std::shared_ptr(二)

Defined in header `<memory>`

template< class T > class shared_ptr;		(since C++11)

std::shared_ptr is a smart pointer that retains shared ownership of an object through a pointer. Severalshared_ptr objects may own the same object; the object is destroyed when the last remaining shared_ptr pointing to it is destroyed or reset. The object is destroyed using delete-expression or a custom deleter that is supplied toshared_ptr during construction.

A shared_ptr may also own no objects, in which case it is called empty.

shared_ptr meets the requirements of CopyConstructible and CopyAssignable.

Member types


Member type	Definition

element_type	T

Member functions


(constructor)	constructs new `shared_ptr` (public member function)

(destructor)	destructs the owned object if no more `shared_ptr`s link to it (public member function)

operator=	assigns the `shared_ptr` (public member function)
Modifiers

reset	replaces the managed object (public member function)

swap	swaps the managed objects (public member function)
Observers

get	returns a pointer to the managed object (public member function)

operator* operator->	dereferences pointer to the managed object (public member function)

use_count	returns the number of `shared_ptr` objects referring to the same managed object (public member function)

unique	checks whether the managed object is managed only by the current `shared_ptr` instance (public member function)

operator bool	checks if there is associated managed object (public member function)

owner_before	provides owner-based ordering of shared pointers (public member function)

Non-member functions


make_shared	creates a shared pointer that manages a new object (function template)

allocate_shared	creates a shared pointer that manages a new object allocated using an allocator (function template)

static_pointer_cast dynamic_pointer_cast const_pointer_cast	applies `static_cast`, `dynamic_cast` or `const_cast` to the type of the managed object (function template)

get_deleter	returns the deleter of specified type, if owned (function template)

operator== operator!= operator< operator<= operator> operator>=	compares with another `shared_ptr` or with nullptr (function template)

operator<<	outputs the value of the managed pointer to an output stream (function template)

std::swap(std::shared_ptr) (C++11)	specializes the std::swap algorithm (function template)

specializes atomic operations
(function template)

Helper classes


std::hash<std::shared_ptr> (C++11)	hash support for std::shared_ptr (class template specialization)

Implementation notes

In a typical implementation, std::shared_ptr holds only two pointers:

a pointer to the referenced object
a pointer to control block

Where the control block is a dynamically-allocated object that holds:

a pointer to the managed object or the managed object itself
the deleter
the allocator
the number of shared_ptrs that own the managed object
the number of weak_ptrs that refer to the managed object

When shared_ptr is created by calling std::make_shared or std::allocate_shared, the control block holds the managed object directly, as a data member. When shared_ptr is created by calling a constructor, a pointer is stored.

The pointer held by the shared_ptr directly is the one returned by get(), while the pointer/object held by the control block is the one that will be deleted when the number of shared owners reaches zero: these pointers are not necessarily equal.

The destructor of shared_ptr decrements the number of shared owners of the control block, and if that reaches zero, the control block calls the destructor of the managed object, but the control block does not deallocate itself until thestd::weak_ptr counter reaches zero as well.

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