;
*p3=p1+p2;
p3->pr();
return 0;
}

Its output looks like that:

constr B(10)
constr D(-59)
constr B(10)
constr D(32)
constr B(10)
constr D(10)
constr B(-26)
operator + from D
operator = from D
-26

I don't understand why is "constr B(-26)" called (and also: why is it called before + operator)? ok, the lvalue needed to be type B, but in B there is no constructor with a type D parameter. shouldn't this type cast be illegal? I know that where B i s expected, it can be D. but i don't think it's correct the other way (to pass B where D is expected)

[firedraco]

Well, that is because inside of your operator + you are constructing a B object before outputting "inside operator +".