NAME¶

std::weak_ptr::use_count - std::weak_ptr::use_count

Synopsis¶

long use_count() const noexcept; (since C++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¶

expired checks whether the referenced object was already deleted
(public member function)