C++ virtual method overriding [duplicate]

Question

Possible Duplicate:
Calling virtual functions inside constructors

main.cpp

#include <iostream>

class BaseClass {

    public:

    BaseClass() {
        init();
    }

    virtual ~BaseClass() {
        deinit();
    }

    virtual void init() {
        std::cout << "BaseClass::init()\n";
    }

    virtual void deinit() {
        std::cout << "BaseClass::deinit()\n";
    }

};

class SubClass : public BaseClass {

    public:

    virtual void init() {
        std::cout << "SubClass::init()\n";
    }

    virtual void deinit() {
        std::cout << "SubClass::deinit()\n";
    }

};

int main() {
    SubClass* cls = new SubClass;
    delete cls;
    return 0;
}

Why is init() and deinit() not properly overriden and the BaseClasses' methods are called instead of the SubClasses ones? What are the requirements to make it work?

BaseClass::init()
BaseClass::deinit()

Answer 1

Because you are calling a virtual method inside a constructor. While constructing Base class the derived one (SubClass) isn't still constructed, so actually it doesn't still exist.

It's generally a good practice to avoid calling virtual methods inside constructors.

Answer 2

They are overridden just fine.

But you've invoked them from the base constructor, and when the base constructor is executing, the derived part of the object does not yet exist.

So this is a largely a safety feature, and it's mandated by the C++ standard.