c++智能指针的使用,shared_ptr,unique_ptr,weak_ptr

c++智能指针的使用

官方参考

普通指针的烦恼:内存泄漏,多次释放,提前释放

智能指针 负责自动释放所指向的对象。

三种智能指针 shared_ptr,unique_ptr,weak_ptr;

将shared_ptr存放在一个容器中,不再需要它的时候,要erase掉。

allocator负责封装堆内存管理的对象,它们在整个标准库中使用,特别是STL容器使用它们来管理容器内部的所有内存分配,大部份情况下,程序员不用理会,标准容器使用默认的分配器称为std :: allocator。

shared_ptr

shared_ptr

多个指针指向相同的对象;

使用引用计数,引用计数是线程安全的,但是对象的读写需要加锁。

不可以直接将指针直接赋值给一个智能指针,因为指针指针是一个类。

get获取原始指针

最大的陷阱就是循环引用,这会导致内存无法正确释放,导致内存泄漏

#include <iostream>
#include <memory>
#include <thread>
#include <chrono>
#include <mutex>
 
struct Base
{
    Base() { std::cout << "  Base::Base()\n"; }
    // 注意:此处非虚析构函数 OK
    ~Base() { std::cout << "  Base::~Base()\n"; }
};
 
struct Derived: public Base
{
    Derived() { std::cout << "  Derived::Derived()\n"; }
    ~Derived() { std::cout << "  Derived::~Derived()\n"; }
};
 
void thr(std::shared_ptr<Base> p)
{
    std::this_thread::sleep_for(std::chrono::seconds(1));
    std::shared_ptr<Base> lp = p; // 线程安全,虽然自增共享的 use_count
    {
        static std::mutex io_mutex;
        std::lock_guard<std::mutex> lk(io_mutex);
        std::cout << "local pointer in a thread:\n"
                  << "  lp.get() = " << lp.get()
                  << ", lp.use_count() = " << lp.use_count() << '\n';
    }
}
 
int main()
{
    std::shared_ptr<Base> p = std::make_shared<Derived>();
 
    std::cout << "Created a shared Derived (as a pointer to Base)\n"
              << "  p.get() = " << p.get()
              << ", p.use_count() = " << p.use_count() << '\n';
    std::thread t1(thr, p), t2(thr, p), t3(thr, p);
    p.reset(); // 从 main 释放所有权
    std::cout << "Shared ownership between 3 threads and released\n"
              << "ownership from main:\n"
              << "  p.get() = " << p.get()
              << ", p.use_count() = " << p.use_count() << '\n';
    t1.join(); t2.join(); t3.join();
    std::cout << "All threads completed, the last one deleted Derived\n";
}

可能的输出:

Base::Base()
  Derived::Derived()
Created a shared Derived (as a pointer to Base)
  p.get() = 0x2299b30, p.use_count() = 1
Shared ownership between 3 threads and released
ownership from main:
  p.get() = 0, p.use_count() = 0
local pointer in a thread:
  lp.get() = 0x2299b30, lp.use_count() = 5
local pointer in a thread:
  lp.get() = 0x2299b30, lp.use_count() = 3
local pointer in a thread:
  lp.get() = 0x2299b30, lp.use_count() = 2
  Derived::~Derived()
  Base::~Base()
All threads completed, the last one deleted Derived

weak_ptr

是为了配合shared_ptr而引入的一种智能指针,没有重载operator*和->,它的最大作用在于协助shared_ptr工作,像旁观者那样观测资源的使用情况。

weak_ptr可以从一个shared_ptr或者另一个weak_ptr对象构造,获得资源的观测权。但weak_ptr没有共享资源,它的构造不会引起指针引用计数的增加。

成员函数expired()的功能等价于use_count()==0,

weak_ptr可以使用一个非常重要的成员函数lock()从被观测的shared_ptr获得一个可用的shared_ptr对象

#include <iostream>
#include <memory>
 
std::weak_ptr<int> gw;
 
void observe()
{
    std::cout << "use_count == " << gw.use_count() << ": ";
    if (auto spt = gw.lock()) { // 使用之前必须复制到 shared_ptr
	std::cout << *spt << "\n";
    }
    else {
        std::cout << "gw is expired\n";
    }
}
 
int main()
{
    {
        auto sp = std::make_shared<int>(42);
	gw = sp;
 
	observe();
    }
 
    observe();
}

输出:

use_count == 1: 42
use_count == 0: gw is expired

unique_ptr

unique_ptr

唯一拥有对象

通过reset方法重新指定

通过release方法释放所有权

#include <iostream>
#include <vector>
#include <memory>
#include <cstdio>
#include <fstream>
#include <cassert>
#include <functional>
 
struct B {
  virtual void bar() { std::cout << "B::bar\n"; }
  virtual ~B() = default;//父类的析构函数需要定义为虚函数,防止内存泄漏
};
struct D : B
{
    D() { std::cout << "D::D\n";  }
    ~D() { std::cout << "D::~D\n";  }
    void bar() override { std::cout << "D::bar\n";  }
};
 
// 消费 unique_ptr 的函数能以值或以右值引用接收它
std::unique_ptr<D> pass_through(std::unique_ptr<D> p)
{
    p->bar();
    return p;
}
 
void close_file(std::FILE* fp) { std::fclose(fp); }
 
int main()
{
  std::cout << "unique ownership semantics demo\n";
  {
      auto p = std::make_unique<D>(); // p 是占有 D 的 unique_ptr
      auto q = pass_through(std::move(p)); 
      assert(!p); // 现在 p 不占有任何内容并保有空指针
      q->bar();   // 而 q 占有 D 对象
  } // ~D 调用于此
 
  std::cout << "Runtime polymorphism demo\n";
  {
    std::unique_ptr<B> p = std::make_unique<D>(); // p 是占有 D 的 unique_ptr
                                                  // 作为指向基类的指针
    p->bar(); // 虚派发
 
    std::vector<std::unique_ptr<B>> v;  // unique_ptr 能存储于容器
    v.push_back(std::make_unique<D>());
    v.push_back(std::move(p));
    v.emplace_back(new D);
    for(auto& p: v) p->bar(); // 虚派发
  } // ~D called 3 times
 
  std::cout << "Custom deleter demo\n";
  std::ofstream("demo.txt") << 'x'; // 准备要读的文件
  {
      std::unique_ptr<std::FILE, void (*)(std::FILE*) > fp(std::fopen("demo.txt", "r"),
                                                           close_file);
      if(fp) // fopen 可以打开失败;该情况下 fp 保有空指针
        std::cout << (char)std::fgetc(fp.get()) << '\n';
  } // fclose() 调用于此,但仅若 FILE* 不是空指针
    // (即 fopen 成功)
 
  std::cout << "Custom lambda-expression deleter demo\n";
  {
    std::unique_ptr<D, std::function<void(D*)>> p(new D, [](D* ptr)
        {
            std::cout << "destroying from a custom deleter...\n";
            delete ptr;
        });  // p 占有 D
    p->bar();
  } // 调用上述 lambda 并销毁 D
 
  std::cout << "Array form of unique_ptr demo\n";
  {
      std::unique_ptr<D[]> p{new D[3]};
  } // 调用 ~D 3 次
}

输出:

unique ownership semantics demo
D::D
D::bar
D::bar
D::~D
Runtime polymorphism demo
D::D
D::bar
D::D
D::D
D::bar
D::bar
D::bar
D::~D
D::~D
D::~D
Custom deleter demo
x
Custom lambda-expression deleter demo
D::D
D::bar
destroying from a custom deleter...
D::~D
Array form of unique_ptr demo
D::D
D::D
D::D
D::~D
D::~D
D::~D

shared_ptr循环引用的内存泄漏问题

如下对象建模——家长与子女:a Parent has a Child, a Child knowshis/her Parent。

从程序的运行中可以看到最终资源没有得到释放。

一个智能指针在创建一个对象的时候初始化引用计数为 1,并把自己的指针指向创建的对象。但这个引用计数在何处?在智能指针内部?非也,这个计数是一个单独的对象来实现的,如图1,当另外一个智能指针指向这个对象的时候,便找到与这个对象对应的计数对象,并加一个引用,即 use_count++。这样多个智能指针对象便可以使用相同的引用计数。

下面程序中,当指针p释放时,由于指针c->ParentPtr还在引用着new Child,所以这时(new Child)的use_count从2减为1。同理当指针c释放时,由于p->ChildPtr还在引用着new Parent,所以这时(new Parent)的use_count从2减为1。最终,内存没有被释放完全。

class Child;
class Parent;

class Parent {
private:
    std::shared_ptr<Child> ChildPtr;
public:
    void setChild(std::shared_ptr<Child> child) {
        this->ChildPtr = child;
    }

    void doSomething() {
        if (this->ChildPtr.use_count()) {
        }
    }

    ~Parent() {}
};

class Child {
private:
    std::shared_ptr<Parent> ParentPtr;
public:
    void setPartent(std::shared_ptr<Parent> parent) {
        this->ParentPtr = parent;
    }
    void doSomething() {
        if (this->ParentPtr.use_count()) {
        }
    }
    ~Child() {}
};

int main() {
    std::weak_ptr<Parent> wpp;
    std::weak_ptr<Child> wpc;
    {
        std::shared_ptr<Parent> p(new Parent);
        std::shared_ptr<Child> c(new Child);
        std::cout << "p.use_count() = " << p.use_count() << std::endl; 
        std::cout << "c.use_count() = " << c.use_count() << std::endl;
        p->setChild(c);
        c->setPartent(p);
        std::cout << "p.use_count() = " << p.use_count() << std::endl; 
        std::cout << "c.use_count() = " << c.use_count() << std::endl;
        wpp = p;
        wpc = c;

        std::cout << "p.use_count() = " << p.use_count() << std::endl; // 2
        std::cout << "c.use_count() = " << c.use_count() << std::endl; // 2
        cout<<endl;

    }
        std::cout << "p.use_count() = " << wpp.use_count() << std::endl;  // 1
        std::cout << "c.use_count() = " << wpc.use_count() << std::endl;  // 1
    return 0;
}

运行结果

p.use_count() = 1
c.use_count() = 1
p.use_count() = 2
c.use_count() = 2
p.use_count() = 2
c.use_count() = 2

p.use_count() = 1
c.use_count() = 1

shared_ptr循环引用的内存泄漏问题解决

如下,在两个需要互相引用的类的内部,使用weak_ptr智能指针引用对方,来避免循环引用导致的内存泄漏问题。

#include <iostream>
#include <memory>

class Child;
class Parent;

class Parent {
private:
    //std::shared_ptr<Child> ChildPtr;
    std::weak_ptr<Child> ChildPtr;
public:
    void setChild(std::shared_ptr<Child> child) {
        this->ChildPtr = child;
    }

    void doSomething() {
        //new shared_ptr
        if (this->ChildPtr.lock()) {

        }
    }

    ~Parent() {
    }
};

class Child {
private:
    std::shared_ptr<Parent> ParentPtr;
public:
    void setPartent(std::shared_ptr<Parent> parent) {
        this->ParentPtr = parent;
    }
    void doSomething() {
        if (this->ParentPtr.use_count()) {

        }
    }
    ~Child() {
    }
};

int main() {
    std::weak_ptr<Parent> wpp;
    std::weak_ptr<Child> wpc;
    {
        std::shared_ptr<Parent> p(new Parent);
        std::shared_ptr<Child> c(new Child);
        p->setChild(c);
        c->setPartent(p);
        wpp = p;
        wpc = c;
        std::cout << p.use_count() << std::endl; // 2
        std::cout << c.use_count() << std::endl; // 1
    }
    std::cout << wpp.use_count() << std::endl;  // 0
    std::cout << wpc.use_count() << std::endl;  // 0
    return 0;
}

运行结果

2100

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