Vptr
in [C++]
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From: MC on 10 Aug 2010 05:20 Hi! I have another naive question. I have been trying to understand the polymorphysim implementation in C++. I believe that most of the compilers implement it using vptr and vtables. My question is when for every object of a class (which has a virtual member) if vptr points to the same vtable, why does every object has a copy of the same vptr? For example say X is a class which has a virtual function and a and b are objects of that class both a and b will have a vptr installed in them and this vptr will point to the same vtable Why not have the vptr as a static data member of the class X so that every object of that class can use it. I am sure there is a very good reason for the way it is implemented, but because of my inexperience with C++ I dont see the reason. Can anybody please elaborate on the reason. Thanks, -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: Goran Pusic on 11 Aug 2010 16:08 On Aug 10, 10:20 pm, MC <manan.cho...(a)gmail.com> wrote: > Hi! > I have another naive question. I have been trying to understand the > polymorphysim implementation in C++. > I believe that most of the compilers implement it using vptr and > vtables. Me, too ;-) > My question is when for every object of a class (which has a virtual > member) if vptr points to the same vtable, why does every object has > a copy of the same vptr? > > For example > say X is a class which has a virtual function > and a and b are objects of that class > both a and b will have a vptr installed in them and this vptr will > point to the same vtable > Why not have the vptr as a static data member of the class X so that > every object of that class can use it. Ask yourself this: how will object know which vptr table to take? It has to know that it's type is X, and hence, when virtual function f is called on it, it has to do (metacode): GENERIC_CLASS_STATIC_DATA& sd = this->get_generic__static_data(); ^^^ vptr* p = sd.vptr(); p[index_of_f](this, params); How do you plan to do ^^^ (should resolve to e.g. X::static_data)? Solution is exactly that: store some sort of indication of object's type __in the object__. And that indication is virtual table pointer. (And when you compile with RTTI on, in the vicinity of said vptr table you have object's RTTI, too.) In a way, what you are asking for __is__ what is happening, you are just looking at things with different eyes. :-) Goran. -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: Bo Persson on 11 Aug 2010 16:00 MC wrote: > Hi! > I have another naive question. I have been trying to understand the > polymorphysim implementation in C++. > I believe that most of the compilers implement it using vptr and > vtables. > My question is when for every object of a class (which has a virtual > member) if vptr points to the same vtable, why does every object > has a copy of the same vptr? > > For example > say X is a class which has a virtual function > and a and b are objects of that class > both a and b will have a vptr installed in them and this vptr will > point to the same vtable > Why not have the vptr as a static data member of the class X so that > every object of that class can use it. > > I am sure there is a very good reason for the way it is implemented, > but because of my inexperience with C++ I dont see the reason. Can > anybody please elaborate on the reason. > The vtable is interesting only when you have other classes, like Y and Z, derivning from X. Then a pointer to X could in fact point to an object of either X, Y, or Z. In that case, the vtable will be used to select the proper virtual function to call. If you only have X objects, many compiler will bypass the vtable and call X functions directly. Bo Persson -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: Thomas Richter on 11 Aug 2010 16:18 MC schrieb: > Hi! > I have another naive question. I have been trying to understand the > polymorphysim implementation in C++. > I believe that most of the compilers implement it using vptr and > vtables. Likely, but quite irrelevant. The C++ standard doesn't say how to implement dynamic dispatch and polymorphism. > My question is when for every object of a class (which has a virtual > member) if vptr points to the same vtable, why does every object has > a copy of the same vptr? Because it is one way of how the compiler can deduce the dynamic type of an object. > For example > say X is a class which has a virtual function > and a and b are objects of that class > both a and b will have a vptr installed in them and this vptr will > point to the same vtable > Why not have the vptr as a static data member of the class X so that > every object of that class can use it. Because then polymorphism wouldn't work. If you, at some point, hold a pointer to a class derived from the class of a or b, then this pointer may have the same static type, i.e. pointer to the base class. However, the object it points to is actually of the derived class. Static members are, however, resolved based on the static type, and thus you would get the wrong "vtable". Actually, the purpose of the vtable implmentation strategy is exactly that: Storing some information on the object in the object without requiring static type information deduced at compile time - and this is what is required for polymorphism. So long, Thomas -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: Ulrich Eckhardt on 11 Aug 2010 16:17
MC wrote: > I have another naive question. I have been trying to understand the > polymorphysim implementation in C++. > I believe that most of the compilers implement it using vptr and > vtables. > My question is when for every object of a class (which has a virtual > member) if vptr points to the same vtable, why does every object has > a copy of the same vptr? That's not true, objects of the same class can have different vtables. This happens if they are not exactly of that class but of a derived class. If you have a debugger that shows the vtable pointer, you could output the pointer in the base-class constructor and then again in the derived class constructor for the same object. You would then see that the same pointer has different values. This also means the value changes during construction and destruction. Uli -- Sator Laser GmbH Geschäftsführer: Thorsten Föcking, Amtsgericht Hamburg HR B62 932 [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] |