std::shared_ptr::shared_ptr

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std::shared_ptr
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shared_ptr::shared_ptr
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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 Y* is said to be compatible with a pointer type T* if either Y* is convertible to T* or Y is the array type U[N] and T is U cv [] (where cv is some set of cv-qualifiers).

(since C++17)
1-2) Constructs a shared_ptr with no managed object, i.e. empty shared_ptr
3-7) Constructs a shared_ptr with ptr as the pointer to the managed object.

Y* must be convertible to T*.

(until C++17)

If T is an array type U[N], these constructors do not participate in overload resolution if Y(*)[N] is not convertible to T*. If T is an array type U[], these constructors do not participate in overload resolution if Y(*)[] is not convertible to T*. Otherwise, these constructors do not participate in overload resolution if Y* is not convertible to T*.

(since C++17)
Additionally:
3) Uses the delete-expression 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)
4-5) Uses the specified deleter 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.

Deleter must be CopyConstructible.

(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)
6-7) Same as (4-5), but additionally uses a copy of alloc for allocation of data for internal use. Alloc must be a Allocator.
8) The aliasing constructor: constructs a 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()
9) Constructs a 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*.
10) Move-constructs a 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*.
11) Constructs a 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.
12) Constructs a shared_ptr that stores and owns the object formerly owned by r. Y* must be convertible to T*. After construction, r is empty.
13) Constructs a 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)
If 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

1-2)
noexcept specification:  
noexcept
  
3) std::bad_alloc if required additional memory could not be obtained. May throw implementation-defined exception for other errors. delete ptr if T is not an array type, delete[] ptr otherwise) (since C++17) is called if an exception occurs.
4-7) 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.
8-10)
noexcept specification:  
noexcept
  
11) std::bad_weak_ptr if r.expired() == true. The constructor has no effect in this case.
12) 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.
13) If an exception is thrown, the constructor has no effects.

Example

Run this code
#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)
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