std::shared_ptr<T>::use_count

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< cpp‎ | memory‎ | shared ptr
 
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std::shared_ptr
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shared_ptr::use_count
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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, the value returned by use_count is approximate (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 哋它亢++17) member function unique() is provided for this use case.(until 哋它亢++20) In multithreaded environment, this 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.

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

(until 哋它亢++20)
 checks whether the managed object is managed only by the current shared_ptr object
(public member function)
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