std::defer_lock,std::try_to_lock,std::adopt_lock (3) Linux Manual Page

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();
}

See also

defer_lock_t
try_to_lock_t
adopt_lock_t tag type used to specify locking strategy
              (class)
(C++11)
(C++11)
(C++11)
              constructs a lock_guard, optionally locking the given mutex
constructor (public member function of std::lock_guard<Mutex>)
              constructs a unique_lock, optionally locking the supplied mutex
constructor (public member function of std::unique_lock<Mutex>)