the “inner class” idiom

有些时候我们需要upcast为多种类型，这种情况下除了可以使用multiply inherits还可以inner class。
以下为例子：

//: C10:InnerClassIdiom.cpp
// Example of the "inner class" idiom.
#include <iostream>
#include <string>
using namespace std;

class Poingable {
public:
  virtual void poing() = ;
};
void callPoing(Poingable& p) {
  p.poing();
}

class Bingable {
public:
  virtual void bing() = ;
};
void callBing(Bingable& b) {
  b.bing();
}

class Outer {
  string name;
  // Define one inner class:
  class Inner1;
  friend class Outer::Inner1;
  class Inner1 : public Poingable {
    Outer* parent;
  public:
    Inner1(Outer* p) : parent(p) {}
    void poing() {
      cout << "poing called for "
        << parent->name << endl;
      // Acesses data in the outer class object
    }
  } inner1;

  // Define a second inner class:
  class Inner2;
  friend class Outer::Inner2;
  class Inner2 : public Bingable {
    Outer* parent;
  public:
    Inner2(Outer* p) : parent(p) {}
    void bing() {
      cout << "bing called for "
        << parent->name << endl;
    }
  } inner2;
public:
  Outer(const string& nm)
  : name(nm), inner1(this), inner2(this) {}
  // Return reference to interfaces
  // implemented by the inner classes:
  operator Poingable&() { return inner1; }
  operator Bingable&() { return inner2; }
};

int main() {
  Outer x("Ping Pong");
  // Like upcasting to multiple base types!:
  callPoing(x);
  callBing(x);
} ///:~

注意inner class的使用方法：1、前置声明 2、friend  3、定义 4、访问private变量

内容源自：《TICPP-2nd-ed-Vol-two》