vptr1.cpp

#include <iostream>
#include <stdio.h>
using namespace std;

/**
 * @brief 函数指针
 */
typedef void (*Fun)();

/**
 * @brief 基类
 */
class Base {
public:
  Base(){};
  virtual void fun1() { cout << "Base::fun1()" << endl; }
  virtual void fun2() { cout << "Base::fun2()" << endl; }
  virtual void fun3() {}
  ~Base(){};
};

/**
 * @brief 派生类
 */
class Derived : public Base {
public:
  Derived(){};
  void fun1() { cout << "Derived::fun1()" << endl; }
  void fun2() { cout << "DerivedClass::fun2()" << endl; }
  ~Derived(){};
};

/**
 * @brief
 * 获取vptr地址与func地址,vptr指向的是一块内存，这块内存存放的是虚函数地址，这块内存就是我们所说的虚表
 *
 * @param obj
 * @param offset
 *
 * @return
 */
Fun getAddr(void *obj, unsigned int offset) {
  cout << "=======================" << endl;
  void *vptr_addr =
      (void *)*(unsigned long *)obj; // 64位操作系统，占8字节，通过*(unsigned
                                     // long *)obj取出前8字节，即vptr指针
  printf("vptr_addr:%p\n", vptr_addr);

  /**
   * @brief 通过vptr指针访问virtual
   * table，因为虚表中每个元素(虚函数指针)在64位编译器下是8个字节，因此通过*(unsigned
   * long *)vptr_addr取出前8字节， 后面加上偏移量就是每个函数的地址！
   */
  void *func_addr = (void *)*((unsigned long *)vptr_addr + offset);
  printf("func_addr:%p\n", func_addr);
  return (Fun)func_addr;
}
int main(void) {
  Base ptr;
  Derived d;
  Base *pt = new Derived(); // 基类指针指向派生类实例
  Base &pp = ptr;           // 基类引用指向基类实例
  Base &p = d;              // 基类引用指向派生类实例
  cout << "基类对象直接调用" << endl;
  ptr.fun1();
  cout << "基类引用指向基类实例" << endl;
  pp.fun1();
  cout << "基类指针指向派生类实例并调用虚函数" << endl;
  pt->fun1();
  cout << "基类引用指向派生类实例并调用虚函数" << endl;
  p.fun1();

  // 手动查找vptr 和 vtable
  Fun f1 = getAddr(pt, 0);
  (*f1)();
  Fun f2 = getAddr(pt, 1);
  (*f2)();

  delete pt;
  return 0;
}