Chapter 12. Dynamic Memory

新标准库提供了两种智能指针（smart pointer）类型来管理动态对象。

智能指针的行为类似常规指针，重要的区别是它负责自动释放所指向的对象。

新标准库提供的这两种智能指针的区别在于管理底层指针的方式：

shared_ptr 允许多个指针指向同一个对象；

unique_ptr 则“独占”所指向的对象。

weak_ptr 伴随类，它是一种弱引用，指向shared_ptr所管理的对象。

这三种类型都定义在 memory 头文件中。

shared_ptr<string>p1;    //shared_ptr ,可以指向stringshared_ptr<list<int>>p2; //shared_ptr ,可以指向int 的 listif (p1&&p1->empty())     //如果p1不为空，检查它是否指向一个空string*p1 = "Hi";          //如果p1指向一个空string，解引用p1，将一个新值赋予string

默认初始化的智能指针中保存着一个空指针。

p         //Use p as a condition;true if p points to an object.*p       //Dereference p to get the object to which p points.p.get() //Return the pointer in p.Use with caution;the object to which the returned pointer points   //will disappear when the smart pointer deletes it.

shared_ptr独有的操作：

make_shared<T>(args)  返回一个shared_ptr,指向一个动态分配的类型为T的对象。使用args初始化对象。

Return a shared_ptr pointing to a dynamically allocated object of type T.Useargs to initialize that object.

shared_ptr<T>p(q)     p是shared_ptrq 的拷贝；此操作会递增q中的计数器。q中的指针必须能转换为T*.

p.unique()    Reurnsture if p.use_count() is one;false otherwise.

shared_ptr<int> p3 = make_shared<int>(42);      // shared_ptr that points to an int with value 42shared_ptr<string> p4 = make_shared<string>(10, '9');  // p4 points to a string with value 9999999999shared_ptr<int> p5 = make_shared<int>();   // p5 points to an int that is value initialized to 0

当然，我们通常用auto定义一个对象来保存make_shared的结果，这种结果较为简单：

// p6 points to a dynamically allocated, empty vector<string>auto p6 = make_shared<vector<string>>();

auto p = make_shared<int>(42); // object to which p points has one userauto q(p); // p and q point to the same object// object to which p and q point has two users

When we copy or assign ashared_ptr, eachshared_ptr keeps track of how many othershared_ptrs point to the same object.

auto p = make_shared<int>(42); // object to which p points has one userauto q(p); // p and q point to the same object// object to which p and q point has two users

一旦一个shared_ptr的计数器变为0，它就会自动释放自己所管理的对象。

auto r = make_shared<int>(42); // int to which r points has one userr = q; // assign to r, making it point to a different address// increase the use count for the object to which q points// reduce the use count of the object to which r had pointed// the object r had pointed to has no users; that object is automaticallyfreed

Programs tend to use dynamic memory for one of three purposes:
1. They don’t know how many objects they’ll need.程序不知道自己需要使用多少对象。
2. They don’t know the precise type of the objects they need.程序不知道所需对象的准确类型。
3. They want to share data between several objects.程序需要在多个对象间共享数据。

vector<string> v1; // empty vector空vector{ // new scope新作用域vector<string> v2 = { "a", "an", "the" };v1 = v2; // copies the elements from v2 into v1} // v2 is destroyed, which destroys the elements in v2,,v2被销毁，其中的元素也被销毁  // v1 has three elements, which are copies of the ones originally in v2，v1有三个元素，是原来v2中元素的拷贝