虚函数表(图文详解)

class Base {
public:
  virtual void func1() { cout << "Base::func1" << endl; }
  virtual void func2() { cout << "Base::func2" << endl; }
private:
  int a;
};
class Derive :public Base {
public:
  virtual void func1() { cout << "Derive::func1" << endl; }
  virtual void func3() { cout << "Derive::func3" << endl; }
  virtual void func4() { cout << "Derive::func4" << endl; }
private:
  int b;
};

class Base {
public:
  virtual void func1() { cout << "Base::func1" << endl; }
  virtual void func2() { cout << "Base::func2" << endl; }
private:
  int a;
};
class Derive :public Base {
public:
  virtual void func1() { cout << "Derive::func1" << endl; }
  virtual void func3() { cout << "Derive::func3" << endl; }
  virtual void func4() { cout << "Derive::func4" << endl; }
private:
  int b;
};
typedef void(*VFPTR) ();
void PrintVTable(VFPTR vTable[])
{
  // 依次取虚表中的虚函数指针打印并调用。调用就可以看出存的是哪个函数
  cout << " 虚表地址>" << vTable << endl;
  for (int i = 0; vTable[i] != nullptr; ++i)
  {
    printf(" 第%d个虚函数地址 :0X%x,->", i, vTable[i]);
    VFPTR f = vTable[i];
    f();
  }
  cout << endl;
}
int main()
{
  Base b;
  Derive d;
  // 思路：取出b、d对象的头4bytes，就是虚表的指针，前面我们说了虚函数表本质是一个存虚函数
  //指针的指针数组，这个数组最后面放了一个nullptr
    // 1.先取b的地址，强转成一个int*的指针
    // 2.再解引用取值，就取到了b对象头4bytes的值，这个值就是指向虚表的指针
    // 3.再强转成VFPTR*，因为虚表就是一个存VFPTR类型(虚函数指针类型)的数组。
    // 4.虚表指针传递给PrintVTable进行打印虚表
    // 5.需要说明的是这个打印虚表的代码经常会崩溃，因为编译器有时对虚表的处理不干净，虚表最
    //后面没有放nullptr，导致越界，这是编译器的问题。我们只需要点目录栏的 - 生成 - 清理解决方案，再
    //编译就好了。
  VFPTR * vTableb = (VFPTR*)(*(int*)&b);
  PrintVTable(vTableb);
  VFPTR* vTabled = (VFPTR*)(*(int*)&d);
  PrintVTable(vTabled);
  return 0;
}

class Base1 {
public:
  virtual void func1() { cout << "Base1::func1" << endl; }
  virtual void func2() { cout << "Base1::func2" << endl; }
private:
  int b1;
};
class Base2 {
public:
  virtual void func1() { cout << "Base2::func1" << endl; }
  virtual void func2() { cout << "Base2::func2" << endl; }
private:
  int b2;
};
class Derive : public Base1, public Base2 {
public:
  virtual void func1() { cout << "Derive::func1" << endl; }
  virtual void func3() { cout << "Derive::func3" << endl; }
private:
  int d1;
};
typedef void(*VFPTR) ();
void PrintVTable(VFPTR vTable[])
{
  cout << " 虚表地址>" << vTable << endl;
  for (int i = 0; vTable[i] != nullptr; ++i)
  {
    printf(" 第%d个虚函数地址 :0X%x,->", i, vTable[i]);
    VFPTR f = vTable[i];
    f();
  }
  cout << endl;
}
int main()
{
  Derive d;
  VFPTR* vTableb1 = (VFPTR*)(*(int*)&d);
  PrintVTable(vTableb1);
  VFPTR* vTableb2 = (VFPTR*)(*(int*)((char*)&d + sizeof(Base1)));
  PrintVTable(vTableb2);
  return 0;
}

#include<iostream>
using namespace std;
class A {
public:
  A(const char* s) { cout << s << endl; }
  ~A() {}
};
class B :virtual public A
{
public:
  B(const char* s1, const char* s2) :A(s1) { cout << s2 << endl; }
};
class C :virtual public A
{
public:
  C(const char* s1, const char* s2) :A(s1) { cout << s2 << endl; }
};
class D :public B, public C
{
public:
  D(const char* s1, const char* s2, const char* s3, const char* s4) :B(s1, s2), C(s1, s3), A(s1)
  {
    cout << s4 << endl;
  }
};
int main() {
  D* p = new D("class A", "class B", "class C", "class D");
  delete p;
  return 0;
}

class Base1 { public:  int _b1; };
class Base2 { public:  int _b2; };
class Derive : public Base1, public Base2 { public: int _d; };
int main() {
  Derive d;
  Base1* p1 = &d;
  Base2* p2 = &d;
  Derive* p3 = &d;
  return 0;
}

class A
{
public:
    virtual void func(int val = 1) { std::cout << "A->" << val << std::endl; }
    virtual void test() { func(); }
};
class B : public A
{
public:
    void func(int val = 0) { std::cout << "B->" << val << std::endl; }
};
int main(int argc, char* argv[])
{
    B* p = new B;
    p->test();
    return 0;
}