c++智能指针(smart pointer)详解

2019-11-08 03:15:59

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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_ptrweak_ptrunique_ptr

shared_ptr (共享指针)

referenced-counted smart pointerShared 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_castdynamic_pointer_castconst_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