c++中unique_ptr 的使用和理解

unique_ptr 的使用

std::unique_ptr是c++11起引入的智能指针，为什么必须要在c++11起才有该特性，主要还是c++11增加了move语义，否则无法对对象的所有权进行传递。

unique_ptr 介绍

• 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。这意味着，内存资源所有权将转移到另一 unique_ptr，并且原始 unique_ptr 不再拥有此资源。建议将对象限制为由一个所有者所有，因为多个所有权会使程序逻辑变得复杂。因此，当需要智能指针用于纯 C++ 对象时，可使unique_ptr，而当构造 unique_ptr 时，可使用make_unique 函数。make_unique函数是从C++14之后才有。
• std::unique_ptr 实现了独享所有权的语义。一个非空的 std::unique_ptr 总是拥有它所指向的资源。转移一个 std::unique_ptr 将会把所有权也从源指针转移给目标指针（源指针被置空）。拷贝一个 std::unique_ptr 将不被允许，因为如果你拷贝一个 std::unique_ptr ,那么拷贝结束后，这两个 std::unique_ptr 都会指向相同的资源，它们都认为自己拥有这块资源（所以都会企图释放）。因此 std::unique_ptr 是一个仅能移动（move_only）的类型。当指针析构时，它所拥有的资源也被销毁。默认情况下，资源的析构是伴随着调用 std::unique_ptr 内部的原始指针的 delete 操作的。

示例程序

• 示例程序1

#include <memory>
#include <iostream>
#include <utility>

class Foo{

public:
    Foo() = default;
    Foo(int a):_a(a) {}
    ~Foo() {}
    int get_a(){
        return _a;
    }
    void set_a(int a) {
        _a = a;
    }
private:
    int _a;

};

std::unique_ptr<Foo> change_a(std::unique_ptr<Foo> f)
{
    f->set_a(10);
    return f;
}

int main()
{
    // std::make_unique是c++14才有
    std::unique_ptr<Foo> pf = std::make_unique<Foo>(10);
    // unique_ptr的复制构造函数和拷贝构造函数是删除了的，这样保证了对象独占，如果不是独占，那么跟shared_ptr
    // 就是一样的功能了。
    // std::unique_ptr<Foo> pf1 = pf;  // compile error

    // 按值传参，会有拷贝问题，同上
    // auto p = change_a(pf);   //compile error

    auto p = change_a(std::move(pf));
    std::cout << "get_a = " << p->get_a() << std::endl;
    if(!pf)
    {
        std::cout << "pf is nullptr" << std::endl;
    }
    //owner transfer from function
    std::unique_ptr<Foo> pf2 = std::make_unique<Foo>(11);
    std::unique_ptr<Foo> p2 = change_a(std::move(pf2));
    std::cout << "get_a = " << p2->get_a() << std::endl;
    if(!pf2)
    {
        std::cout << "pf2 is nullptr" << std::endl;
    }

    //使用reset
    pf2.reset(new Foo(12));
    std::cout << "pf2 is not null: " << pf2->get_a() <<  std::endl;

    //release获取原始指针
    Foo* ff = pf2.release();
    if(!pf2)
    {
        std::cout << "pf2 is nullptr" << std::endl;
    }
    std::cout << "ff is not null: " << ff->get_a() <<  std::endl;

    return 0;
}

输出结果

get_a = 10
pf is nullptr
get_a = 10
pf2 is nullptr
pf2 is not null: 12
pf2 is nullptr
ff is not null: 12

• 示例程序2

#include <cassert>
#include <cstdio>
#include <fstream>
#include <iostream>
#include <locale>
#include <memory>
#include <stdexcept>

// helper class for runtime polymorphism demo below
struct B
{
  virtual ~B() = default;

  virtual void bar() { std::cout << "B::bar\n"; }
};

struct D : B
{
  D() { std::cout << "D::D\n"; }
  ~D() { std::cout << "D::~D\n"; }

  void bar() override { std::cout << "D::bar\n"; }
};

// a function consuming a unique_ptr can take it by value or by rvalue reference
std::unique_ptr<D> pass_through(std::unique_ptr<D> p)
{
  p->bar();
  return p;
}

// helper function for the custom deleter demo below
void close_file(std::FILE* fp)
{
  std::fclose(fp);
}

// unique_ptr-based linked list demo
struct List
{
  struct Node
  {
    int data;
    std::unique_ptr<Node> next;
  };

  std::unique_ptr<Node> head;

  ~List()
  {
    // destroy list nodes sequentially in a loop, the default destructor
    // would have invoked its `next`'s destructor recursively, which would
    // cause stack overflow for sufficiently large lists.
    while (head)
    {
      auto next = std::move(head->next);
      head = std::move(next);
    }
  }

  void push(int data)
  {
    head = std::unique_ptr<Node>(new Node{data, std::move(head)});
  }
};

int main()
{
  std::cout << "1) Unique ownership semantics demo\n";
  {
    // Create a (uniquely owned) resource
    std::unique_ptr<D> p = std::make_unique<D>();

    // Transfer ownership to `pass_through`,
    // which in turn transfers ownership back through the return value
    std::unique_ptr<D> q = pass_through(std::move(p));

    // p is now in a moved-from 'empty' state, equal to nullptr
    assert(!p);
  }

  std::cout << "\n" "2) Runtime polymorphism demo\n";
  {
    // Create a derived resource and point to it via base type
    std::unique_ptr<B> p = std::make_unique<D>();

    // Dynamic dispatch works as expected
    p->bar();
  }

  std::cout << "\n" "3) Custom deleter demo\n";
  std::ofstream("demo.txt") << 'x'; // prepare the file to read
  {
    using unique_file_t = std::unique_ptr<std::FILE, decltype(&close_file)>;
    unique_file_t fp(std::fopen("demo.txt", "r"), &close_file);
    if (fp)
      std::cout << char(std::fgetc(fp.get())) << '\n';
  } // `close_file()` called here (if `fp` is not null)

  std::cout << "\n" "4) Custom lambda-expression deleter and exception safety demo\n";
  try
  {
    std::unique_ptr<D, void(*)(D*)> p(new D, [](D* ptr)
                                        {
                                          std::cout << "destroying from a custom deleter...\n";
                                          delete ptr;
                                        });

    throw std::runtime_error(""); // `p` would leak here if it were a plain pointer
  }
  catch (const std::exception&) { std::cout << "Caught exception\n"; }

  std::cout << "\n" "5) Array form of unique_ptr demo\n";
  {
    std::unique_ptr<D[]> p(new D[3]);
  } // `D::~D()` is called 3 times

  std::cout << "\n" "6) Linked list demo\n";
  {
    List wall;
    const int enough{1'000'000};
    for (int beer = 0; beer != enough; ++beer)
      wall.push(beer);

    std::cout.imbue(std::locale("en_US.UTF-8"));
    std::cout << enough << " bottles of beer on the wall...\n";
  } // destroys all the beers
}

输出

1) Unique ownership semantics demo
D::D
D::bar
D::~D

2) Runtime polymorphism demo
D::D
D::bar
D::~D

3) Custom deleter demo
x

4) Custom lambda-expression deleter and exception safety demo
D::D
destroying from a custom deleter...
D::~D
Caught exception

5) Array form of unique_ptr demo
D::D
D::D
D::D
D::~D
D::~D
D::~D

6) Linked list demo
1,000,000 bottles of beer on the wall...