constructor with virtual fuction call in c++

Question

Possible Duplicate:
Calling virtual functions inside constructors

first of all below code is not working visual c++ , but workin with bloodshed

output is 0 , but acc. to me it shud be 1 ; can anyone explain this

#include<iostream>
using namespace std;
class shape
{
public:
    virtual void print() const =0;
    virtual double area() { return 0.0;}
};
class point : public shape
{
    int x;
    int y;
public :
    point(int a=11, int b=11)
    {
        x=a;
        shape *s;
        s=this;
        cout<<s->area();
        y=b;
    }
    double area()const {return 1.0;}
    void print() const
    {
        cout<<"\nPoint\n";
        cout<<x<<"\t"<<y;
    }
};

int main()
{   
    point p(1,2);
    return 0;
}

Answer 1

Calling a virtual function in the constructor of a function acts as if the function is not virtual. It can be confusing, but it's standard behavour in c++

If you want to achieve your aim then you should probably have an "initialise" function that you call after the constructor in which virtual function calls will work as you expect.

Answer 2

You got the correct output. It should be 0. When you call area() in the constructor of your derived class, actually the base version of area() is called instead of the derived version. Calling a virtual function inside a constructor won't call the overriding functions in the derived classes.

Read this.

Answer 3

Calling virtual functions from the constructor is a really bad idea. See this Calling virtual functions from constructor (see 3rd para) for details.

Answer 4

There's a subtle flaw in your code:

double area()const {return 1.0;}

The base class' area() method is not declared const. point::area is therefore not a virtual method. Either declare shape::area const as well or remove const from point::area and it will work as expected.

Answer 5

@nobuz so just i want to clear my point . that in vtable of class point , base::area is there and not point::area as point::area is not a virtual method since this function is const while virtual base::area is not?? I think i got it right??