| Defined in header <mutex> | ||
|---|---|---|
constexpr std::defer_lock_t defer_lock {}; | (since C++11) | |
constexpr std::try_to_lock_t try_to_lock {}; | (since C++11) | |
constexpr std::adopt_lock_t adopt_lock {}; | (since C++11) |
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 and std::unique_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)
{
// 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) | |
constructs a unique_lock, optionally locking the supplied mutex (public member function of std::unique_lock) |
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