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...
