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std::shared_ptr::use_count

From cppreference.com
< cpp‎ | memory‎ | shared ptr
 
 
 
Dynamic memory management
Uninitialized storage
(C++17)
(deprecated since c++17)
(deprecated since c++17)
(deprecated since c++17)
Garbage collection support
Miscellaneous
(C++11)
(C++11)
C Library
Low level memory management
 
 
long use_count() const;

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)

Contents

[edit] Parameters

(none)

[edit] Return value

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

[edit] Exceptions

noexcept specification:  
noexcept
  

[edit] 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 null). However, this is not guaranteed in multithreaded environment.
  • comparison with 1. If use_count returns 1, there are no other owners, which may indicate that the managed object is safe to modify. The member function unique() is provided for this use case. However, this is not guaranteed in multithreaded environment.

[edit] Example

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

Output:

sp1.use_count() == 1
fun: sp.use_count() == 2

[edit] See also

(deprecated)
checks whether the managed object is managed only by the current shared_ptr instance
(public member function) [edit]