constexpr shared_ptr() noexcept; | (1) | |
constexpr shared_ptr( std::nullptr_t ) noexcept; | (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 ) noexcept; | (8) | |
template< class Y > shared_ptr( shared_ptr<Y>&& r, element_type* ptr ) noexcept; | (8) | (since C++20) |
shared_ptr( const shared_ptr& r ) noexcept; | (9) | |
template< class Y > shared_ptr( const shared_ptr<Y>& r ) noexcept; | (9) | |
shared_ptr( shared_ptr&& r ) noexcept; | (10) | |
template< class Y > shared_ptr( shared_ptr<Y>&& r ) noexcept; | (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) | (removed in 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. For (3-4,6), | (until C++17) |
If | (since C++17) |
delete ptr
if 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 copied from it must not throw exceptions.
| (until C++17) |
These constructors additionally do not participate in overload resolution if the expression | (since C++17) |
alloc
for allocation of data for internal use. Alloc
must be an Allocator.shared_ptr
which shares ownership information with the initial value of r
, but holds an unrelated and unmanaged pointer ptr
. If this shared_ptr
is the last of the group to go out of scope, it will call the stored deleter for the object originally managed by r
. However, calling get()
on this shared_ptr
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()
For the second overload taking an rvalue, r
is empty and r.get() == nullptr
after the call. (since C++20)
shared_ptr
which shares ownership of the object managed by r
. If r
manages no object, *this
manages no object either. 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 participates in overload resolution only 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, it is equivalent to shared_ptr(r.release(), std::ref(r.get_deleter())
. Otherwise, it is equivalent to shared_ptr(r.release(), std::move(r.get_deleter()))
.When T
is not an array type, 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()
.
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 |
std::bad_alloc
if required additional memory could not be obtained. May throw implementation-defined exception for other errors. If an exception occurs, this calls delete ptr
if T
is not an array type, and calls delete[] ptr
otherwise (since C++17).std::bad_alloc
if required additional memory could not be obtained. May throw implementation-defined exception for other errors. d(ptr)
is called if an exception occurs.std::bad_alloc
if required additional memory could not be obtained. May throw implementation-defined exception for other errors. This constructor has no effect if an exception occurs.A constructor enables shared_from_this
with a pointer ptr
of type U*
means that it determines if U
has an unambiguous and accessible (since C++17) 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::enable_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 std::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 std::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.
#include <iostream> #include <memory> struct Foo { int id{0}; Foo(int i = 0) : id{i} { std::cout << "Foo::Foo(" << i << ")\n"; } ~Foo() { std::cout << "Foo::~Foo(), id=" << id << '\n'; } }; struct D { void operator()(Foo* p) const { std::cout << "Call delete from function object. Foo::id=" << p->id << '\n'; delete p; } }; int main() { { std::cout << "1) constructor with no managed object\n"; std::shared_ptr<Foo> sh1; } { std::cout << "2) constructor with object\n"; std::shared_ptr<Foo> sh2(new Foo{10}); std::cout << "sh2.use_count(): " << sh2.use_count() << '\n'; std::shared_ptr<Foo> sh3(sh2); std::cout << "sh2.use_count(): " << sh2.use_count() << '\n'; std::cout << "sh3.use_count(): " << sh3.use_count() << '\n'; } { std::cout << "3) constructor with object and deleter\n"; std::shared_ptr<Foo> sh4(new Foo{11}, D()); std::shared_ptr<Foo> sh5(new Foo{12}, [](auto p) { std::cout << "Call delete from lambda... p->id=" << p->id << '\n'; delete p; }); } }
Output:
1) constructor with no managed object 2) constructor with object Foo::Foo(10) sh2.use_count(): 1 sh2.use_count(): 2 sh3.use_count(): 2 Foo::~Foo(), id=10 3) constructor with object and deleter Foo::Foo(11) Foo::Foo(12) Call delete from lambda... p->id=12 Foo::~Foo(), id=12 Call delete from function object. Foo::id=11 Foo::~Foo(), id=11
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
DR | Applied to | Behavior as published | Correct behavior |
---|---|---|---|
LWG 3548 | C++11 | the constructor from unique_ptr copy-constructed the deleter | move-constructs instead |
(C++20) | creates a shared pointer that manages a new object (function template) |
(C++20) | creates a shared pointer that manages a new object allocated using an allocator (function template) |
(C++11) | allows an object to create a shared_ptr referring to itself (class template) |
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