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# std::shared_ptr::shared_ptr

From cppreference.com
< cpp‎ | memory‎ | 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 `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())

### [edit] 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.

### [edit] 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

### [edit] 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.

### [edit] 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..

### [edit] See also

make_shared | creates a shared pointer that manages a new object (function template) [edit]
allocate_shared | creates a shared pointer that manages a new object allocated using an allocator (function template) [edit]

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