std::defer_lock_t, std::try_to_lock_t, std::adopt_lock_t
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> ) |
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