When I read the document in cppreference here https://en.cppreference.com/w/cpp/memory/shared_ptr#Example
I am wondering what the possible value of the first lp.use_count()
printed out is? I marked it with "<<<<<" in output content.
#include <iostream> #include <memory> #include <thread> #include <chrono> #include <mutex> struct Base { Base() { std::cout << " Base::Base()\n"; } // Note: non-virtual destructor is OK here ~Base() { std::cout << " Base::~Base()\n"; } }; struct Derived: public Base { Derived() { std::cout << " Derived::Derived()\n"; } ~Derived() { std::cout << " Derived::~Derived()\n"; } }; void thr(std::shared_ptr<Base> p) { std::this_thread::sleep_for(std::chrono::seconds(1)); std::shared_ptr<Base> lp = p; // thread-safe, even though the // shared use_count is incremented { static std::mutex io_mutex; std::lock_guard<std::mutex> lk(io_mutex); std::cout << "local pointer in a thread:\n" << " lp.get() = " << lp.get() << ", lp.use_count() = " << lp.use_count() << '\n'; } } int main() { std::shared_ptr<Base> p = std::make_shared<Derived>(); std::cout << "Created a shared Derived (as a pointer to Base)\n" << " p.get() = " << p.get() << ", p.use_count() = " << p.use_count() << '\n'; std::thread t1(thr, p), t2(thr, p), t3(thr, p); p.reset(); // release ownership from main std::cout << "Shared ownership between 3 threads and released\n" << "ownership from main:\n" << " p.get() = " << p.get() << ", p.use_count() = " << p.use_count() << '\n'; t1.join(); t2.join(); t3.join(); std::cout << "All threads completed, the last one deleted Derived\n"; }
Possible output:
Base::Base()
Derived::Derived()
Created a shared Derived (as a pointer to Base)
p.get() = 0x2299b30, p.use_count() = 1
Shared ownership between 3 threads and released
ownership from main:
p.get() = 0, p.use_count() = 0
local pointer in a thread:
lp.get() = 0x2299b30, lp.use_count() = 5 <<<<<<<< HERE <<<<<<
local pointer in a thread:
lp.get() = 0x2299b30, lp.use_count() = 3
local pointer in a thread:
lp.get() = 0x2299b30, lp.use_count() = 2
Derived::~Derived()
Base::~Base()
All threads completed, the last one deleted Derived
@user2452809 's answer is very appreciated, which pointed out an important feature of use_count()
.
Supposing use_count()
would return an accurate count, what would be the answer?