Virtual call in constructor

Question

class A{
  int i;
  public:
  A():i(0){cout<<" A : "<<i<<endl;}
  A(int a);
    virtual int val(){ return i+1; }

};

class B :public A{
 int i;
  public:
  B():A(),i(0){cout<<" B : "<<i<<endl;}
  B(int a):A(a),i(a) {
  }

  int val(){  return i;}
};

 A::A(int a):i(a){
   cout<<" A ; "<<i<<endl;
   A* ap = new B;
   cout<<" from A "<<ap->val()<<endl; ///expect 1 but got 0;
}

int main(){
  A a(5);
}

I expected the return val of call ap->val() to be 1, since I thought virtual mechanism doesn't work in constructor, but here I got value to be 0, since ap->val() resulted in call to B::val(). Why ?

You're instantiating it with `new B`, so it's an instance of `B`, not `A`. When you call `val()`, it uses the actual type, not declared type. — Krease, Jan 05 '16 at 18:15
Downvoting, as this is actually a poorly researched question. — πάντα ῥεῖ, Jan 05 '16 at 18:15

score 2 · Answer 1 · answered Jan 05 '16 at 18:13

2

In this constructor

 A::A(int a):i(a){
   cout<<" A ; "<<i<<endl;
   A* ap = new B;
   cout<<" from A "<<ap->val()<<endl; ///expect 1 but got 0;
}

function call ap->val() is not applied to an object under construction.

In this statement

   A* ap = new B;

an object of type B was already created using its own constructor call and the function call is applied to the object of type B that was already created.

answered Jan 05 '16 at 18:13

Vlad from Moscow

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I understand this, so if object of B is successfully created in constructor of A, virtual call mechanism works even in constructor of A ?, I though in constructor it should default to local function call ? – shanker861 Jan 05 '16 at 18:19
1

@shanker861 If the function call would be applied to the object under construction then you are right. – Vlad from Moscow Jan 05 '16 at 18:30
okay that make sense. – shanker861 Jan 05 '16 at 18:57

Virtual call in constructor

1 Answers1