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

< 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

#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

See also

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