std::shared_ptr 详解

一、介绍

shared_ptr是一种智能指针（smart pointer），作用有如同指针，但会记录有多少个shared_ptrs共同指向一个对象。这便是所谓的引用计数（reference counting）。
一旦最后一个这样的指针被销毁，也就是一旦某个对象的引用计数变为0，这个对象会被自动删除。

二、shared_ptr的构造和析构

#include <iostream>
#include <memory>struct C {int* data;};int main () {auto deleter = [](int*p){std::cout << "[deleter called]\n"; delete p;};//Labmbda表达式std::shared_ptr<int> p1;//默认构造，没有获取任何指针的所有权，引用计数为0std::shared_ptr<int> p2 (nullptr);//同1std::shared_ptr<int> p3 (new int);//拥有指向int的指针所有权，引用计数为1std::shared_ptr<int> p4 (new int, deleter);//作用同3，但是拥有自己的析构方法，如果指针所指向对象为复杂结构C//结构C里有指针，默认析构函数不会将结构C里的指针data所指向的内存释放,这时需要自己使用自己的析构函数（删除器）std::shared_ptr<int> p5 (new int, [](int* p){delete p;}, std::allocator<int>());//同4,但拥有自己的分配器（构造函数），如成员中有指针，可以为指针分配内存，原理跟浅拷贝和深拷贝类似std::shared_ptr<int> p6 (p5);//如果p5引用计数不为0，则引用计数加1，否则同样为0std::shared_ptr<int> p7 (std::move(p6));//获取p6的引用计数，p6引用计数为0std::shared_ptr<int> p8 (std::unique_ptr<int>(new int));//p8获取所有权，引用计数设置为1std::shared_ptr<C> obj (new C);std::shared_ptr<int> p9 (obj, obj->data);//同6一样，只不过拥有自己的删除器与4一样std::cout << "use_count:\n";std::cout << "p1: " << p1.use_count() << '\n';std::cout << "p2: " << p2.use_count() << '\n';std::cout << "p3: " << p3.use_count() << '\n';std::cout << "p4: " << p4.use_count() << '\n';std::cout << "p5: " << p5.use_count() << '\n';std::cout << "p6: " << p6.use_count() << '\n';std::cout << "p7: " << p7.use_count() << '\n';std::cout << "p8: " << p8.use_count() << '\n';std::cout << "p9: " << p9.use_count() << '\n';return 0;
}
/*
Output:
use_count:
p1: 0
p2: 0
p3: 1
p4: 1
p5: 2
p6: 0
p7: 2
p8: 1
p9: 2
*/

三、shared_ptr赋值

给shared_ptr赋值有三种方式，如下

#include <iostream>
#include <memory>int main () {std::shared_ptr<int> foo;std::shared_ptr<int> bar (new int(10));foo = bar;                          // 拷贝，引用计数加1bar = std::make_shared<int> (20);   // 移动
//unique_ptr 不共享它的指针。它无法复制到其他 unique_ptr，无法通过值传递到函数，也无法用于需要副本的任何标准模板库 (STL) 算法。只能移动unique_ptrstd::unique_ptr<int> unique (new int(30));foo = std::move(unique);            // move from unique_ptr，引用计数转移std::cout << "*foo: " << *foo << '\n';std::cout << "*bar: " << *bar << '\n';return 0;
}
Output:
/*
*foo: 30
*bar: 20
*/

四、成员函数功能

1、swap:交换内存
void swap (shared_ptr& x)

  std::shared_ptr<int> foo (new int(10));std::shared_ptr<int> bar (new int(20));foo.swap(bar);//此时foo和bar所指向的内存发生交换

2、reset：引用计数减1，并可以重新赋值
void reset()

  std::shared_ptr<int> sp;  // sp.reset (new int);       // 获取指针的所有权*sp=10;std::cout << *sp << '\n';sp.reset (new int);       // 之前的指针的引用计数减1，并获取当前指针的所有权*sp=20;std::cout << *sp << '\n';sp.reset();  //引用计数减1，释放内存

3、get：获取原始指针
element_type* get()；

// shared_ptr::get example
#include <iostream>
#include <memory>int main () {int* p = new int (10);std::shared_ptr<int> a (p);if (a.get()==p)std::cout << "a and p point to the same location\n";// three ways of accessing the same address:std::cout << *a.get() << "\n";    //10std::cout << *a << "\n";        //10std::cout << *p << "\n";        //10return 0;
}

4、use_count：获取引用计数值，共享指针为空返回0
long int use_count(）；

5、unique ：检测是否唯一
bool unique ：

#include <iostream>
#include <memory>int main () {std::shared_ptr<int> foo;std::shared_ptr<int> bar (new int);std::cout << "foo unique?\n" << std::boolalpha;std::cout << "1: " << foo.unique() << '\n';  // false (empty)foo = bar;std::cout << "2: " << foo.unique() << '\n';  // false (shared with bar)bar = nullptr;std::cout << "3: " << foo.unique() << '\n';  // truereturn 0;
}

6、make_shared：构造共享指针
shared_ptr make_shared (Args&&… args);

#include <iostream>
#include <memory>int main () {std::shared_ptr<int> foo = std::make_shared<int> (10);// same as:std::shared_ptr<int> foo2 (new int(10));auto bar = std::make_shared<int> (20);//创建内存，并返回共享指针，只创建一次内存，make_shared详细原理可以看其他博客auto baz = std::make_shared<std::pair<int,int>> (30,40);std::cout << "*foo: " << *foo << '\n';std::cout << "*bar: " << *bar << '\n';std::cout << "*baz: " << baz->first << ' ' << baz->second << '\n';return 0;
}

7、static_pointer_cast：转换指针类型
template <class T, class U>
shared_ptr static_pointer_cast (const shared_ptr& sp)

#include <iostream>
#include <memory>struct A {static const char* static_type;const char* dynamic_type;A() { dynamic_type = static_type; }
};
struct B: A {static const char* static_type;B() { dynamic_type = static_type; }
};const char* A::static_type = "class A";
const char* B::static_type = "class B";int main () {std::shared_ptr<A> foo;std::shared_ptr<B> bar;foo = std::make_shared<A>();// cast of potentially incomplete object, but ok as a static cast://bar = std::static_pointer_cast<B>(foo);//父类转子类，强制转换，没有进行类型检查//foo = std::dynamic_pointer_cast<A>(bar);//子类转父类，动态转换，进行了类型检查std::cout << "foo's static  type: " << foo->static_type << '\n';std::cout << "foo's dynamic type: " << foo->dynamic_type << '\n';std::cout << "bar's static  type: " << bar->static_type << '\n';std::cout << "bar's dynamic type: " << bar->dynamic_type << '\n';return 0;
}
Output:
foo's static  type: class A
foo's dynamic type: class A
bar's static  type: class B
bar's dynamic type: class A