CODEWITHSUNDEEP
c++inheritance
Dmitriy Kachko
Dmitriy Kachko

Reputation: 2914

Calling virtual method from destructor - workaround?

I need to declare finalizing method finalize() for all descendants of the base class Base, that should be called during destruction, and my intent was to call pure virtual void Base::finalize() = 0 from the ~Base(), but c++ forbids such a thing. So my question is

How can we oblige descendants to do some finalizing work in right and preliminary defined way?

That code cannot be compiled:

#include <QDebug>
class Base {
        public:
            Base(){}
            virtual ~Base(){
                qDebug("deleting b");
                finalize();
            }
            virtual void finalize() = 0;

    };

class A : public Base
    {
    public:
        A(){}
        ~A(){}
        void finalize(){qDebug("called finalize in a");}
    };

    int main(int argc, char *argv[])
    {
        Base *b = new A;
        delete b;
    }

If I make Base::finalize() not pure virtual, it is called from ~Base() without dispatching to child since it have been already destructed.

I can call finalize() from child's destructor but question is how to force to do that. In other words my question is: is it possible to oblige people who will write descendants of the Base class to use finalizing method, well, in another way than commenting it in a documentation? :)

Upvotes: 7

Views: 5455

Answers (4)

Antonio P&#233;rez
Antonio P&#233;rez

Reputation: 6972

Destructors are the right place to release acquired resources, but each class is responsible to release its own resources. Resources acquired by class A should not (and just can not) be released by class Base.

Defining virtual destructors allows class A's destructor to be called when deleting a pointer to class Base pointing to a class A object

Base* p = new A;
delete p; // Both A and Base destructors are sequencially called!

So to achieve proper resource release you just have to release each class' resources in its own destructor.

Upvotes: 9

unknownfrog
unknownfrog

Reputation: 35

Why not use the desctructor of Base? That what are destructors are made for.

Make search RAII and discover one of the finest things in c++.

Most people used to use other languages have to discover this. Resource management in c++ is completely different than in most other computer languages

Upvotes: 0

Mark B
Mark B

Reputation: 96241

Make the base class destructor pure virtual with a body, which should do exactly what you want.

Upvotes: 2

Luchian Grigore
Luchian Grigore

Reputation: 258588

That's what a virtual destructor is for:

class A
{
public:
   virtual ~A() {}
};

class B : public A
{
public:
   virtual ~B() {}
};

When an object of type B is destroyed, regardless of whether from a pointer to a B or a pointer to an A, both destructors will be called. First, B::~B() and then A::~A().

Upvotes: 3

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