std::shared_ptr<T>::use_count

 

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std::shared_ptr

Member functions

shared_ptr::shared_ptr

shared_ptr::~shared_ptr

shared_ptr::operator=

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shared_ptr::reset

shared_ptr::swap

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shared_ptr::get

shared_ptr::operator*shared_ptr::operator->

shared_ptr::operator[] (C++17)

shared_ptr::use_count

shared_ptr::unique (until C++20*)

shared_ptr::operator bool

shared_ptr::owner_before

shared_ptr::owner_hash (C++26)

shared_ptr::owner_equal (C++26)

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swap(std::shared_ptr)

make_sharedmake_shared_for_overwrite (C++20)

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operator<<

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atomic<std::shared_ptr> (C++20)

hash<std::shared_ptr>

Deduction guides(C++17)

 

long use_count() const noexcept;

Returns the number of different shared_ptr instances (*this included) managing the current object. If there is no managed object, ​0​ is returned.

In multithreaded environment, use_count atomically retrieves the number of instances (typical implementations use a memory_order_relaxed load).

Parameters

(none)

Return value

The number of std::shared_ptr instances managing the current object or ​0​ if there is no managed object.

Notes

Common use cases include

• comparison with ​0​. If use_count returns zero, the shared pointer is empty and manages no objects (whether or not its stored pointer is nullptr).
• comparison with 1. If use_count returns 1, there are no other owners. The deprecated(since C++17) member function unique() is provided for this use case.(until C++20)

In multithreaded environment

The value returned by use_count should be considered approximate, as the number of shared owners might change in other threads between the atomic retrieval and meaningful use of the value. When use_count returns 1, it does not imply that the object is safe to modify because accesses to the managed object by former shared owners may not have completed, and because new shared owners may be introduced concurrently, such as by std::weak_ptr::lock. Only when use_count returns 0 is the count accurate.

Example

Run this code

#include <iostream>
#include <memory>
 
void fun(std::shared_ptr<int> sp)
{
    std::cout << "in fun(): sp.use_count() == " << sp.use_count()
              << " (object @ " << sp << ")\n";
}
 
int main()
{
    auto sp1 = std::make_shared<int>(5);
    std::cout << "in main(): sp1.use_count() == " << sp1.use_count()
              << " (object @ " << sp1 << ")\n";
 
    fun(sp1);
}

Possible output:

in main(): sp1.use_count() == 1 (object @ 0x20eec30)
in fun(): sp.use_count() == 2 (object @ 0x20eec30)

See also

unique (until C++20)

checks whether the managed object is managed only by the current shared_ptr object (public member function)