shared_ptr class

Wraps a reference-counted smart pointer around a dynamically allocated object.

Syntax

template <class T>
class shared_ptr;

Remarks

The shared_ptr class describes an object that uses reference counting to manage resources. A shared_ptr object effectively holds a pointer to the resource that it owns or holds a null pointer. A resource can be owned by more than one shared_ptr object; when the last shared_ptr object that owns a particular resource is destroyed, the resource is freed.

A shared_ptr stops owning a resource when it's reassigned or reset.

The template argument T might be an incomplete type except as noted for certain member functions.

When a shared_ptr<T> object is constructed from a resource pointer of type G* or from a shared_ptr<G>, the pointer type G* must be convertible to T*. If it's not convertible, the code will not compile. For example:

#include <memory>
using namespace std;

class F {};
class G : public F {};

shared_ptr<G> sp0(new G);   // okay, template parameter G and argument G*
shared_ptr<G> sp1(sp0);     // okay, template parameter G and argument shared_ptr<G>
shared_ptr<F> sp2(new G);   // okay, G* convertible to F*
shared_ptr<F> sp3(sp0);     // okay, template parameter F and argument shared_ptr<G>
shared_ptr<F> sp4(sp2);     // okay, template parameter F and argument shared_ptr<F>
shared_ptr<int> sp5(new G); // error, G* not convertible to int*
shared_ptr<int> sp6(sp2);   // error, template parameter int and argument shared_ptr<F>

A shared_ptr object owns a resource:

  • if it was constructed with a pointer to that resource,

  • if it was constructed from a shared_ptr object that owns that resource,

  • if it was constructed from a weak_ptr object that points to that resource, or

  • if ownership of that resource was assigned to it, either with shared_ptr::operator= or by calling the member function shared_ptr::reset.

The shared_ptr objects that own a resource share a control block. The control block holds:

  • the number of shared_ptr objects that own the resource,

  • the number of weak_ptr objects that point to the resource,

  • the deleter for that resource if it has one,

  • the custom allocator for the control block if it has one.

A shared_ptr object that is initialized by using a null pointer has a control block and is not empty. After a shared_ptr object releases a resource, it no longer owns that resource. After a weak_ptr object releases a resource, it no longer points to that resource.

When the number of shared_ptr objects that own a resource becomes zero, the resource is freed, either by deleting it or by passing its address to a deleter, depending on how ownership of the resource was originally created. When the number of shared_ptr objects that own a resource is zero, and the number of weak_ptr objects that point to that resource is zero, the control block is freed, using the custom allocator for the control block if it has one.

An empty shared_ptr object does not own any resources and has no control block.

A deleter is a function object that has a member function operator(). Its type must be copy constructible, and its copy constructor and destructor must not throw exceptions. It accepts one parameter, the object to be deleted.

Some functions take an argument list that defines properties of the resulting shared_ptr<T> or weak_ptr<T> object. You can specify such an argument list in several ways:

no arguments -- the resulting object is an empty shared_ptr object or an empty weak_ptr object.

ptr -- a pointer of type Other* to the resource to be managed. T must be a complete type. If the function fails (because the control block can't be allocated), it evaluates the expression delete ptr.

ptr, deleter -- a pointer of type Other* to the resource to be managed and a deleter for that resource. If the function fails (because the control block can't be allocated), it calls deleter(ptr), which must be well-defined.

ptr, deleter, alloc -- a pointer of type Other* to the resource to be managed, a deleter for that resource, and an allocator to manage any storage that must be allocated and freed. If the function fails (because the control block can't be allocated), it calls deleter(ptr), which must be well-defined.

sp -- a shared_ptr<Other> object that owns the resource to be managed.

wp -- a weak_ptr<Other> object that points to the resource to be managed.

ap -- an auto_ptr<Other> object that holds a pointer to the resource to be managed. If the function succeeds, it calls ap.release(); otherwise it leaves ap unchanged.

In all cases, the pointer type Other* must be convertible to T*.

Thread Safety

Multiple threads can read and write different shared_ptr objects at the same time, even when the objects are copies that share ownership.

Members

Constructors
shared_ptr Constructs a shared_ptr.
~shared_ptr Destroys a shared_ptr.
Typedefs
element_type The type of an element.
weak_type The type of a weak pointer to an element.
Member functions
get Gets address of owned resource.
owner_before Returns true if this shared_ptr is ordered before (or less than) the provided pointer.
reset Replace owned resource.
swap Swaps two shared_ptr objects.
unique Tests if owned resource is unique.
use_count Counts numbers of resource owners.
Operators
operator bool Tests if an owned resource exists.
operator* Gets the designated value.
operator= Replaces the owned resource.
operator-> Gets a pointer to the designated value.

element_type

The type of an element.

typedef T element_type;                  // before C++17
using element_type = remove_extent_t<T>; // C++17

Remarks

The element_type type is a synonym for the template parameter T.

Example

// std__memory__shared_ptr_element_type.cpp
// compile with: /EHsc
#include <memory>
#include <iostream>

int main()
{
    std::shared_ptr<int> sp0(new int(5));
    std::shared_ptr<int>::element_type val = *sp0;

    std::cout << "*sp0 == " << val << std::endl;

    return (0);
}
*sp0 == 5

get

Gets address of owned resource.

element_type* get() const noexcept;

Remarks

The member function returns the address of the owned resource. If the object does not own a resource, it returns 0.

Example

// std__memory__shared_ptr_get.cpp
// compile with: /EHsc
#include <memory>
#include <iostream>

int main()
{
    std::shared_ptr<int> sp0;
    std::shared_ptr<int> sp1(new int(5));

    std::cout << "sp0.get() == 0 == " << std::boolalpha
        << (sp0.get() == 0) << std::endl;
    std::cout << "*sp1.get() == " << *sp1.get() << std::endl;

    return (0);
}
sp0.get() == 0 == true
*sp1.get() == 5

operator bool

Tests if an owned resource exists.

explicit operator bool() const noexcept;

Remarks

The operator returns a value of true when get() != nullptr, otherwise false.

Example

// std__memory__shared_ptr_operator_bool.cpp
// compile with: /EHsc
#include <memory>
#include <iostream>

int main()
{
    std::shared_ptr<int> sp0;
    std::shared_ptr<int> sp1(new int(5));

    std::cout << "(bool)sp0 == " << std::boolalpha
        << (bool)sp0 << std::endl;
    std::cout << "(bool)sp1 == " << std::boolalpha
        << (bool)sp1 << std::endl;

    return (0);
}
(bool)sp0 == false
(bool)sp1 == true

operator*

Gets the designated value.

T& operator*() const noexcept;

Remarks

The indirection operator returns *get(). Hence, the stored pointer must not be null.

Example

// std__memory__shared_ptr_operator_st.cpp
// compile with: /EHsc
#include <memory>
#include <iostream>

int main()
{
    std::shared_ptr<int> sp0(new int(5));

    std::cout << "*sp0 == " << *sp0 << std::endl;

    return (0);
}
*sp0 == 5

operator=

Replaces the owned resource.

shared_ptr& operator=(const shared_ptr& sp) noexcept;

shared_ptr& operator=(shared_ptr&& sp) noexcept;

template <class Other>
shared_ptr& operator=(const shared_ptr<Other>& sp) noexcept;

template <class Other>
shared_ptr& operator=(shared_ptr<Other>&& sp) noexcept;

template <class Other>
shared_ptr& operator=(auto_ptr<Other>&& ap);    // deprecated in C++11, removed in C++17

template <class Other, class Deleter>
shared_ptr& operator=(unique_ptr<Other, Deleter>&& up);

Parameters

sp
The shared pointer to copy or move from.

ap
The auto pointer to move. The auto_ptr overload is deprecated in C++11 and removed in C++17.

up
The unique pointer to the object to adopt ownership of. up owns no object after the call.

Other
The type of the object pointed to by sp, ap, or up.

Deleter
The type of the deleter of the owned object, stored for later deletion of the object.

Remarks

The operators all decrement the reference count for the resource currently owned by *this and assign ownership of the resource named by the operand sequence to *this. If the reference count falls to zero, the resource is released. If an operator fails, it leaves *this unchanged.

Example

// std__memory__shared_ptr_operator_as.cpp
// compile with: /EHsc
#include <memory>
#include <iostream>

int main()
{
    std::shared_ptr<int> sp0;
    std::shared_ptr<int> sp1(new int(5));
    std::unique_ptr<int> up(new int(10));

    sp0 = sp1;
    std::cout << "*sp0 == " << *sp0 << std::endl;

    sp0 = up;
    std::cout << "*sp0 == " << *sp0 << std::endl;

    return (0);
}
*sp0 == 5
*sp0 == 10

operator->

Gets a pointer to the designated value.

T* operator->() const noexcept;

Remarks

The selection operator returns get(), so that the expression sp->member behaves the same as (sp.get())->member where sp is an object of class shared_ptr<T>. Hence, the stored pointer must not be null, and T must be a class, structure, or union type with a member member.

Example

// std__memory__shared_ptr_operator_ar.cpp
// compile with: /EHsc
#include <memory>
#include <iostream>

typedef std::pair<int, int> Mypair;
int main()
{
    std::shared_ptr<Mypair> sp0(new Mypair(1, 2));

    std::cout << "sp0->first == " << sp0->first << std::endl;
    std::cout << "sp0->second == " << sp0->second << std::endl;

    return (0);
}
sp0->first == 1
sp0->second == 2

owner_before

Returns true if this shared_ptr is ordered before (or less than) the provided pointer.

template <class Other>
bool owner_before(const shared_ptr<Other>& ptr) const noexcept;

template <class Other>
bool owner_before(const weak_ptr<Other>& ptr) const noexcept;

Parameters

ptr
An lvalue reference to either a shared_ptr or a weak_ptr.

Remarks

The template member function returns true if *this is ordered before ptr.

reset

Replace owned resource.

void reset() noexcept;

template <class Other>
void reset(Other *ptr);

template <class Other, class Deleter>
void reset(
    Other *ptr,
    Deleter deleter);

template <class Other, class Deleter, class Allocator>
void reset(
    Other *ptr,
    Deleter deleter,
    Allocator alloc);

Parameters

Other
The type controlled by the argument pointer.

Deleter
The type of the deleter.

ptr
The pointer to copy.

deleter
The deleter to copy.

Allocator
The type of the allocator.

alloc
The allocator to copy.

Remarks

The operators all decrement the reference count for the resource currently owned by *this and assign ownership of the resource named by the operand sequence to *this. If the reference count falls to zero, the resource is released. If an operator fails, it leaves *this unchanged.

Example

// std__memory__shared_ptr_reset.cpp
// compile with: /EHsc
#include <memory>
#include <iostream>

struct deleter
{
    void operator()(int *p)
    {
        delete p;
    }
};

int main()
{
    std::shared_ptr<int> sp(new int(5));

    std::cout << "*sp == " << std::boolalpha
        << *sp << std::endl;

    sp.reset();
    std::cout << "(bool)sp == " << std::boolalpha
        << (bool)sp << std::endl;

    sp.reset(new int(10));
    std::cout << "*sp == " << std::boolalpha
        << *sp << std::endl;

    sp.reset(new int(15), deleter());
    std::cout << "*sp == " << std::boolalpha
        << *sp << std::endl;

    return (0);
}
*sp == 5
(bool)sp == false
*sp == 10
*sp == 15

shared_ptr

Constructs a shared_ptr.

constexpr shared_ptr() noexcept;

constexpr shared_ptr(nullptr_t) noexcept : shared_ptr() {}

shared_ptr(const shared_ptr& sp) noexcept;

shared_ptr(shared_ptr&& sp) noexcept;

template <class Other>
explicit shared_ptr(Other* ptr);

template <class Other, class Deleter>
shared_ptr(
    Other* ptr,
    Deleter deleter);

template <class Deleter>
shared_ptr(
    nullptr_t ptr,
    Deleter deleter);

template <class Other, class Deleter, class Allocator>
shared_ptr(
    Other* ptr,
    Deleter deleter,
    Allocator alloc);

template <class Deleter, class Allocator>
shared_ptr(
    nullptr_t ptr,
    Deleter deleter,
    Allocator alloc);

template <class Other>
shared_ptr(
    const shared_ptr<Other>& sp) noexcept;

template <class Other>
explicit shared_ptr(
    const weak_ptr<Other>& wp);

template <class &>
shared_ptr(
    std::auto_ptr<Other>& ap);

template <class &>
shared_ptr(
    std::auto_ptr<Other>&& ap);

template <class Other, class Deleter>
shared_ptr(
    unique_ptr<Other, Deleter>&& up);

template <class Other>
shared_ptr(
    const shared_ptr<Other>& sp,
    element_type* ptr) noexcept;

template <class Other>
shared_ptr(
    shared_ptr<Other>&& sp,
    element_type* ptr) noexcept;

template <class Other, class Deleter>
shared_ptr(
    const unique_ptr<Other, Deleter>& up) = delete;

Parameters

Other
The type controlled by the argument pointer.

ptr
The pointer to copy.

Deleter
The type of the deleter.

Allocator
The type of the allocator.

deleter
The deleter.

alloc
The allocator.

sp
The smart pointer to copy.

wp
The weak pointer.

ap
The auto pointer to copy.

Remarks

The constructors each construct an object that owns the resource named by the operand sequence. The constructor shared_ptr(const weak_ptr<Other>& wp) throws an exception object of type bad_weak_ptr if wp.expired().

Example

// std__memory__shared_ptr_construct.cpp
// compile with: /EHsc
#include <memory>
#include <iostream>

struct deleter
{
    void operator()(int *p)
    {
        delete p;
    }
};

int main()
{
    std::shared_ptr<int> sp0;
    std::cout << "(bool)sp0 == " << std::boolalpha
        << (bool)sp0 << std::endl;

    std::shared_ptr<int> sp1(new int(5));
    std::cout << "*sp1 == " << *sp1 << std::endl;

    std::shared_ptr<int> sp2(new int(10), deleter());
    std::cout << "*sp2 == " << *sp2 << std::endl;

    std::shared_ptr<int> sp3(sp2);
    std::cout << "*sp3 == " << *sp3 << std::endl;

    std::weak_ptr<int> wp(sp3);
    std::shared_ptr<int> sp4(wp);
    std::cout << "*sp4 == " << *sp4 << std::endl;

    std::auto_ptr<int> ap(new int(15));
    std::shared_ptr<int> sp5(ap);
    std::cout << "*sp5 == " << *sp5 << std::endl;

    return (0);
}
(bool)sp0 == false
*sp1 == 5
*sp2 == 10
*sp3 == 10
*sp4 == 10
*sp5 == 15

~shared_ptr

Destroys a shared_ptr.

~shared_ptr();

Remarks

The destructor decrements the reference count for the resource currently owned by *this. If the reference count falls to zero, the resource is released.

Example

// std__memory__shared_ptr_destroy.cpp
// compile with: /EHsc
#include <memory>
#include <iostream>

int main()
{
    std::shared_ptr<int> sp1(new int(5));
    std::cout << "*sp1 == " << *sp1 << std::endl;
    std::cout << "use count == " << sp1.use_count() << std::endl;

    {
        std::shared_ptr<int> sp2(sp1);
        std::cout << "*sp2 == " << *sp2 << std::endl;
        std::cout << "use count == " << sp1.use_count() << std::endl;
    }

    // check use count after sp2 is destroyed
    std::cout << "use count == " << sp1.use_count() << std::endl;

    return (0);
}
*sp1 == 5
use count == 1
*sp2 == 5
use count == 2
use count == 1

swap

Swaps two shared_ptr objects.

void swap(shared_ptr& sp) noexcept;

Parameters

sp
The shared pointer to swap with.

Remarks

The member function leaves the resource originally owned by *this subsequently owned by sp, and the resource originally owned by sp subsequently owned by *this. The function does not change the reference counts for the two resources and it does not throw any exceptions.

Example

// std__memory__shared_ptr_swap.cpp
// compile with: /EHsc
#include <memory>
#include <iostream>

int main()
{
    std::shared_ptr<int> sp1(new int(5));
    std::shared_ptr<int> sp2(new int(10));
    std::cout << "*sp1 == " << *sp1 << std::endl;

    sp1.swap(sp2);
    std::cout << "*sp1 == " << *sp1 << std::endl;

    swap(sp1, sp2);
    std::cout << "*sp1 == " << *sp1 << std::endl;
    std::cout << std::endl;

    std::weak_ptr<int> wp1(sp1);
    std::weak_ptr<int> wp2(sp2);
    std::cout << "*wp1 == " << *wp1.lock() << std::endl;

    wp1.swap(wp2);
    std::cout << "*wp1 == " << *wp1.lock() << std::endl;

    swap(wp1, wp2);
    std::cout << "*wp1 == " << *wp1.lock() << std::endl;

    return (0);
}
*sp1 == 5
*sp1 == 10
*sp1 == 5
*wp1 == 5
*wp1 == 10
*wp1 == 5

unique

Tests if owned resource is unique. This function was deprecated in C++17, and removed in C++20.

bool unique() const noexcept;

Remarks

The member function returns true if no other shared_ptr object owns the resource that is owned by *this, otherwise false.

Example

// std__memory__shared_ptr_unique.cpp
// compile with: /EHsc
#include <memory>
#include <iostream>

int main()
{
    std::shared_ptr<int> sp1(new int(5));
    std::cout << "sp1.unique() == " << std::boolalpha
        << sp1.unique() << std::endl;

    std::shared_ptr<int> sp2(sp1);
    std::cout << "sp1.unique() == " << std::boolalpha
        << sp1.unique() << std::endl;

    return (0);
}
sp1.unique() == true
sp1.unique() == false

use_count

Counts numbers of resource owners.

long use_count() const noexcept;

Remarks

The member function returns the number of shared_ptr objects that own the resource that is owned by *this.

Example

// std__memory__shared_ptr_use_count.cpp
// compile with: /EHsc
#include <memory>
#include <iostream>

int main()
{
    std::shared_ptr<int> sp1(new int(5));
    std::cout << "sp1.use_count() == "
        << sp1.use_count() << std::endl;

    std::shared_ptr<int> sp2(sp1);
    std::cout << "sp1.use_count() == "
        << sp1.use_count() << std::endl;

    return (0);
}
sp1.use_count() == 1
sp1.use_count() == 2

weak_type

The type of a weak pointer to an element.

using weak_type = weak_ptr<T>; // C++17

Remarks

The weak_type definition was added in C++17.

See also

Header Files Reference
<memory>
unique_ptr
weak_ptr class