Question

Suppose I have two C++ classes:

class A
{
public:
  A() { fn(); }

  virtual void fn() { _n = 1; }
  int getn() { return _n; }

protected:
  int _n;
};

class B : public A
{
public:
  B() : A() {}

  virtual void fn() { _n = 2; }
};

If I write the following code:

int main()
{
  B b;
  int n = b.getn();
}

One might expect that n is set to 2.

It turns out that n is set to 1. Why?

Answer 1

Calling virtual functions from a constructor or destructor is dangerous and should be avoided whenever possible. All C++ implementations should call the version of the function defined at the level of the hierarchy in the current constructor and no further.

The C++ FAQ Lite covers this in section 23.7 in pretty good detail. I suggest reading that (and the rest of the FAQ) for a followup.

Excerpt:

[…] 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”.

[…]

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.

EDIT Corrected Most to All (thanks litb)