C++ vfptr, vftable, virtual functions
C++ compiler creates a hidden class member called virtual-pointer or in short vfptr when there are one or more virtual functions. This vfptr is a pointer that points to a table of function pointers. This table is also created by compiler and called virtual function table or vftable. Each row of the vftable is a function pointer pointing to a corresponding virtual function.
Base object vptr vtable:
|vfptr ->||base::Vftable ->||base::funct1()|
Derived object vptr vtable:
|vfptr ->||derived::Vftable ->||derived::funct1()|
To accomplish late binding, the compiler creates this vftable table for each class that contains virtual functions and for the class derived from it. The compiler places the addresses of the virtual functions for that particular class in "vftable".
When virtual function call is made through a base-class pointer, the compiler quietly inserts code to fetch the VFPTR and look up the function address in the VFTABLE, thus calling the right function and causing late/dynamic binding to take place.
Compiler assigns vptr to vtable
vfptr, vtables pictorial view
Here is a pictorial view of C++ object with virtual functions. Objects is containing vptr member and how it is pointing to vtable. Further how virtual functions are pointed from vtable entries.
Print vfptr, vtables using C++
Here is a C++ program to print vfptr/vptr and vftable/vtable and function addresses of the vtable.
Print vfptr, vtables using C++ output
The object construction happens in the sequence starting from base class to the derived class. First the base class object is constructed and vptr will point to base::vtable during this time. Later derived class is constructed and this time vptr pointer changes to derived::vtable location. We have taken C style function pointers and assigns these pointers from vtable and vtable. Then we are calling these function addresses using these function pointers.
Object base constructed Object address is 0x7ffeeaf43b28 vfptr address is 0x104cbe148 base::funct1 address is 0x104cbcf40 base::funct2 address is 0x104cbcf80 Calling vfunctions using vptr and vtable: base::funct1 base::funct2 Object derived constructed Object address is 0x7ffeeaf43b28 vfptr address is 0x104cbe100 derived::funct1 address is 0x104cbc950 derived::funct2 address is 0x104cbca60 Calling vfunctions using vptr and vtable: derived::funct1 derived::funct2 From main Address of d is 0x7ffeeaf43b28 d.vfptr is 0x104cbe100 Address of d.funct1 is 0x104cbc950 Address of d.funct2 is 0x104cbca60 Calling vfunctions using vptr and vtable: derived::funct1 derived::funct2
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