C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  virtual function table ptr stored to file and then again rer          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Guest Posted: Tue Aug 08, 2006 3:31 am    Post subject: virtual function table ptr stored to file and then again rer Hello All, I am defining a class with one virtual function and storing its first 4 bytes ie. the address of the virtual function table to a file.I am again rereading the file in the same program and calling the virtual function.The function gets called nicely but it shows the junk values for the member variables of the class of which the function is a member..below is the code //Program start #include "iostream.h" #include "fstream.h" struct vptrtable { void (*fptr)(); }; class one { int i; public: one(){i = 9; } virtual void fun() { cout<<"i is "<<i<<"\ninside fun\n"; } }; int main() { one o; fstream f("c:\\1.txt",ios::out); //storing the vptr into the file f.write((char*)&o,sizeof(int)); f.close(); int p=0; fstream g("c:\\1.txt",ios::in); //getting back the fptr from the file g.read((char*)&p,sizeof(int)); vptrtable * v=(vptrtable*)p; //calling the function. v->fptr(); return 1; } //Program end The output is i is 1769172585 inside fun i should be 9..but the function is getting called nicely.. what is going wrong here? If it has to fail then why the function is getting called correctly? Thanks and Regards, Yogesh Joshi [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Thomas J. Gritzan Guest Posted: Tue Aug 08, 2006 5:58 am    Post subject: Re: virtual function table ptr stored to file and then again ypjofficial (AT) indiatimes (DOT) com schrieb: Quote: Hello All, I am defining a class with one virtual function and storing its first 4 bytes ie. the address of the virtual function table to a file.I am again rereading the file in the same program and calling the virtual function.The function gets called nicely but it shows the junk values for the member variables of the class of which the function is a member..below is the code Your code invokes undefined behaviour. There is no (portable) way to write the vtable to disk, using it later. Even if it would be possible, it is a bad idea, since when you rebuild your programm, the compiler could relocate the table, so the vtable pointer may become invalid. [snipped code] Quote: //Program end The output is i is 1769172585 inside fun i should be 9..but the function is getting called nicely.. what is going wrong here? If it has to fail then why the function is getting called correctly? The simple explanation: Your code invokes undefined behaviour, it does what it does. The offtopic explanation: You created some sort of a copy of the original object but left out the "int i". The function tries to access some garbage value in memory. -- Thomas [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Ismail Pazarbasi Guest Posted: Tue Aug 08, 2006 6:04 am    Post subject: Re: virtual function table ptr stored to file and then again <ypjofficial (AT) indiatimes (DOT) com> wrote in message news:1154983384.091386.297620 (AT) i42g2000cwa (DOT) googlegroups.com... Quote: Hello All, I am defining a class with one virtual function and storing its first 4 bytes ie. the address of the virtual function table to a file. No offence, but I hope you are well aware of what you are trying to do. I also hope you already know there has been some talks about those who write pointers to disk and read them back (first thing I remember is Herb's talks). Without looking at the piece of code below, I am guessing that you forgot "this" pointer and read a junk memory, fortunately, somewhere that does not cause a page fault (i.e. access violation, segfault). You need correct "this" pointer to play with (super-fragile) vtable properly. However, those extreme tricks are not reputable as far as standard is concerned, as standard does not define vtable layout or internal implementation details. In multiple-inheritance cases, vtable layout may become extremely complex and even brings experts down their knees. Quote: I am again rereading the file in the same program and calling the virtual function.The function gets called nicely but it shows the junk values for the member variables of the class of which the function is a member..below is the code //Program start #include "iostream.h" #include "fstream.h" struct vptrtable { void (*fptr)(); }; class one { int i; public: one(){i = 9; } virtual void fun() { cout<<"i is "<<i<<"\ninside fun\n"; } }; int main() { one o; fstream f("c:\\1.txt",ios::out); //storing the vptr into the file f.write((char*)&o,sizeof(int)); f.close(); int p=0; fstream g("c:\\1.txt",ios::in); //getting back the fptr from the file g.read((char*)&p,sizeof(int)); vptrtable * v=(vptrtable*)p; //calling the function. v->fptr(); return 1; } //Program end The output is i is 1769172585 inside fun i should be 9..but the function is getting called nicely.. what is going wrong here? If it has to fail then why the function is getting called correctly? Thanks and Regards, Yogesh Joshi I see no (run-time) polymorphism here. In fact, I think I have understood what you want, but the term "vtable" is usually used with polymorphism. Anyway, one short part of the answer to your question is (IMHO); you jump to code, but you do not have a correct object and this pointer, so the "i" is loaded with garbage. "this" pointer is required to find correct address of member "i" (well, actually for virtual functions as well) - it's about memory layout of the object. If you derive a class from "class one" and call "fun" member function polymorphically, your call will still go to one::fun rather than the last override - and you will, of course, not haveany member of the class. This is not portable, not standard, not good, too fragile, too evil (and you have used it wrong). Ismail [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Francis Glassborow Guest Posted: Tue Aug 08, 2006 5:47 pm    Post subject: Re: virtual function table ptr stored to file and then again In article <1154983384.091386.297620 (AT) i42g2000cwa (DOT) googlegroups.com>, ypjofficial (AT) indiatimes (DOT) com writes Quote: Hello All, I am defining a class with one virtual function and storing its first 4 bytes ie. the address of the virtual function table to a file.I am again rereading the file in the same program and calling the virtual function.The function gets called nicely but it shows the junk values for the member variables of the class of which the function is a member..below is the code The real question is why yyou expect your code to work Quote: //Program start #include "iostream.h" Not a standard header #include "fstream.h" Not a standard header Quote: struct vptrtable { void (*fptr)(); }; What is this for? And what makes you think that there is such a thing as a virtual function pointer table. VFPT are just an implementation detail. Quote: class one { int i; public: one(){i = 9; } Please learn to write constructors using constructor initialiser lists: one(): i(9) {} Quote: virtual void fun() Note that as a member function this function can only be called using an object of type one (or via a reference or pointer to such an object Quote: { cout<<"i is "<<i<<"\ninside fun\n"; } }; int main() { one o; fstream f("c:\\1.txt",ios::out); //storing the vptr into the file f.write((char*)&o,sizeof(int)); No, all this does is to store the address of o as an array of char. Whether that is, or is not the address of a vfpt is entirely implementation dependant. However, much more important is your assumptions that 1) the address of an object has the same format as the address of a pointer to a table of function pointers. 2) that the size of a pointer is the same as the size of an int. Quote: f.close(); int p=0; fstream g("c:\\1.txt",ios::in); //getting back the fptr from the file g.read((char*)&p,sizeof(int)); And here you go again, there is absolutely no requirement in C++ that the bits of a pointer can be stored in a int lvalue. Quote: vptrtable * v=(vptrtable*)p; However even if all your other assumptions happen to work for the implementation you are using all you have done here is to create a pointer to a nonexistent object of type vptrtable Quote: //calling the function. v->fptr(); This has so many problems that it is hard to know where to start. As you do have an object of type vptrtable v is pointing to some arbitrary storage. It is pure coincidence that that expression seems to call a member function of one. But as there is no object of type one anywhere, directly or indirectly, in that expression statement you are deep in undefined behaviour and are a little lucky that you have actually done no damage. Quote: return 1; } //Program end The output is i is 1769172585 inside fun i should be 9..but the function is getting called nicely.. what is going wrong here? If it has to fail then why the function is getting called correctly? Begs the question. It wasn't called correctly so anything can happen including reformatting your hard drive or causing some other chaos to your computer. -- Francis Glassborow ACCU Author of 'You Can Do It!' and "You Can Program in C++" see http://www.spellen.org/youcandoit For project ideas and contributions: http://www.spellen.org/youcandoit/projects [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Guest Posted: Wed Aug 09, 2006 3:05 am    Post subject: Re: virtual function table ptr stored to file and then again Hi All, Thanks for the debate.. actually i tried this ..because I am learning COM programming and it is based on the principle that the first 3 entry of the virtual function table should always be same.So I was just trying to simulate this behaviour in my own way by storing the virtual function table pointer to a file and then later on accessing it.. Thanks for all your help.. Thanks and Regards, Yogesh Joshi [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Ismail Pazarbasi Guest Posted: Wed Aug 09, 2006 7:02 am    Post subject: Re: virtual function table ptr stored to file and then again <ypjofficial (AT) indiatimes (DOT) com> wrote in message news:1155073830.997481.186940 (AT) 75g2000cwc (DOT) googlegroups.com... Quote: Hi All, Thanks for the debate.. actually i tried this ..because I am learning COM programming and it is based on the principle that the first 3 entry of the virtual function table should always be same.So I was just trying to simulate this behaviour in my own way by storing the virtual function table pointer to a file and then later on accessing it.. Thanks for all your help.. Thanks and Regards, Yogesh Joshi "First three entry of the vtable should always be the same", I don't remember such a rule. Could that actually mean; "all objects in COM world derive from IUnknown - directly or indirectly -, which has three methods"? I recommend you to examine the your object's memory layout in your IDE's "memory" window (with help of "watch" window as well). Unfortunately, your question is not applicable in general, so bear with Visual C++ (or Studio) IDE to investigate vendor-specific things (like vtable layout) further. This is Microsoft specific question and I think people at Microsoft news groups would help more. Visual C++: microsoft.public.vc ATL library (and COM stuff): microsoft.public.vc.atl Cheers! Ismail [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Guest Posted: Wed Aug 09, 2006 9:10 am    Post subject: Re: virtual function table ptr stored to file and then again Quote: "First three entry of the vtable should always be the same", I don't remember such a rule. Could that actually mean; "all objects in COM world derive from IUnknown - directly or indirectly -, which has three methods"? Since this is not a topic to be discussed in this forum I will make it look like typical C++ discussion so that other won't take any objection.. Interface is an abstract class and abstract class is nothing but a collection of pure virtual function...so internally the compiler creates an array(i.e table) of pointers to virtual function (VTABLE) and store the address of the array(vfptr) as a member of the abstract class..so when you derive any class from this abstract class , it inherits the vtable also and the compiler stores the addresses of the overriden function to the vtable of the derived class..The functions inside the vtable are always accessed by index.. So coming back to specific COM .. " Every interface should inherit from the IUNKnown" means the first three entries of the vtable must be of QueryInterface,AddRef and Release... Quote: I recommend you to examine the your object's memory layout in your IDE's "memory" window (with help of "watch" window as well). Unfortunately, your question is not applicable in general, so bear with Visual C++ (or Studio) IDE to investigate vendor-specific things (like vtable layout) further. This is Microsoft specific question and I think people at Microsoft news groups would help more..c++.moderated. First time posters: Do this! ] Yes..I know..thats why I put the problem without using any platform specific words.. Thanks for the discussion... Yogesh Joshi [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Marco Manfredini Guest Posted: Wed Aug 09, 2006 5:35 pm    Post subject: Re: virtual function table ptr stored to file and then again Ismail Pazarbasi wrote: Quote: "First three entry of the vtable should always be the same", I don't remember such a rule. Could that actually mean; "all objects in COM world derive from IUnknown - directly or indirectly -, which has three methods"? I recommend you to examine the your object's memory layout in your IDE's "memory" window (with help of "watch" window as well). This isn't neccessary at all. COM mandates a specific vtable layout and calling convention. If one looks at the headers of the Windows SDK which define the COM Objects of the runtime, one will see that every interface is defined twice. Once as an abstract class in C++ and then again as a C struct with a table of function pointers a the first and only member. The OP should look at these (and the output of MIDL.EXE which generates the same sort of setup) to learn about the vtables COM expects. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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