Analysis of the bottom layer of the virtual function table

Analysis of the bottom layer of the virtual function table

Class composition structure

 

The structure of the virtual function table

 

Code display

#include <iostream>
using namespace std;

class Base
{
public:
	virtual void ShowInf()
	{
		cout << "Base" << endl;
	}
	virtual void ShowName()
	{
		cout << "Name" << endl;
	}
};

using Func = void(*)(void);

int main()
{
	Base obj;
	cout << &obj << endl; // 对象的地址
	cout << (int*)&obj << endl; // 虚函数表的地址
	cout << (int*)*(int*)&obj << endl; // 访问虚函数表的第一个元素，即第一个虚函数的地址
	Func fptr = (Func)*(int*)*(int*)&obj; // 访问第一个成员函数
	Func fptr1 = (Func)*((int*)*(int*)&obj + 1); // 访问第二个成员函数
	fptr(); // 调用第一个成员函数
	fptr1(); // 调用第二个成员函数
}

 

 

Why (int*)&obj and &obj dereference results are different, but the output pointer address is the same?

(int*)&obj casts a pointer to a pointer to a variable of type int. We know that a variable of type int occupies 4 bytes, so it happens that the pointer points to the virtual function table in the obj class object. After dereference, it points to the virtual function table. The header element.

After &obj is dereferenced, it points to sizeof(class type) bytes, which points to the entire class.