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>
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| 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
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do not acquire ownership of the mutex |
try_to_lock_t
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try to acquire ownership of the mutex without blocking |
adopt_lock_t
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assume the calling thread already has ownership of the mutex |
Example
Run this code
#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
See also
constructs a lock_guard, optionally locking the given mutex (public member function of std::lock_guard<Mutex>)
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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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