Do not call virtual functions in constructors and destructors

The reference to constructors and destructors must be well understood, but can you call virtual functions in constructors or destructors?

The answer is never to do this, and it won't get the results you want.

First, mention the constructor, the order of the constructors is to construct the-> subclass from the base class, if the virtual function is invoked in the base class, because the constructor base class has only its own

(or its parent, if present), is not invoked according to the rules of the virtual function table. See the following example

Class Base {

Public

Base () {callvirtual ();}

Protected

Virtual Callvirtual () {fprintf (stdout, base::callvirtual () \ n);}

};

Class Derived:public Base {

Public

Derived () {}

Protected

Virtual Callvirtual () {fprintf (stdout, derived::callvirtual () \ n);}

};

int main () {

Derived D;

}

The result is base::callvirtual (), because a derived class has not been constructed during the base class construction, and the functions of the derived class in the virtual function table are not joined

So when you look up a function, you can only find the call function of the base class

Similarly: Because the destructor of a subclass has already been invoked in a destructor, the virtual function table of a subclass cannot be accessed in the destructor of a parent class