where is the vptr (virtual pointer) initialized in a class having only parameterized constructors?

Question

Suppose I have a class like this

class Base
{
    private:
        int i;
        int j;
    public:
        Base(int i)
        {
            this->i = i;
            j = 0;
        }
        Base(int i, int j)
        {
            this->i = i;
            this->j = j;
        }
        virtual void f()
        {
            cout<<"in base f()"<<endl;
        }
};

VPTR gets initialized in the beginning of the constructor. But in this case there is no default constructor, only 2 parameterized constructors. Where will the VPTR be initialized?

Why do you assign some special importance to *default* constructor? All constructors initialize vptr in exactly the same way. There's nothing special about the default one in this respect. — AnT stands with Russia, Nov 22 '12 at 07:26
@AndreyT: It didn't strike my mind that all constructors will initialize vptr. Later, after seeing the answers, my doubt is cleared. — bibbsey, Nov 22 '12 at 07:34

Basile Starynkevitch · Answer 1 · 2012-11-22T11:40:39.803

3

Every constructor will first initialize the vptr pseudo-field. You can imagine that it is the first hidden field of every C++ class having some virtual member functions.

^{It is in theory possible to implement virtual functions without a virtual table pointer, but I know no common C++ implementation doing that.}

edited Nov 22 '12 at 11:40

answered Nov 22 '12 at 07:18

Basile Starynkevitch

223,805
18
296
547

Alok Save · Accepted Answer · 2012-11-22T07:25:33.180

Where is the vptr (virtual pointer) initialized in a class having only parameterized constructors?

To be strictly technical this is completely implementation defined.
However, almost known compilers implement dynamic dispatch through vptr and v-table mechanism. All of these compilers will initialize the vptr to point to its own v-table in each constructor's member initialization list.

Something like:

Base::Base(...arbitrary params...)
   : __vptr(&Base::__vtable[0])  ← supplied by the compiler, hidden from the programmer
 {

 }

This C++ FAQ explains a gist of what exactly happens.

where is the vptr (virtual pointer) initialized in a class having only parameterized constructors?

2 Answers2