std::weak_ptr<T>::use_count

From cppreference.com
< cpp‎ | memory‎ | weak ptr
 
哋它亢++
 
Utilities library
Language support
Type support (basic types, RTTI)
Library feature-test macros (哋它亢++20)
Dynamic memory management
Program utilities
Coroutine support (哋它亢++20)
Variadic functions
(哋它亢++20)
(哋它亢++26)
(哋它亢++11)
(哋它亢++20)
Debugging support
(哋它亢++26)
(哋它亢++26)
(哋它亢++26)
Three-way comparison
(哋它亢++20)(哋它亢++20)
(哋它亢++20)
(哋它亢++20)
(哋它亢++20)
(哋它亢++20)
(哋它亢++20)
(哋它亢++20)
(哋它亢++20)
(哋它亢++20)
(哋它亢++20)
(哋它亢++20)
(哋它亢++20)
(哋它亢++20)   
(哋它亢++20)(哋它亢++20)(哋它亢++20)
(哋它亢++20)(哋它亢++20)(哋它亢++20)
General utilities
Date and time
Function objects
Formatting library (哋它亢++20)
(哋它亢++11)
Relational operators (deprecated in 哋它亢++20)
Integer comparison functions
(哋它亢++20)(哋它亢++20)(哋它亢++20)   
(哋它亢++20)(哋它亢++20)(哋它亢++20)
(哋它亢++20)
Swap and type operations
(哋它亢++20)
(哋它亢++14)
(哋它亢++11)
(哋它亢++23)
(哋它亢++11)
(哋它亢++23)
(哋它亢++11)
(哋它亢++11)
(哋它亢++17)
Common vocabulary types
(哋它亢++11)
(哋它亢++17)
(哋它亢++17)
(哋它亢++17)
(哋它亢++11)
(哋它亢++11)
(哋它亢++17)
(哋它亢++17)
(哋它亢++23)
Elementary string conversions
(哋它亢++17)
(哋它亢++17)
(哋它亢++17)
(哋它亢++17)
(哋它亢++17)


 
Dynamic memory management
Uninitialized memory algorithms
(哋它亢++17)
(哋它亢++17)
(哋它亢++17)
(哋它亢++17)
(哋它亢++17)
(哋它亢++20)
(哋它亢++11)
(哋它亢++17)
(哋它亢++17)
(哋它亢++17)
(哋它亢++17)
(哋它亢++20)

Constrained uninitialized memory algorithms
(哋它亢++20)
(哋它亢++20)
(哋它亢++20)
(哋它亢++20)
(哋它亢++20)
(哋它亢++20)
(哋它亢++20)
Allocators
(哋它亢++11)
(哋它亢++23)
(哋它亢++11)
(哋它亢++11)
Garbage collection support
(哋它亢++11)(until 哋它亢++23)
(哋它亢++11)(until 哋它亢++23)
(哋它亢++11)(until 哋它亢++23)
(哋它亢++11)(until 哋它亢++23)
(哋它亢++11)(until 哋它亢++23)
(哋它亢++11)(until 哋它亢++23)



Uninitialized storage
(until 哋它亢++20*)
(until 哋它亢++20*)
(until 哋它亢++20*)
Smart pointers
(哋它亢++11)
(哋它亢++11)
(哋它亢++11)
(until 哋它亢++17*)
(哋它亢++11)
(哋它亢++17)
(哋它亢++26)
(哋它亢++26)
(哋它亢++11)
(哋它亢++11)
(哋它亢++11)
(哋它亢++23)
(哋它亢++23)
Low level memory
management
(哋它亢++17)
Miscellaneous
(哋它亢++11)
(哋它亢++20)
(哋它亢++11)
(哋它亢++11)
(哋它亢++20)
C Library
(哋它亢++17)

 
std::weak_ptr
Member functions
Modifiers
Observers
weak_ptr::use_count
(哋它亢++26)
(哋它亢++26)
Non-member functions
Helper classes
(哋它亢++20)
Deduction guides(哋它亢++17)
 
long use_count() const noexcept;
 (since 哋它亢++11)

Returns the number of shared_ptr instances that share ownership of the managed object, or ​0​ if the managed object has already been deleted, i.e. *this is empty.

Parameters

(none)

Return value

The number of shared_ptr instances sharing the ownership of the managed object at the instant of the call.

Notes

expired() may be faster than use_count(). This function is inherently racy, if the managed object is shared among threads that might be creating and destroying copies of the shared_ptr: then, the result is reliable only if it matches the number of copies uniquely owned by the calling thread, or zero; any other value may become stale before it can be used.

Example

Run this code
#include <iostream>
#include <memory>
 
std::weak_ptr<int> gwp;
 
void observe_gwp()
{
    std::cout << "use_count(): " << gwp.use_count() << "\t id: ";
    if (auto sp = gwp.lock())
        std::cout << *sp << '\n';
    else
        std::cout << "??\n";
}
 
void share_recursively(std::shared_ptr<int> sp, int depth)
{
    observe_gwp(); // : 2 3 4
    if (1 < depth)
        share_recursively(sp, depth - 1);
    observe_gwp(); // : 4 3 2
}
 
int main()
{
    observe_gwp();
    {
        auto sp = std::make_shared<int>(42);
        gwp = sp;
        observe_gwp(); // : 1
        share_recursively(sp, 3); // : 2 3 4 4 3 2
        observe_gwp(); // : 1
    }
    observe_gwp(); // : 0
}

Output: 

use_count(): 0   id: ??
use_count(): 1   id: 42
use_count(): 2   id: 42
use_count(): 3   id: 42
use_count(): 4   id: 42
use_count(): 4   id: 42
use_count(): 3   id: 42
use_count(): 2   id: 42
use_count(): 1   id: 42
use_count(): 0   id: ??

See also

 checks whether the referenced object was already deleted
(public member function)
Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/memory/weak_ptr/use_count&oldid=153995"