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?
The reason is that C++ objects are constructed like onions, from the inside out. Base classes are constructed before derived classes. So, before a B can be made, an A must be made. When A's constructor is called, it's not a B yet, so the virtual function table still has the entry for A's copy of fn().
Do you know the crash error from Windows explorer?! "Pure virtual function call ..."
Same problem ...
class AbstractClass
{
public:
AbstractClass( ){
//if you call pureVitualFunction I will crash...
}
virtual void pureVitualFunction() = 0;
};
Because there is no implemetation for the function pureVitualFunction() and the function is called in the constructor the program will crash.