shared_ptr Class
The latest version of this topic can be found at 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 is 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 is not, the code will not compile. For example:
class F {};
#include <memory>
using namespace std;
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_ptrobject that owns that resource,if it was constructed from a weak_ptr Class 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_ptrobjects that own the resource,the number of
weak_ptrobjects 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 cannot be allocated) it evaluates the expression delete ptr.
ptr, dtor -- 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 cannot be allocated), it calls dtor(ptr), which must be well defined.
ptr, dtor, 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 dtor(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::shared_ptr | Constructs a shared_ptr. |
| shared_ptr::~shared_ptr | Destroys a shared_ptr. |
Methods
| shared_ptr::element_type | The type of an element. |
| shared_ptr::get | Gets address of owned resource. |
| shared_ptr::owner_before | Returns true if this shared_ptr is ordered before (or less than) the provided pointer. |
| shared_ptr::reset | Replace owned resource. |
| shared_ptr::swap | Swaps two shared_ptr objects. |
| shared_ptr::unique | Tests if owned resource is unique. |
| shared_ptr::use_count | Counts numbers of resource owners. |
Operators
| shared_ptr::operator boolean-type | Tests if an owned resource exists. |
| shared_ptr::operator* | Gets the designated value. |
| shared_ptr::operator= | Replaces the owned resource. |
| shared_ptr::operator-> | Gets a pointer to the designated value. |
Requirements
Header: <memory>
Namespace: std
shared_ptr::element_type
The type of an element.
typedef T element_type;
Remarks
The type is a synonym for the template parameter T.
Example
// std_tr1__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
shared_ptr::get
Gets address of owned resource.
T *get() const;
Remarks
The member function returns the address of the owned resource. If the object does not own a resource it returns 0.
Example
// std_tr1__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
shared_ptr::operator boolean-type
Tests if an owned resource exists.
operator boolean-type
() const;
Remarks
The operator returns a value of a type that is convertible to bool. The result of the conversion to bool is true when get() != 0, otherwise false.
Example
// std_tr1__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
shared_ptr::operator*
Gets the designated value.
T& operator*() const;
Remarks
The indirection operator returns *get(). Hence, the stored pointer must not be null.
Example
// std_tr1__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
shared_ptr::operator=
Replaces the owned resource.
shared_ptr& operator=(const shared_ptr& sp);
template <class Other>
shared_ptr& operator=(const shared_ptr<Other>& sp);
template <class Other>
shared_ptr& operator=(auto_ptr<Other>& ap);
template <class Other>
shared_ptr& operator=(auto_ptr<Other>& ap);
template <class Other>
shared_ptr& operator=(auto_ptr<Other>&& ap);
template <class Other, class Deletor>
shared_ptr& operator=(unique_ptr<Other, Deletor>&& ap);
Parameters
sp
The shared pointer to copy.
ap
The auto pointer 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_tr1__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::auto_ptr<int> ap(new int(10));
sp0 = sp1;
std::cout << "*sp0 == " << *sp0 << std::endl;
sp0 = ap;
std::cout << "*sp0 == " << *sp0 << std::endl;
return (0);
}
*sp0 == 5
*sp0 == 10
shared_ptr::operator->
Gets a pointer to the designated value.
T * operator->() const;
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_tr1__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
shared_ptr::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);
template <class Other>
bool owner_before(const weak_ptr<Other>& ptr);
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.
shared_ptr::reset
Replace owned resource.
void reset();
template <class Other>
void reset(Other *ptr;);
template <class Other, class D>
void reset(Other *ptr, D dtor);
template <class Other, class D, class A>
void reset(Other *ptr, D dtor, A alloc);
Parameters
Other
The type controlled by the argument pointer.
D
The type of the deleter.
ptr
The pointer to copy.
dtor
The deleter to copy.
A
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_tr1__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::shared_ptr
Constructs a shared_ptr.
shared_ptr();
shared_ptr(nullptr_t);
shared_ptr(const shared_ptr& sp);
shared_ptr(shared_ptr&& sp);
template <class Other>
explicit shared_ptr(Other* ptr);
template <class Other, class D>
shared_ptr(Other* ptr, D dtor);
template <class D>
shared_ptr(nullptr_t ptr, D dtor);
template <class Other, class D, class A>
shared_ptr(Other* ptr, D dtor, A alloc);
template <class D, class A>
shared_ptr(nullptr_t ptr, D dtor, A alloc);
template <class Other>
shared_ptr(const shared_ptr<Other>& sp);
template <class Other>
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 D>
shared_ptr(unique_ptr<Other, D>&& up);
template <class Other>
shared_ptr(const shared_ptr<Other>& sp, T* ptr);
template <class Other, class D>
shared_ptr(const unique_ptr<Other, D>& up) = delete;
Parameters
Other
The type controlled by the argument pointer.
ptr
The pointer to copy.
D
The type of the deleter.
A
The type of the allocator.
dtor
The deleter.
ator
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 Class if wp.expired().
Example
// std_tr1__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::~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_tr1__memory__shared_ptr_destroy.cpp
// compile with: /EHsc
#include <memory>
#include <iostream>
struct deleter
{
void operator()(int *p)
{
delete p;
}
};
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
shared_ptr::swap
Swaps two shared_ptr objects.
void swap(shared_ptr& sp);
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_tr1__memory__shared_ptr_swap.cpp
// compile with: /EHsc
#include <memory>
#include <iostream>
struct deleter
{
void operator()(int *p)
{
delete p;
}
};
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
shared_ptr::unique
Tests if owned resource is unique.
bool unique() const;
Remarks
The member function returns true if no other shared_ptr object owns the resource that is owned by *this, otherwise false.
Example
// std_tr1__memory__shared_ptr_unique.cpp
// compile with: /EHsc
#include <memory>
#include <iostream>
struct deleter
{
void operator()(int *p)
{
delete p;
}
};
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
shared_ptr::use_count
Counts numbers of resource owners.
long use_count() const;
Remarks
The member function returns the number of shared_ptr objects that own the resource that is owned by *this.
Example
// std_tr1__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