构造与析构函数与=不能被继承，以及内部类的用法

//: NINHERIT.CPP -- Non-inherited functions
#include <fstream.h>

class root {
public:
  root() { cout << "root()\n"; }
  root(root&) { cout << "root(root&)\n"; }
  root(int) { cout << "root(int)\n"; }
  root& operator=(const root&) {
    cout << "root::operator=()\n";
    return *this;
  }

  class other {}; // 定义内部类
  operator other() const {
    cout << "root::operator other()\n";
    return other();
  }
  ~root() { cout << "~root()\n"; }
};

class derived : public root {};

void f(root::other) {} // 使用内部类

main() {
  derived d1;  // Default constructor
  derived d2 = d1; // Copy-constructor
//! derived d3(1); // Error: no int constructor
  d1 = d2; // Operator= not inherited

  f(d1); // Type-conversion IS inherited
}