C++多继承下，派生类对象有几张虚函数表？

#include <iostream>
#include <string>
#include <typeinfo>

using namespace std;

//基类
class Base1
{
public:
  Base1() : x(1) {}
  virtual void play() { cout << "Base1::play basketball" << endl; }
  virtual void dance() { cout << "Base1::dance dance" << endl; }
private:
  int x;
};

//基类
class Base2
{
public:
  Base2() : y(2) {}
  virtual void print() { cout << "Base2::print hello world" << endl; }
private:
  int y;
};

class Base3{
public:
  Base3() : z(3) {}
  virtual void sing() { cout << "Base3::sing song" << endl; }

private:
  int z;
};

//派生类多继承
class Derived : public Base1, public Base2, public Base3{
public:
  Derived() : w(4) {}
  virtual void play() { cout << "Derived::play basketball" << endl; }
  virtual void print() { cout << "Derived::print hello world" << endl; }
  virtual void sing() { cout << "Derived::sing song" << endl; }
private:
  int w;
};

int main(){
  Base1* p1 = new Derived();
  Base2* p2 = new Derived();
  Base3* p3 = new Derived();

  p1->play();
  p2->print();
  p3->sing();
  p1->dance();

  return 0;
}

Derived::play basketball                // 派生类重写的虚函数，vftable记录的是重写方法的地址
Derived::print hello world              // 派生类重写的虚函数，vftable记录的是重写方法的地址
Derived::sing song                      // 派生类重写的虚函数，vftable记录的是重写方法的地址
Base1::dance dance                      // 派生类没有重写dance，vftable记录的是基类方法的地址

查看类空间布局：

cl main.cpp /d1reportSingleClassLayoutDerived

派生类对象有三个vfptr，分别对应于三个基类

这些东西在编译阶段就生成指令了，哪个类型的基类指针，访问的就是对应的vfptr，找对应的vftable，取虚函数地址

单继承中，基类有虚函数时，派生类会继承其vfptr，如果派生类有重写虚函数，vftable中就会更新重写方法的地址，如果没有重写虚函数，vftable记录的依然是基类中函数的地址