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::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 |
#include <mutex>
#include <thread>
#include <iostream>
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 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
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 |
|
(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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