vptr - Developer IT

Doubt in Vptr -want to know how it is getting Vtable address

- by gobi

Hi all, i wanted to know how vptr getting vtable base address. for instance class A { virtual getdata(); int a; } now, A obj; /// here vptr getting vtable's base address. i wanted to know this mystery.. Thanks in advance

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Getting list of all existing vtables.

- by Patrick

In my application I have quite some void-pointers (this is because of historical reasons, application was originally written in pure C). In one of my modules I know that the void-pointers points to instances of classes that could inherit from a known base class, but I cannot be 100% sure of it. Therefore, doing a dynamic_cast on the void-pointer might give problems. Possibly, the void-pointer even points to a plain-struct (so no vptr in the struct). I would like to investigate the first 4 bytes of the memory the void-pointer is pointing to, to see if this is the address of the valid vtable. I know this is platform, maybe even compiler-version-specific, but it could help me in moving the application forward, and getting rid of all the void-pointers over a limited time period (let's say 3 years). Is there a way to get a list of all vtables in the application, or a way to check whether a pointer points to a valid vtable, and whether that instance pointing to the vtable inherits from a known base class?

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help needed on deciphering the g++ vtable dumps

- by Ganesh Kundapur

Hi, for the fallow class hierarchy class W { public: virtual void f() { cout << "W::f()" << endl; } virtual void g() { cout << "W::g()" << endl; } }; class AW : public virtual W { public: void g() { cout << "AW::g()" << endl; } }; class BW : public virtual W { public: void f() { cout << "BW::f()" << endl; } }; class CW : public AW, public BW { }; g++ -fdump-class-hierarchy is Vtable for W W::_ZTV1W: 4u entries 0 (int ()(...))0 4 (int ()(...))(& _ZTI1W) 8 W::f 12 W::g Class W size=4 align=4 base size=4 base align=4 W (0xb6e3da50) 0 nearly-empty vptr=((& W::_ZTV1W) + 8u) Vtable for AW AW::_ZTV2AW: 7u entries 0 0u 4 0u 8 0u 12 (int ()(...))0 16 (int ()(...))(& _ZTI2AW) 20 W::f 24 AW::g VTT for AW AW::_ZTT2AW: 2u entries 0 ((& AW::_ZTV2AW) + 20u) 4 ((& AW::_ZTV2AW) + 20u) Class AW size=4 align=4 base size=4 base align=4 AW (0xb6dbf6c0) 0 nearly-empty vptridx=0u vptr=((& AW::_ZTV2AW) + 20u) W (0xb6e3da8c) 0 nearly-empty virtual primary-for AW (0xb6dbf6c0) vptridx=4u vbaseoffset=-0x00000000000000014 Vtable for BW BW::_ZTV2BW: 7u entries 0 0u 4 0u 8 0u 12 (int ()(...))0 16 (int ()(...))(& _ZTI2BW) 20 BW::f 24 W::g VTT for BW BW::_ZTT2BW: 2u entries 0 ((& BW::_ZTV2BW) + 20u) 4 ((& BW::_ZTV2BW) + 20u) Class BW size=4 align=4 base size=4 base align=4 BW (0xb6dbf7c0) 0 nearly-empty vptridx=0u vptr=((& BW::_ZTV2BW) + 20u) W (0xb6e3dac8) 0 nearly-empty virtual primary-for BW (0xb6dbf7c0) vptridx=4u vbaseoffset=-0x00000000000000014 Vtable for CW CW::_ZTV2CW: 14u entries 0 0u 4 0u 8 4u 12 (int ()(...))0 16 (int ()(...))(& _ZTI2CW) 20 BW::_ZTv0_n12_N2BW1fEv 24 AW::g 28 4294967292u 32 4294967292u 36 0u 40 (int ()(...))-0x00000000000000004 44 (int ()(...))(& _ZTI2CW) 48 BW::f 52 0u Construction vtable for AW (0xb6dbf8c0 instance) in CW CW::_ZTC2CW0_2AW: 7u entries 0 0u 4 0u 8 0u 12 (int ()(...))0 16 (int ()(...))(& _ZTI2AW) 20 W::f 24 AW::g Construction vtable for BW (0xb6dbf900 instance) in CW CW::_ZTC2CW4_2BW: 13u entries 0 4294967292u 4 4294967292u 8 0u 12 (int ()(...))0 16 (int ()(...))(& _ZTI2BW) 20 BW::f 24 0u 28 0u 32 4u 36 (int ()(...))4 40 (int ()(...))(& _ZTI2BW) 44 BW::_ZTv0_n12_N2BW1fEv 48 W::g VTT for CW CW::_ZTT2CW: 7u entries 0 ((& CW::_ZTV2CW) + 20u) 4 ((& CW::_ZTC2CW0_2AW) + 20u) 8 ((& CW::_ZTC2CW0_2AW) + 20u) 12 ((& CW::_ZTC2CW4_2BW) + 20u) 16 ((& CW::_ZTC2CW4_2BW) + 44u) 20 ((& CW::_ZTV2CW) + 20u) 24 ((& CW::_ZTV2CW) + 48u) Class CW size=8 align=4 base size=8 base align=4 CW (0xb6bea2d0) 0 vptridx=0u vptr=((& CW::_ZTV2CW) + 20u) AW (0xb6dbf8c0) 0 nearly-empty primary-for CW (0xb6bea2d0) subvttidx=4u W (0xb6e3db04) 0 nearly-empty virtual primary-for AW (0xb6dbf8c0) vptridx=20u vbaseoffset=-0x00000000000000014 BW (0xb6dbf900) 4 nearly-empty lost-primary subvttidx=12u vptridx=24u vptr=((& CW::_ZTV2CW) + 48u) W (0xb6e3db04) alternative-path what are each entries in Vtable for AW AW::_ZTV2AW: 7u entries 0 0u // ? 4 0u // ? 8 0u // ? Vtable for CW CW::_ZTV2CW: 14u entries 0 0u // ? 4 0u // ? 8 4u // ? 12 (int ()(...))0 16 (int ()(...))(& _ZTI2CW) 20 BW::_ZTv0_n12_N2BW1fEv // ? 24 AW::g 28 4294967292u // ? 32 4294967292u // ? 36 0u // ? 40 (int ()(...))-0x00000000000000004 // some delta 44 (int ()(...))(& _ZTI2CW) 48 BW::f 52 0u // ? Thanks, Ganesh

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Size of a class with 'this' pointer

- by psvaibhav

The size of a class with no data members is returned as 1 byte, even though there is an implicit 'this' pointer declared. Shouldn't the size returned be 4 bytes(on a 32 bit machine)? I came across articles which indicated that 'this' pointer is not counted for calculating the size of the object. But I am unable to understand the reason for this. Also, if any member function is declared virtual, the size of the class is now returned as 4 bytes. This means that the vptr is counted for calculating the size of the object. Why is the vptr considered and 'this' pointer ignored for calculating the size of object?

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Object of an abstract class

- by webgenius

Why is it not possible to create an object of an abstract class? I understand that the compiler creates a VTABLE with VPTR pointing to NULL. Does this prevent from creating an object of an abstract class?

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