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std::weak_ptr<T>::use_count

Since C++11
long use_count() const noexcept;

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

#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
}
Result
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: ??

std::weak_ptr<T>::use_count

Since C++11
long use_count() const noexcept;

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

#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
}
Result
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: ??