Re: What is a virtual thunk

[John V. Shahid]

On Thu, 2007-10-04 at 11:10 -0700, parag_paul@hotmail.com wrote:
> I am seeing the following, for GDB  step into a function.

What function ?

>  Somehow the
> debug information is not there and it shows me some other file.
> Please help with me with this.
>   I have compiled with -g all the way.
> 
> #0  0x42bb6e76 in virtual thunk to
> vhpiVssPortDeclC::vhpiGetValue(vhpiValueT*) () at util.hh:178

The "virtual thunk" or "adjuster thunk" is a piece of code that adjusts
the offset of a pointer or reference to an object. For example:

========================= Test.cpp ============================
class P {
        public:
                void pvf(int x) {this.x = x;}
        private:
                int x;
};
class R {
        public:
                void pvf(int x) {//do nothing}
};
class S : P, R {
        public:
                void pvf(int x) { this.x = x * 2;}
};
#include <stdlib.h>
int main(void)
{
        S s;
        S* ps = &s;
        P* pp = ps;//A pointer to a S object is also a pointer to a P object
        //By polymorphism the following call should be dispatched to the
        //overriding version of pvf (i.e. x = x * 2). But this function 
        //expects a pointer to a S object (i.e. the "this" pointer), yet
        //the function can be called on either a S object pointer, P 
        //object pointer or R object pointer due to multiple 
        //inheritance.
        //which requires the adjuster thunk to technically changes the 
        //pointer from either a P pointer or R pointer to S pointer.
        //The adjuster thunk is called first to change the offset and 
        //then the function is called.
        pp->pvf(2);
        return EXIT_SUCCESS;
}
=========================== End of Test.cpp ==========================

Stepping a couple of steps through the adjuster thunk should eventually
take you to the beginning of your function.

Cheers,
-- 
John V. Shahid <jvshahid@gmail.com>