virtualconstructor.cpp

Think in C++ 第二版源码

//: C25:VirtualConstructor.cpp
// From Thinking in C++, 2nd Edition
// Available at http://www.BruceEckel.com
// (c) Bruce Eckel 1999
// Copyright notice in Copyright.txt
#include <iostream>
#include <string>
#include <exception>
#include <vector>
using namespace std;

class Shape {
  Shape* s;
  // Prevent copy-construction & operator=
  Shape(Shape&);
  Shape operator=(Shape&);
protected:
  Shape() { s = 0; };
public:
  virtual void draw() { s->draw(); }
  virtual void erase() { s->erase(); }
  virtual void test() { s->test(); };
  virtual ~Shape() {
    cout << "~Shape\n";
    if(s) {
      cout << "Making virtual call: ";
      s->erase(); // Virtual call
    }
    cout << "delete s: ";
    delete s; // The polymorphic deletion
  }
  class BadShapeCreation : public exception {
    string reason;
  public:
    BadShapeCreation(string type) {
      reason = "Cannot create type " + type;
    }
    const char *what() const { 
      return reason.c_str(); 
    }
  };
  Shape(string type) throw(BadShapeCreation);
};

class Circle : public Shape {
  Circle(Circle&);
  Circle operator=(Circle&);
  Circle() {} // Private constructor
  friend class Shape;
public:
  void draw() { cout << "Circle::draw\n"; }
  void erase() { cout << "Circle::erase\n"; }
  void test() { draw(); }
  ~Circle() { cout << "Circle::~Circle\n"; }
};

class Square : public Shape {
  Square(Square&);
  Square operator=(Square&);
  Square() {}
  friend class Shape;
public:
  void draw() { cout << "Square::draw\n"; }
  void erase() { cout << "Square::erase\n"; }
  void test() { draw(); }
  ~Square() { cout << "Square::~Square\n"; }
};

Shape::Shape(string type) 
  throw(Shape::BadShapeCreation) {
  if(type == "Circle") 
    s = new Circle();
  else if(type == "Square")
    s = new Square();
  else throw BadShapeCreation(type);
  draw();  // Virtual call in the constructor
}

char* shlist[] = { "Circle", "Square", "Square",
  "Circle", "Circle", "Circle", "Square", "" };

int main() {
  vector<Shape*> shapes;
  cout << "virtual constructor calls:" << endl;
  try {
    for(char** cp = shlist; **cp; cp++)
      shapes.push_back(new Shape(*cp));
  } catch(Shape::BadShapeCreation e) {
    cout << e.what() << endl;
    return 1;
  }
  for(int i = 0; i < shapes.size(); i++) {
    shapes[i]->draw();
    cout << "test\n";
    shapes[i]->test();
    cout << "end test\n";
    shapes[i]->erase();
  }
  Shape c("Circle"); // Create on the stack
  cout << "destructor calls:" << endl;
  for(int j = 0; j < shapes.size(); j++) {
    delete shapes[j];
    cout << "\n------------\n";
  }
} ///:~