std::shared_ptr::shared_ptr(3)

NAME
std::shared_ptr::shared_ptr - std::shared_ptr::shared_ptr

Synopsis
   constexpr shared_ptr() noexcept; (1)
   constexpr shared_ptr( std::nullptr_t ) noexcept; (2)
   template< class Y > (3)
   explicit shared_ptr( Y* ptr );
   template< class Y, class Deleter > (4)
   shared_ptr( Y* ptr, Deleter d );
   template< class Deleter > (5)
   shared_ptr( std::nullptr_t ptr, Deleter d );
   template< class Y, class Deleter, class Alloc > (6)
   shared_ptr( Y* ptr, Deleter d, Alloc alloc );
   template< class Deleter, class Alloc > (7)
   shared_ptr( std::nullptr_t ptr, Deleter d, Alloc alloc );
   template< class Y >
   shared_ptr( const shared_ptr<Y>& r, element_type* ptr ) (8)
   noexcept;
   template< class Y > (8) (since C++20)
   shared_ptr( shared_ptr<Y>&& r, element_type* ptr ) noexcept;
   shared_ptr( const shared_ptr& r ) noexcept; (9)
   template< class Y > (9)
   shared_ptr( const shared_ptr<Y>& r ) noexcept;
   shared_ptr( shared_ptr&& r ) noexcept; (10)
   template< class Y > (10)
   shared_ptr( shared_ptr<Y>&& r ) noexcept;
   template< class Y > (11)
   explicit shared_ptr( const std::weak_ptr<Y>& r );
   template< class Y > (12) (removed in C++17)
   shared_ptr( std::auto_ptr<Y>&& r );
   template< class Y, class Deleter > (13)
   shared_ptr( std::unique_ptr<Y, Deleter>&& r );
   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 (since C++17)
   T* or Y is the array type U[N] and T is U cv [] (where cv is some set
   of cv-qualifiers).
   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.
   For (3,4,6), Y* must be convertible to T*. (until C++17)
   If T is an array type U[N], (3,4,6) do not participate in overload
   resolution if Y(*)[N] is an invalid type or not convertible to T*. If
   T is an array type U[], (3,4,6) do not participate in overload (since C++17)
   resolution if Y(*)[] is an invalid type or not convertible to T*.
   Otherwise, (3,4,6) do not participate in overload resolution if Y* is
   not convertible to T*.
   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 copied from it 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 (since C++17)
   std::is_move_constructible_v<D> is false.
   6,7) Same as (4,5), but additionally uses a copy of alloc for allocation of data for
   internal use. Alloc must be an Allocator.
   8) The aliasing constructor: constructs a 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)
   9) Constructs a 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*.
   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 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.
   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 (since C++17)
   r.get() is a null pointer, this overload is equivalent to the default
   constructor (1).
   If 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 (3,4,6) enable shared_from_this with ptr,
   and the overload (13) enables shared_from_this with the pointer returned by
   r.release().

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
   3) 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).
   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.
   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.

Notes
   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.

Example
// Run this code
 #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
   Defect reports
   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 move-constructs instead
                       copy-constructed the deleter

See also
   make_shared creates a shared pointer that manages a new object
   make_shared_for_overwrite (function template)
   (C++20)
   allocate_shared creates a shared pointer that manages a new object
   allocate_shared_for_overwrite allocated using an allocator
   (C++20) (function template)
   enable_shared_from_this allows an object to create a shared_ptr referring to
   (C++11) itself
                        (class template)