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

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< cpp‎ | thread

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

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