C + + virtual functions and virtual function tables

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The above blog post describes the structure of virtual function tables in base classes and derived classes when virtual functions exist in a base class.

What is the structure of a function table when a derived class also defines a virtual function?

Let's look at the following sample code:

1#include <iostream>2 3 using namespacestd;4 5 classA6 {7  Public:8     Virtual voidFunca () {cout<<"A"<<Endl;}9 };Ten  One classB A { -  Public: -     Virtual voidFUNCB () {cout<<"B"<<Endl;} the }; -  - classE | PublicA PublicB - { +  Public: -     Virtual voidFUNCC () {cout<<"C"<<Endl;} + }; A  at intMain () - { - c C; -Cin.Get(); -}

Both Class A and Class B have a virtual function, and class C inherits A and B. In VS2010, look at the variables:

, only the virtual function table addresses that inherit from a and B are shown in the local variables. So what about C's own virtual function?

First, look at the address of A virtual function table:

As you can see, the first 4 bytes in the virtual function table are the addresses of the virtual functions in A (red box). At the same time, followed by 4 bytes with content, and then the virtual function table is the end of the 4 0.

It can be guessed that this should be the virtual function address of C. Let's take a look at the virtual function table for B:

As you can see, only the virtual function of B in the virtual function table is an address. To confirm the above guess, the function pointer is incremented back from A::funca one time, which should be the call to C::func:

int Main () {    void(*pfun) ();    c C;     = &C;     = (pfun) * * ((int*) p)    ; // Call A::funca = (pfun) * (* (int1);          // Call C::FUNCC     cin. Get ();}

Output: Confirms our guess.

What if you add a class D to inherit C?

class  Public c{    virtualvoid  Funcd () {}}; int Main () {    d D;    Cin. Get ();}

Variable:

As shown in C, there are only two virtual function tables.

Memory:

As you can see, there are two addresses after A::funca, which can be inferred as C::FUNCC and D::funcd's address.

Summarize:

1. In multi-inheritance, the number of virtual function tables of a derived class is determined by the number of "top-level" base classes it inherits: How many of these base classes there are virtual function tables.

2. A derived class's own virtual function is appended to the first virtual function table.

For example, the following inheritance:

Assuming that each class X has a funcx virtual function, the virtual and virtual function tables in G are as follows:

C + + virtual and virtual function tables