Why is a call to a virtual member function in the constructor a non-virtual call?
c++, constructor, member-functions, overriding, virtual-functions
Solution
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)
Problem
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?