Thread: Calling virtual function from Constructor

What happens if we call virtual function from constructor?

Yes, but be careful. It may not do what you expect. In a constructor, the virtual call mechanism is disabled because overriding from derived classes hasn't yet happened. Objects are constructed from the base up, "base before derived".

Consider:

Code:

#include<string>
#include<iostream>
using namespace std;

class B {
public:
    B(const string& ss) { cout << "B constructor\n"; f(ss); }
    virtual void f(const string&) { cout << "B::f\n";}
};

class D : public B {
public:
    D(const string & ss) :B(ss) { cout << "D constructor\n";}
    void f(const string& ss) { cout << "D::f\n"; s = ss; }
private:
    string s;
};

int main()
{
    D d("Hello");
}

the program compiles and produce

Code:

B constructor
B::f
D constructor

Note not D::f. Consider what would happen if the rule were different so that D::f() was called from B::B(): Because the constructor D::D() hadn't yet been run, D::f() would try to assign its argument to an uninitialized string s. The result would most likely be an immediate crash.

Destruction is done "derived class before base class", so virtual functions behave as in constructors: Only the local definitions are used - and no calls are made to overriding functions to avoid touching the (now destroyed) derived class part of the object.