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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
< cpp‎ | thread
 
 
Concurrency support library
Threads
(C++11)
(C++20)
(C++20)
this_thread namespace
(C++11)
(C++11)
(C++11)
Mutual exclusion
(C++11)
(C++11)  
Generic lock management
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
defer_locktry_to_lockadopt_lockdefer_lock_ttry_to_lock_tadopt_lock_t
(C++11)(C++11)(C++11)(C++11)(C++11)(C++11)
Condition variables
(C++11)
Semaphores
Latches and Barriers
(C++20)
(C++20)
Futures
(C++11)
(C++11)
(C++11)
(C++11)
Safe Reclamation
(C++26)
(C++26)
Hazard Pointers





Atomic types
(C++11)
(C++20)
Initialization of atomic types
(C++11)(deprecated in C++20)
(C++11)(deprecated in C++20)
Memory ordering
Free functions for atomic operations
Free functions for atomic flags
 
Defined in header <mutex>
struct defer_lock_t { explicit defer_lock_t() = default; };
(1) (since C++11)
(2)
constexpr std::defer_lock_t defer_lock {};
(since C++11)
(until C++17)
inline constexpr std::defer_lock_t defer_lock {};
(since C++17)
struct try_to_lock_t { explicit try_to_lock_t() = default; };
(3) (since C++11)
(4)
constexpr std::try_to_lock_t try_to_lock {};
(since C++11)
(until C++17)
inline constexpr std::try_to_lock_t try_to_lock {};
(since C++17)
struct adopt_lock_t { explicit adopt_lock_t() = default; };
(5) (since C++11)
(6)
constexpr std::adopt_lock_t adopt_lock {};
(since C++11)
(until C++17)
inline constexpr std::adopt_lock_t adopt_lock {};
(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

[edit] Example

#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

[edit] See also

constructs a lock_guard, optionally locking the given mutex
(public member function of std::lock_guard<Mutex>) [edit]
constructs a unique_lock, optionally locking (i.e., taking ownership of) the supplied mutex
(public member function of std::unique_lock<Mutex>) [edit]