【C++11】 智能指针 std::shared_ptr 详解

参考：

1. https://blog.csdn.net/baidu_31541363/article/details/95802210
2. 智能指针之shared_ptr基本概述

• 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。
• shared_ptr 是在C++标准库的<memory>头文件中定义的。

1. 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0
  
  std::shared_ptr<int> p2 (nullptr);//同1
  
  std::shared_ptr<int> p3 (new int);
  //拥有指向int的指针所有权，引用计数为1
  
  std::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引用计数为0，则引用计数加1，否则同样为0
  
  std::shared_ptr<int> p7 (std::move(p6));
  //获取p6的引用计数，p6引用计数为0
  
  std::shared_ptr<int> p8 (std::unique_ptr<int>(new int));
  //p8获取所有权，引用计数设置为1
  
  std::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;
}
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/*
Output:
use_count:
p1: 0
p2: 0
p3: 1
p4: 1
p5: 2
p6: 0
p7: 2
p8: 1
p9: 2
*/
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2. 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1

  bar = 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_ptr
  std::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;
}
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Output:
/*
*foo: 30
*bar: 20
*/
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3. 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
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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
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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";    //10
  std::cout << *a << "\n";        //10
  std::cout << *p << "\n";        //10

  return 0;
}
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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';  // true

  return 0;
}
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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;
}
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7. static_pointer_cast、dynamic_pointer_cast：转换指针类型

• template <class T, class U>
  shared_ptr static_pointer_cast (const shared_ptr& sp) noexcept;

#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;
}
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# 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
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• template <class T, class U>
  shared_ptr dynamic_pointer_cast (const shared_ptr& sp) noexcept;

// static_pointer_cast example
#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;

  bar = std::make_shared<B>();

  foo = std::dynamic_pointer_cast<A>(bar);

  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;
}
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# Output:
foo's static  type: class A
foo's dynamic type: class B
bar's static  type: class B
bar's dynamic type: class B
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