std::shared_ptr::shared_ptr
constexpr shared_ptr(); |
(1) | |
constexpr shared_ptr( std::nullptr_t ); |
(2) | |
template< class Y > explicit shared_ptr( Y* ptr ); |
(3) | |
template< class Y, class Deleter > shared_ptr( Y* ptr, Deleter d ); |
(4) | |
template< class Deleter > shared_ptr( std::nullptr_t ptr, Deleter d ); |
(5) | |
template< class Y, class Deleter, class Alloc > shared_ptr( Y* ptr, Deleter d, Alloc alloc ); |
(6) | |
template< class Deleter, class Alloc > shared_ptr( std::nullptr_t ptr, Deleter d, Alloc alloc ); |
(7) | |
template< class Y > shared_ptr( const shared_ptr<Y>& r, element_type *ptr ); |
(8) | |
shared_ptr( const shared_ptr& r ); |
(9) | |
template< class Y > shared_ptr( const shared_ptr<Y>& r ); |
(9) | |
shared_ptr( shared_ptr&& r ); |
(10) | |
template< class Y > shared_ptr( shared_ptr<Y>&& r ); |
(10) | |
template< class Y > explicit shared_ptr( const std::weak_ptr<Y>& r ); |
(11) | |
template< class Y > shared_ptr( std::auto_ptr<Y>&& r ); |
(12) | (until C++17) |
template< class Y, class Deleter > shared_ptr( std::unique_ptr<Y,Deleter>&& r ); |
(13) | |
Constructs new shared_ptr
from a variety of pointer types that refer to an object to manage.
For the purposes of the description below, a pointer type |
(since C++17) |
shared_ptr
with no managed object, i.e. empty shared_ptr
shared_ptr
with ptr
as the pointer to the managed object.
|
(until C++17) |
If |
(since C++17) |
T
is not an array type; delete[] ptr if T
is an array type (since C++17) as the deleter. Y
must be a complete type. The delete expression must be well-formed, have well-defined behavior and not throw any exceptions. This constructor additionally does not participate in overload resolution if the delete expression is not well-formed. (since C++17)d
as the deleter. The expression d(ptr) must be well formed, have well-defined behavior and not throw any exceptions. The construction of d
and of the stored deleter from d
must not throw exceptions.
|
(until C++17) |
These constructors additionally do not participate in overload resolution if the expression d(ptr) is not well-formed, or if std::is_move_constructible<D>::value is false. |
(since C++17) |
alloc
for allocation of data for internal use. Alloc
must be a Allocator
.shared_ptr
which shares ownership information with r
, but holds an unrelated and unmanaged pointer ptr
. Even if this shared_ptr
is the last of the group to go out of scope, it will call the destructor for the object originally managed by r
. However, calling get()
on this will always return a copy of ptr
. It is the responsibility of the programmer to make sure that this ptr
remains valid as long as this shared_ptr exists, such as in the typical use cases where ptr
is a member of the object managed by r
or is an alias (e.g., downcast) of r.get()
shared_ptr
which shares ownership of the object managed by r
. If r
manages no object, *this
manages no object too. The template overload doesn't participate in overload resolution if Y*
is not implicitly convertible to (until C++17)compatible with (since C++17) T*
.shared_ptr
from r
. After the construction, *this contains a copy of the previous state of r
, r
is empty and its stored pointer is null. The template overload doesn't participate in overload resolution if Y*
is not implicitly convertible to (until C++17)compatible with (since C++17) T*
.shared_ptr
which shares ownership of the object managed by r
. Y*
must be implicitly convertible to T*
. (until C++17)This overload only participates in overload resolution if Y*
is compatible with T*
. (since C++17) Note that r.lock() may be used for the same purpose: the difference is that this constructor throws an exception if the argument is empty, while std::weak_ptr<T>::lock() constructs an empty std::shared_ptr
in that case.shared_ptr
that stores and owns the object formerly owned by r
. Y*
must be convertible to T*
. After construction, r
is empty.shared_ptr
which manages the object currently managed by r
. The deleter associated with r
is stored for future deletion of the managed object. r
manages no object after the call. This overload doesn't participate in overload resolution if std::unique_ptr<Y, Deleter>::pointer is not compatible with T* .
If r.get() is a null pointer, this overload is equivalent to the default constructor (1). |
(since C++17) |
Deleter
is a reference type, equivalent to shared_ptr(r.release(), std::ref(r.get_deleter()). Otherwise, equivalent to shared_ptr(r.release(), r.get_deleter()) Notes
The overloads (3), (4), and (6) enable shared_from_this with ptr
, and the overload (13) enables shared_from_this with the pointer returned by r.release().
A constructor enables shared_from_this
with a pointer ptr
of type U*
means that it determines if U
has an unambiguous and accessible base class that is a specialization of std::enable_shared_from_this, and if so, the constructor evaluates the statement:
if (ptr != nullptr && ptr->weak_this.expired()) ptr->weak_this = std::shared_ptr<std::remove_cv_t<U>>(*this, const_cast<std::remove_cv_t<U>*>(ptr));
Where weak_this
is the hidden mutable std::weak_ptr
member of std::shared_from_this. The assignment to the weak_this member is not atomic and conflicts with any potentially concurrent access to the same object. This ensures that future calls to shared_from_this() would share ownership with the shared_ptr
created by this raw pointer constructor.
The test ptr->weak_this.expired()
in the exposition code above makes sure that weak_this is not reassigned if it already indicates an owner. This test is required as of C++17.
The raw pointer overloads assume ownership of the pointed-to object. Therefore, constructing a shared_ptr
using the raw pointer overload for an object that is already managed by a shared_ptr
, such as by shared_ptr(ptr.get()) is likely to lead to undefined behavior, even if the object is of a type derived from std::enable_shared_from_this.
Because the default constructor is constexpr
, static shared_ptrs are initialized as part of static non-local initialization, before any dynamic non-local initialization begins. This makes it safe to use a shared_ptr in a constructor of any static object.
In C++11 and C++14 it is valid to construct a std::shared_ptr<T> from a std::unique_ptr<T[]>:
std::unique_ptr<int[]> arr(new int[1]); std::shared_ptr<int> ptr(std::move(arr));
Since the shared_ptr
obtains its deleter (a std::default_delete<T[]> object) from the unique_ptr
, the array will be correctly deallocated.
This is no longer allowed in C++17. Instead the array form std::shared_ptr<T[]> should be used.
Parameters
ptr | - | a pointer to an object to manage |
d | - | a deleter to use to destroy the object |
alloc | - | an allocator to use for allocations of data for internal use |
r | - | another smart pointer to share the ownership to or acquire the ownership from |
Exceptions
T
is not an array type, delete[] ptr otherwise) (since C++17) is called if an exception occurs.Example
#include <memory> #include <iostream> struct Foo { Foo() { std::cout << "Foo...\n"; } ~Foo() { std::cout << "~Foo...\n"; } }; struct D { void operator()(Foo* p) const { std::cout << "Call delete from function object...\n"; delete p; } }; int main() { { std::cout << "constructor with no managed object\n"; std::shared_ptr<Foo> sh1; } { std::cout << "constructor with object\n"; std::shared_ptr<Foo> sh2(new Foo); std::shared_ptr<Foo> sh3(sh2); std::cout << sh2.use_count() << '\n'; std::cout << sh3.use_count() << '\n'; } { std::cout << "constructor with object and deleter\n"; std::shared_ptr<Foo> sh4(new Foo, D()); std::shared_ptr<Foo> sh5(new Foo, [](auto p) { std::cout << "Call delete from lambda...\n"; delete p; }); } }
Output:
constructor with no managed object constructor with object Foo... 2 2 ~Foo... constructor with object and deleter Foo... Foo... Call delete from lambda... ~Foo... Call delete from function object... ~Foo..
See also
creates a shared pointer that manages a new object (function template) | |
creates a shared pointer that manages a new object allocated using an allocator (function template) |