Vptr, vtable, virtual base class table

 
# Include <iostream> using namespace STD; Class X {int x, y, z ;}; class Y: Public Virtual X {}; Class Z: public Virtual X {virtual void F () {}}; Class A: Public y, public Z {}; int main () {A * P; cout <"pointer: "<sizeof (p) <Endl; cout <" int: "<sizeof (INT) <Endl; cout <" X: "<sizeof (x) <Endl; cout <" Y: "<sizeof (y) <Endl; cout <" Z: "<sizeof (z) <Endl; cout <" A: "<sizeof (a) <Endl ;}
 

The output of the above Code on 64-bit Linux is as follows: (G ++)

The output on 32-bit windows is: (vs2013)

The parsing in Linux is as follows:

The output of X is 3*4 = 12. No problem.

If 'y' is inherited from 'x', 'y' will have a vptr, where the pointer occupies 8 bytes. In addition, because Y is a null class, the compiler automatically adds a byte. Add three bytes with the upper boundary alignment. Therefore, 8 + 3*4 + 1 + 3 = 24.

Z is the same as Y.

A inherits multiple values from Y and Z. There are two vptr. In addition, A is an empty class, so the compiler automatically adds a byte. Add three bytes with the upper boundary alignment. Therefore, 8 + 8 + 3*4 + 1 + 3 = 32.

 

 

The parsing in Windows is as follows:

The key is Y, Z, and.

Y is similar to Y in Linux. However, vs does not generate a byte for the empty class. Therefore, 3*4 + 4 = 16.

In the Z, vs compiler, virtual base class table and virtual function table are separated, because there are both virtual base classes and virtual functions, therefore, virtual function table pointers and virtual base class Table pointers are required. Therefore, 3*4 + 4 + 4 = 20.

A multiple inheritance, 3*4 + 4 + (4 + 4) = 24.

Vptr, vtable, virtual base class table