# std::defer_lock_t, std::try_to_lock_t, std::adopt_lock_t

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< cpp‎ | thread

Defined in header `<mutex>` |  |
struct defer_lock_t { explicit defer_lock_t() = default; }; struct try_to_lock_t { explicit try_to_lock_t() = default; }; struct adopt_lock_t { explicit adopt_lock_t() = default; }; |  | (since C++11)

`std::defer_lock_t`, `std::try_to_lock_t` and `std::adopt_lock_t` are empty class tag types used to specify locking strategy for 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>
    #include <iostream>

    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)
    {
        if(&from == &to) return; // avoid deadlock in case of self transfer

        // 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 lock1{from.m, std::adopt_lock};
        std::lock_guard 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();

        std::cout << "my_account.balance = " << my_account.balance << "\n"
                     "your_account.balance = " << your_account.balance << '\n';
    }

Output:

    my_account.balance = 95
    your_account.balance = 55

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 2510 | C++11 | the default constructors were non-explicit, which could lead to ambiguity | made explicit

### See also

defer_lock try_to_lock adopt_lock (C++11)(C++11)(C++11) | tag constants used to specify locking strategy (constant)
(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 (i.e., taking ownership of) the supplied mutex (public member function of `std::unique_lock<Mutex>`)

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