std::weak_ptr<T>::use_count
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
}
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: ??