当基类被继承后this指针将代表派生类的this指针

假如有一派生类Derived，初始化时需要调用基类Base的构造函数，基类的构造函数中的this将代表派生类的this，但它的typeid还是基类。例如：

基类：

class Base{
public:
  Base()
  {
    cout << "Base this : " << typeid(this).name() << endl;
    cout << "Base this =" << this << endl;
  }
  void fun()
  {
    cout << "fun this : " << typeid(this).name() << endl;
    cout << "fun this =" << this << endl;
  }
};

派生类：

class Derived : public Base{
public:
  Derived(){
    cout << "Derived this : " << typeid(this).name() << endl;
    cout << "Derived this = " << this << endl;
  }
};

main函数：

int main(int argc, char* argv[])
{
  std::cout << "*****************Derived**********************\n";
  Derived dev;
  dev.fun();
  std::cout << "\n*****************Base**********************\n";
  Base base;
  base.fun();
  system("pause");
  return 0;
}

结果为：

可以看到，当派生类调用基类的构造函数时，this指针是派生类的this指针，typeid还是基类。

MFC就是这样获得当前WinApp对象指针的。如下：

#include <iostream>
using namespace std;

class CWinApp;
CWinApp *m_pCurrentWinApp;

class CWinApp{
public:

  CWinApp()
  {
    cout << "CWinApp this : " << typeid(this).name() << endl;
    cout << "CWinApp this =" << this << endl;
    m_pCurrentWinApp = this;
    cout << endl;
  }
  virtual void fun()
  {
    cout << "CWinApp fun this : " << typeid(this).name() << endl;
    cout << "CWinApp fun this =" << this << endl;
    cout << endl;
  }
};

class CMyWinApp : public CWinApp{
public:
  CMyWinApp() {
    cout << "CMyWinApp this : " << typeid(this).name() << endl;
    cout << "CMyWinApp this = " << this << endl;
    cout << endl;
  }

  virtual void fun()
  {
    cout << "CMyWinApp fun this : " << typeid(this).name() << endl;
    cout << "CMyWinApp fun this =" << this << endl;
    cout << endl;
  }
};

/*这是个全局变量，因此会调用CMyWinApp构造函数，由于基类为CWinApp，所以先调用CWinApp构造函数，
这样，通过m_pCurrentWinApp = this,m_pCurrentWinApp就指向了CMyWinApp对象。
（也就是说，m_pCurrentWinApp是一个基类指针，指向派生类对象，调用虚函数时，就会调用派生类的对应虚函数。）*/
CMyWinApp theApp;

int main()
{
  std::cout << "m_pCurrentWinApp=" << m_pCurrentWinApp << endl;;
  m_pCurrentWinApp->fun();

  system("pause");
  return 0;
}

结果：