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
|
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 <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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