No Virtual constructors but virtual destructor

问题

If we dont have virtual constructors then why we have virtual destructors? Can constructors also be virtual?

回答1:

• There is no point in virtual constructor - you declare exactly what type is created, and it is well known in compile time. The compiler do not need [and actually cannot, since the dynamic dispatch is based on information which is created only after the object was created]. So there are no virtual constructors.
• Virtual destructors are important to prevent memory leaks, and monitor the system. Assume you have A* a = new B; [B inherits from A], and you later delete a; - the compiler has no way of knowing a is a B [in the general case], and will invoke A's destructor - if it wasn't virtual, and you might get a memory leak, or other faults.
• Using virtual destructor - you ensure that B's destructor is invoked, since a B object is being destroyed.

回答2:

Virtual destructors are needed because at destruction time, you don't always know what type you're dealing with:

Base *make_me_an_object()
{
    if (the_moon_is_full())
        return new Derived();
    else
        return new Base();
}

int main()
{
    Base *p = make_me_an_object();
    delete p;
}

The delete in the above program's main doesn't know whether its p points to a Base or a Derived object, but if the Base destructor is virtual (as it should be), then delete can use *p's vtable to find the right destructor.

By contrast, at construction time, you always know what kind of object you're creating. (And in case you don't, then you can create a factory or "virtual constructor" that does know.)

回答3:

#include<iostream>
using namespace std;
class base {
   protected:
    int a;
};
class derived : public base {

};
int main() {
    base * pointer_of_base = new derived;
    delete pointer_of_base; // this will delete the base calss not the derived

}

The constructors are called on one time when we create the object of the class so when we inherit the base class constructors calls only one time so no need to be virtual.

But when we accessing the derived class from the pointer of the base class, if we want to delete the object of derived class we delete it by the pointer of base class but delete(pointer_of_base) will call the destructor of the base class but the actual motto is to delete the derived class . thus we need the destructor be virtual in nature.

来源：https://stackoverflow.com/questions/9923959/no-virtual-constructors-but-virtual-destructor