C++模拟Java“内部”类

代码思路来自Thinking in C++ 10.13.1内部类方法。类似多重继承，但是类型转换是单向的：Outer支持“向上”转型，但是不能“向下”转型回Outer。

#include <iostream>
#include <string>

using namespace std;

class Interface1
{
public:
  virtual void function1() = 0;
};

void CallInterface1(Interface1& if1)
{
  if1.function1();
}

class Interface2
{
public:
  virtual void function2() = 0;
};

void CallInterface2(Interface2& if2)
{
  if2.function2();
}

// Multiple Inheritance
class Derived : public Interface1,
                public Interface2
{
public:
  Derived(const string& nm) : name(nm) {}
  virtual void function1()
  {
    cout << "Call " << name << "'s " << __func__ << endl;
  }
  virtual void function2()
  {
    cout << "Call " << name << "'s " << __func__ << endl;
  }
private:
  string name;
};

// Mock Inner Class
class Outer {
public:
  Outer(const string& nm) : name(nm), inner1(this), inner2(this) {}
  operator Interface1&() {return inner1;}
  operator Interface2&() {return inner2;}
private:
  string name;
  class Inner1;
  friend class Outer::Inner1;
  class Inner1 : public Interface1
  {
  public:
    Inner1(Outer* p) : parent(p) {}
    virtual void function1()
    {
      cout << "Call " << parent->name << "'s function1" << endl;
    }
  private:
    Outer* parent;
  } inner1;
  class Inner2;
  friend class Outer::Inner2;
  class Inner2 : public Interface2
  {
  public:
    Inner2(Outer* p) : parent(p) {}
    virtual void function2()
    {
      cout << "Call " << parent->name << "'s function2" << endl;
    }
  private:
    Outer* parent;
  } inner2;
};

int main() {
  Derived d("Derived Class");
  CallInterface1(d);
  CallInterface2(d);

  Outer o("Outer Class");
  CallInterface1(o);
  CallInterface2(o);
  return 0;
}

使用聚合代替继承，时间空间的折衷。