3 顶层父类

目录

• 1 准则
• 2 顶层父类的创建

1 准则

• 尽量使用单重继承的方式进行系统设计
• 尽量保持系统中只存在单一的继承树
• 尽量使用组合关系代替继承关系

2 顶层父类的创建

• 创建 DTLib::Object 类的意义

  • 所有数据结构都继承自 Object 类
  • 统一动态内存申请的行为，提高代码的移植性（自定义 new ，delete 操作符）
  • 在堆中创建 Object 子类的对象，失败时返回 NULL 值，而不是抛出标准库中的一个异常出来
  • Object 类为纯虚父类，所有子类都能进行动态类型识别

• 顶层父类的接口定义

  class Object
  {
  public:
      void* operator new (unsigned int size) throw();
      void operator delete (void* p );
      void* operator new[] (unsigned int size) throw();
      void operator delete[] (void* p);
      virtual ~Object() = 0;
  }
  

• Demo

  //Object.h
  #pragma once
  namespace DTLib
  {
  	class Object
  	{
      public:
          void* operator new (unsigned int size) throw();
          void operator delete (void* p);
          void* operator new[] (unsigned int size) throw();
          void operator delete[] (void* p);
          virtual ~Object() = 0;
  	};
  }
  
  
  //Object.cpp
  #include "Object.h"
  #include <cstdlib>
  namespace DTLib
  {
      //throw() 为异常规格说明，表明当前这个函数不会抛出任何异常
  	void* Object::operator new (unsigned int size) throw() {
  		return malloc(size);
  	}
  
  	void Object::operator delete (void* p) {
          free(p);
  	}
  
      //数组版本
  	void* Object::operator new[](unsigned int size) throw() {
  		return malloc(size);
  	}
  
  	void Object::operator delete[] (void* p) {
  		free(p);
  	}
  
  	Object:: ~Object() {
  
  	}
  }
  

• 使用

  • 先修改下 Object.cpp 的代码
  • 可以看到运行结果，说明这里使用的不是C++编译器默认的 new ，delete 的实现，而是在顶层父类 Object 中自定义的实现

  //Object.cpp
  #include "Object.h"
  #include <cstdlib>
  #include <iostream>
  using namespace std;
  namespace DTLib
  {
  	//单一版本
  	void* Object::operator new (unsigned int size) throw() {
  		cout << "Object::operator new: " << size << endl;
  		return malloc(size);
  	}
  
  	void Object::operator delete (void* p) {
  		cout << "Object::operator delete: " << p << endl;
  		free(p);
  	}
  
  	//数组版本
  	void* Object::operator new[](unsigned int size) throw() {
  		return malloc(size);
  	}
  
  	void Object::operator delete[](void* p) {
  		free(p);
  	}
  
  	Object:: ~Object() {
  
  	}
  }
  
  //main.cpp
  #include <iostream>
  #include "SmartPointer.h"
  #include "Object.h"
  using namespace std;
  using namespace DTLib;
  
  //定义两个Object子类
  class Test :public Object
  {
  public:
  	int i;
  	int j;
  };
  
  class Child :public Test
  {
  public:
  	int k;
  };
  
  int main()
  {
  	Object* obj1 = new Test();
  	Object* obj2 = new Child();
  
  	//......
  
  	cout << "obj1 = " << obj1 << endl;
  	cout << "obj2 = " << obj2 << endl;
  
  	delete obj1;
  	delete obj2;
  
  	return 0;
  }
  //输出结果
  Object::operator new: 12  //int*2+虚函数标指针
  Object::operator new: 16  //int*3+虚函数标指针
  obj1 = 00A3DB40
  obj2 = 00A3BED8
  Object::operator delete: 00A3DB40
  Object::operator delete: 00A3BED8