Number of vptr created if derived class also has a virtual function which is not present in Base class

Question

class Base
{
    public:
    
    virtual void func1()
    {
        std::cout<<"Base func1"<<std::endl;
    }
   //virtual destructor

};

class Derived : public Base
{
    public:
    
    virtual void func1()
    {
        std::cout<<"Derived Base func1"<<std::endl;
    }
    
    virtual void func2()
    {
        std::cout<<"Derived func2"<<std::endl;
    }
};


int main()
{
    Derived *d = new Derived;
    delete d;
}

I want to know if there will be two "vptr"s created for resolving virtual functions, one in the Base class which will be inherited in Derived class object for func1 and other one in the Derived object for func2.

Have you checked sizes of both classes? wandbox.org/permlink/6znTdbKbkv5Yn6IB. It will not provide a generic answer, but might help. In linked implementation, there is only one vptr in Derived. — Daniel Langr
– Daniel Langr, Commented Apr 3, 2019 at 10:00
I have checked the size of the base class object and derived class object, it appears same on my system "8 bytes". But according to me, 2 vptr should be created in derived class object and one in base class object. — Naresh
– Naresh, Commented Apr 3, 2019 at 10:04
Why do you think so? If GCC creates just one vptr in Derived and we suppose it's Stdndard-compliant, then the Standard cannot require 2 vptrs. — Daniel Langr
– Daniel Langr, Commented Apr 3, 2019 at 10:06
BTW, terms "vptr", "vtable", and "virtual table" are not at all mentioned in the Standard. The virtual table mechanism is seemingly an implementation issue. — Daniel Langr
– Daniel Langr, Commented Apr 3, 2019 at 10:12

HolyBlackCat · Accepted Answer · 2019-04-03 10:03:19Z

1

On my GCC:

std::cout << sizeof(void*) << ' ' << sizeof(Derived) << '\n';
// Prints 8 8

So one vtable pointer is enough. I'd expect most other compilers to behave in the same way.

derived class also has a virtual function which is not present in Base class

Virtual functions added in Derived are probably simply placed at the end of Derived vtable, after functions inherited from Base.

answered Apr 3, 2019 at 10:03

HolyBlackCat

102k13 gold badges182 silver badges306 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Red.Wave · Accepted Answer · 2019-04-03 22:21:03Z

0

vtable/vptr issues are platform specific implementation details, not generally covered by std. I can imagine implementations with no in-object vptr at all. But on most platforms I know, exactly one vptr entry serves all purposes for single inheritance and virtual inheritance. However there is no warranty for this to be portable behavior.

answered Apr 3, 2019 at 22:21

Red.Wave

4,68914 silver badges21 bronze badges

17 Comments

curiousguy Over a year ago

"I can imagine How would such impl work? Would that leak memory, in general?

Red.Wave Over a year ago

@curiousguy Why should it leak? One possible implementation is a static associative array that keeps a record of <object,type> pairs(static map<void*,type_info*> rtti;).

curiousguy Over a year ago

It would leak if objects are not destroyed properly.

Red.Wave Over a year ago

@curiousguy it is irrelevant to vt. The memory mgmt mechanism is responsible for proper destruction and deallocation; whether or not the type is polymorphic is irrelevant.

curiousguy Over a year ago

How is the map cleaned up?

|

Collectives™ on Stack Overflow

Number of vptr created if derived class also has a virtual function which is not present in Base class

2 Answers 2

Comments

17 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

17 Comments

Your Answer

Sign up or log in

Post as a guest

Related