NAME¶

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

Synopsis¶

Defined in header <mutex>
struct defer_lock_t { explicit defer_lock_t() = default; };

struct try_to_lock_t { explicit try_to_lock_t() = default; }; (since C++11)

struct adopt_lock_t { explicit adopt_lock_t() = default; };

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¶

// 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, made explicit
which could lead to ambiguity

See also¶

defer_lock
try_to_lock
adopt_lock tag constants used to specify locking strategy
(C++11) (constant)
(C++11)
(C++11)
constructor 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
constructor of) the supplied mutex
(public member function of std::unique_lock<Mutex>)