c++智能指针(smart pointer)详解

Smart Pointer

Deal with c++11’s smart pointer facility.

brief

Smart pointers are class objects that behave like built-in pointers but also manage objects that you create with new so that you don’t have to worry about when and whether to delete them - the smart pointers automatically delete the
managed object for you at the appropriate time.

• shared_ptr
• weak_ptr
• unique_ptr

shared_ptr (共享指针)

• referenced-counted smart pointer
• Shared Ownership with shared_ptr

引用计数智能指针:可以和其他 boost::shared_ptr 类型的智能指针共享所有权。 在这种情况下，当引用对象的最后一个智能指针销毁后或者被重新赋值或者使用了reset()，对象才会被释放。多个shared_ptr对象可以拥有同一个对象。

在继承中的例子：

    shared_ptr<Thing> base_ptr(new Thing(2));    shared_ptr<Food> derived_ptr;    ///if static_cast<Derived* >(base_ptr.get()) is valid, then the following is valid:    base_ptr->showID();    ///cast failed    derived_ptr = static_pointer_cast<Food>(base_ptr);    shared_ptr<Food> a(new Food);//    a->showID();    derived_ptr->showID();

使用 make_shared 更加高效

There are actually two dynamic memory allocations that happen: one for the object itself from the new, and then a second for the manager object created by the shared_ptr constructor. Since memory allocations are slow, this means that creating a shared_ptr is slow relative to using either a raw pointer, or a so-called “intrusive” reference- counted smart pointer where the reference count is a member variable of the object. To address this problem, C++11 includes a function template make_shared that does a single memory allocation big enough to hold both the manager object and the new object, passing along any constructor parameters that you specify, and returns a shared_ptr of the specified type, which can then be used to initialize the shared_ptr that you are creating (with efficient move semantics).

    shared_ptr<Thing> p(new Thing); // ouch - two allocations    shared_ptr<Thing> p1(make_shared<Thing>()); // only one allocation!

注意

• 使用 share_ptr copy assignment 或者构造函数会使shared_ptr的引用计数加1
• 使用 reset() 成员函数可以使当前share_ptr为空，删除指向对象的指针
• 通过给share_ptr赋值 nullptr 可以达到第二条的效果
• 不允许原生指针与智能指针之间的直接赋值转换
• 不要直接从原生指针构造两个功能相同的smart pointer，否则会造成double-deletion错误
• 可以通过 get() 函数获得原生指针
• 在继承关系或者其他的转换时，可以使用
  
  • static_pointer_cast
  • dynamic_pointer_cast
  • const_pointer_cast

weak_ptr

Weak pointers just “observe” the managed object; they don’t “keep it alive” or affect its lifetime. Unlike shared_ptrs, when the last weak_ptr goes out of scope or disappears, the pointed-to object can still exist because
the weak_ptrs do not affect the lifetime of the object - they have no ownership rights. But the weak_ptr can be used to determine whether the object exists, and to provide a shared_ptr that can be used to refer to it.

仅仅观察被管理的对象，对其生命周期不产生任何影响

1.weak_ptr build-in-pointer might zero.

void do_it(weak_ptr<Thing> wp){shared_ptr<Thing> sp = wp.lock();// get shared_ptr from weak_ptr         if(sp)          sp->defrangulate(); // tell the Thing to do something        else          cout << "The Thing is gone!" << endl;}

2.This approach is useful as a way to simply ask whether the pointed-to object still exists.

 bool is_it_there(weak_ptr<Thing> wp) {       if(wp.expired()) {          cout << "The Thing is gone!" << endl;          return false;          }       return true;   }

3.if the weak_ptr is expired, an exception is thrown, of type
std::bad_weak_ptr.

void do_it(weak_ptr<Thing> wp){shared_ptr<Thing> sp(wp); // construct shared_ptr from weak_ptr // exception thrown if wp is expired, so if here, sp is good to go sp->defrangulate();  // tell the Thing to do something}   try {          do_it(wpx);       }       catch(bad_weak_ptr&)       {          cout << "A Thing (or something else) has disappeared!" << endl;       }

4.inherit from enabled_shared_from_this\

否则会出现的错误 error:pointer being freed was not allocated

class Thing:public enable_shared_from_this<Thing>{public:    int id;public:    virtual void showID() const;    Thing();    Thing(int _id);    void foo();};void Thing::foo() {    shared_ptr<Thing> t1 = shared_from_this();    t1->showID();}

公有继承enable_shared_from_this\ ,则Thing类有了一个weak_ptr 作为成员变量。当第一个shared_ptr创建时，从第一个shared_ptr中初始化该weak_ptr\,当需要一个指向this的share_ptr时调用shared_from_this()成员函数,返回一个由weak_prt\构造而来的shared_ptr\,使得返回的shared_ptr与第一次的shared_ptr是相同的 manage object.

注意

• weak_ptr 与 share_ptr 结合使用，仅通过从share_ptr的复制和赋值，或者来源与其他weak_ptr。
• lock() 函数检查weak_ptr指向的对象是否存在，如果不存在返回一个空的share_ptr，否则返回一个指向该对象的share_ptr.
• 不能使用给weak_ptr赋值nullptr的方式，只能通过reset()方法
• expired()函数返回weak_ptr是否为存在非空对象。
• 在构造函数中不可以使用shared_from_this
• 尽可能多的搭配使用share_ptr和weak_ptr,自动化内存管理。

unique_ptr

With a unique_ptr, you can point to an allocated object, and when the unique_ptr goes out of scope, the pointed-to object gets deleted, and
this happens regardless of how we leave the function, either by a return or an exception being thrown somewhere.

unique_ptr implements a unique ownership concept - an object can be owned by only one unique_ptr at a time - the opposite of shared ownership.

unique_ptr 隐式的删除了copy构造函数，和copy assignment操作符，不允许一个对象同时被多个unique_ptr拥有这恰恰与shared_ptr相反。

The unique ownership is enforced by disallowing (with =delete) copy construction and copy assignment.So unlike built-in pointers or shared_ptr, you can’t copy or assign a unique_ptr to another unique_ptr.

move semantics: the move constructor and move assignment operator are defined for unique_ptr so that they transfer ownership from the original owner to the new owner.

可以通过move构造函数和move assignment 操作符使得unique_ptr的所属权从原来的转移到新的。转移之后原来的unique_ptr不包含任何对象。

隐式的从右值转换

 unique_ptr<Thing> create_Thing()   {       unique_ptr<Thing> local_ptr(new Thing);       return local_ptr;  // local_ptr will surrender ownership   }void foo() {       unique_ptr<Thing> p1(create_Thing()); // move ctor from returned rvalue        // p1 now owns the Thing        unique_ptr<Thing> p2; // default ctor'd; owns nothing        p2 = create_Thing(); // move assignment from returned       rvalue // p2 now owns the second Thing}

显式的使用move assignment 和 move construction进行转换

unique_ptr<Thing> p1(new Thing); // p1 owns the Thingunique_ptr<Thing> p2; // p2 owns nothing// invoke move assignment explicitlyp2 = std::move(p1); // now p2 owns it, p1 owns nothing// invoke move construction explicitlyunique_ptr<Thing> p3(std::move(p2)); // now p3 owns it, p2 and p1 own nothing 

注意

• 通过reset()函数或者给unique_ptr赋值nullptr，可以手工的删除对象。

reference

c++11 smart pointer