std::defer_lock,std::try_to_lock,std::adopt_lock (3) - Linux Manuals

std::defer_lock,std::try_to_lock,std::adopt_lock: std::defer_lock,std::try_to_lock,std::adopt_lock

NAME

std::defer_lock,std::try_to_lock,std::adopt_lock - std::defer_lock,std::try_to_lock,std::adopt_lock

Synopsis

Defined in header <mutex>
constexpr std::defer_lock_t defer_lock {}; (since C++11)
                                                     (until C++17)
inline constexpr std::defer_lock_t defer_lock {}; (since C++17)
constexpr std::try_to_lock_t try_to_lock {}; (since C++11)
                                                     (until C++17)
inline constexpr std::try_to_lock_t try_to_lock {}; (since C++17)
constexpr std::adopt_lock_t adopt_lock {}; (since C++11)
                                                     (until C++17)
inline constexpr std::adopt_lock_t adopt_lock {}; (since C++17)

std::defer_lock, std::try_to_lock and std::adopt_lock are instances of empty struct tag types std::defer_lock_t, std::try_to_lock_t and std::adopt_lock_t respectively.
They are used to specify locking strategies for std::lock_guard, std::unique_lock and std::shared_lock.

Type Effect(s)
defer_lock_t do not acquire ownership of the mutex
try_to_lock_t try to acquire ownership of the mutex without blocking
adopt_lock_t assume the calling thread already has ownership of the mutex

Example

// Run this code

  #include <mutex>
  #include <thread>

  struct bank_account {
      explicit bank_account(int balance) : balance(balance) {}
      int balance;
      std::mutex m;
  };

  void transfer(bank_account &from, bank_account &to, int amount)
  {
      // lock both mutexes without deadlock
      std::lock(from.m, to.m);
      // make sure both already-locked mutexes are unlocked at the end of scope
      std::lock_guard<std::mutex> lock1(from.m, std::adopt_lock);
      std::lock_guard<std::mutex> lock2(to.m, std::adopt_lock);

  // equivalent approach:
  // std::unique_lock<std::mutex> lock1(from.m, std::defer_lock);
  // std::unique_lock<std::mutex> lock2(to.m, std::defer_lock);
  // std::lock(lock1, lock2);

      from.balance -= amount;
      to.balance += amount;
  }

  int main()
  {
      bank_account my_account(100);
      bank_account your_account(50);

      std::thread t1(transfer, std::ref(my_account), std::ref(your_account), 10);
      std::thread t2(transfer, std::ref(your_account), std::ref(my_account), 5);

      t1.join();
      t2.join();
  }