【面向对象设计C++--翁恺】09-new and delete+10-访问限制

09-new and delete

dynamic memory allocation 在堆里面申请空间

new

• new int; // 分配一块 int 空间
• new Stash; // 分配Stash对象的空间 然后调用构造函数初始化
• new int[10] // 分配10个int 空间

delete // 给它一个地址

• delete p; //
• delete[] p;

相当于c中malloce和free
任何运算都有结果，new操作返回的是地址

动态制造对象。 new 带着[ ], delete 也要带着[]。

delete p所指的对象时候，析构函数先被调用，然后空间被回收。

new and delete

• new is the way to allocate memory as a program runs. Pointers become the only access to that memory
• delete enables you to return memory to the memory pool when you are finished with it

dynamic arrays

int * psome = new int [10];  // 动态申请数组

• the new operator returns the address of the first element of the block

delete [] psome;  // []释放，10个析构都会被调用， 不带[]只有一个被析构

• the presence of the brackets tells the program that it should free the whole array,not just the element

tips for new an delete

1. don’t use delete to free memory that new didn’t allocate
2. don’t use delete to free the same block of memory twice in succession(连续)
3. use delete[] if you used new [] to allocate an array
4. use delete (no brackets) if you used new to allocate an array
5. it’s safe to apply delete to the null pointer (noting happens)
   

new了对象但是不delete，因为我们现在的程序都是跑一下，我们的操作系统都是多进程，程序每次运行都是一个进程，操作系统给分配了虚拟空间，当程序结束后，空间会被回收，即使申请了内存没有释放，打开文件没有关闭，但是程序结束后都结束了。

但是如果程序要一直运行，那么会存在内存泄漏问题。

10-访问限制

setting limits

• to keep the client programmer’s hands off members they shouldn’t touch
• to allow the library designer to change the internal working of the structure without worrying about how it will affect the client programmer

C++ access control

• the members of a class can be cataloged,marked as:
  • public
  • private
  • protected

public公开的。所有的都可以访问

private私有的。类的成员函数可以访问这些私有的成员函数或者成员变量;
private是对类来说的不是对对象来说的，同一个类的对象是可以互相访问私有变量的，private的这种限制仅仅是在编译时刻，运行时刻仍然有办法访问别的类的私有成员

protected。类自己以及子子孙孙可以访问

public

• public means all member declarations that follow are available to everyone

private

• the private keyword means that no one can access that member except inside function members of that type
  自己指的是这个类的成员函数。

private是对类来说的，不是对对象的。同一个类的对象之间是可以互相访问私有的成员变量的。private的限制仅仅在编译时刻，意味着，运行时刻可以访问私有的东西。C++的OOP 仅在源代码时候存在，编译后的 .o 和其他语言是一样的，不会在运行时刻检查访问限制。

friends

• to explicitly grant access to a function that isn’t member of the structure
• the class itself controls which code has access to its members
• can declare a global function as a friend, as well as a member function of another class,or even an entire class,as a friend

别的类，别的函数，别的类内的某个函数

friend的授权也是在编译时刻检查的。运算符重载时候用的较多。

class vs. struct

• class defaults to private
• struct defaults to public