std::defer_lock, std::try_to_lock, std::adopt_lock, std::defer_lock_t, std::try_to_lock_t, std::adopt_lock

std::defer_lock, std::try_to_lock, std::adopt_lock, std::defer_lock_t, std::try_to_lock_t, std::adopt_lock_t

From cppreference.com

Defined in header `<mutex>`
`struct defer_lock_t { explicit defer_lock_t() = default; };`	(1)	(since C++11)
`constexpr std::defer_lock_t defer_lock {};`	(2)	(since C++11) (inline since C++17)
`struct try_to_lock_t { explicit try_to_lock_t() = default; };`	(3)	(since C++11)
`constexpr std::try_to_lock_t try_to_lock {};`	(4)	(since C++11) (inline since C++17)
`struct adopt_lock_t { explicit adopt_lock_t() = default; };`	(5)	(since C++11)
`constexpr std::adopt_lock_t adopt_lock {};`	(6)	(since C++11) (inline since C++17)

1,3,5) The empty class tag types std::defer_lock_t, std::try_to_lock_t and std::adopt_lock_t can be used in the constructor's parameter list for std::unique_lock and std::shared_lock to specify locking strategy.

2,4,6) The corresponding std::defer_lock, std::try_to_lock and std::adopt_lock instances of (1,3,5) can be passed to the constructors to indicate the type of locking strategy.

One of the constructors of the class template std::lock_guard only accepts the tag std::adopt_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

#include <iostream>
#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)
{
    if (&from == &to) // avoid deadlock in case of self transfer
        return;

    // 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

	constructs a `lock_guard`, optionally locking the given mutex (public member function of `std::lock_guard<Mutex>`) [edit]
	constructs a `unique_lock`, optionally locking (i.e., taking ownership of) the supplied mutex (public member function of `std::unique_lock<Mutex>`) [edit]

Example

See also