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Showing content from https://cplusplus.com/reference/mutex/unique_lock/mutex/ below:

public member function

<mutex>

mutex_type* mutex() const noexcept;

Get mutex

Note that the unique_lock does not release ownership of the managed mutex object: i.e., if it owns a lock on the mutex, it is still responsible for unlocking it at some point (such as when it is destroyed).

To retrieve and acquire ownership of the managed mutex object see unique_lock::release.

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// unique_lock::mutex example
#include <iostream>       // std::cout
#include <thread>         // std::thread
#include <mutex>          // std::mutex, std::unique_lock, std::defer_lock

class MyMutex : public std::mutex {
  int _id;
public:
  MyMutex (int id) : _id(id) {}
  int id() {return _id;}
};

MyMutex mtx (101);

void print_ids (int id) {
  std::unique_lock<MyMutex> lck (mtx);
  std::cout << "thread #" << id << " locked mutex " << lck.mutex()->id() << '\n';
}

int main ()
{
  std::thread threads[10];
  // spawn 10 threads:
  for (int i=0; i<10; ++i)
    threads[i] = std::thread(print_ids,i+1);

  for (auto& th : threads) th.join();

  return 0;
}

thread #2 locked mutex 101
thread #3 locked mutex 101
thread #1 locked mutex 101
thread #4 locked mutex 101
thread #5 locked mutex 101
thread #6 locked mutex 101
thread #7 locked mutex 101
thread #8 locked mutex 101
thread #9 locked mutex 101
thread #10 locked mutex 101

unique_lock::release

Release mutex (public member function)

unique_lock::owns_lock

Owns lock (public member function)

unique_lock::operator bool

Return whether it owns a lock (public member function)

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