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

< 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::lock_guard, std::scoped_lock, 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>
 
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<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();
}

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

(C++11)(C++11)(C++11)
 tag constants used to specify locking strategy
(constant)
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>)