std::defer_lock, std::try_to_lock, std::adopt_lock
Defined in header <mutex>
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constexpr std::defer_lock_t defer_lock {}; |
(since C++11) (until C++17) |
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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) |
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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) |
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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
Run this code
#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) { // 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(); }
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 the supplied mutex (public member function of std::unique_lock<Mutex> ) |