std::defer_lock, std::try_to_lock, std::adopt_lock

Defined in header `<mutex>`
constexpr std::defer_lock_t defer_lock {};		(since C++11) (until C++17)
inline constexpr std::defer_lock_t defer_lock {};		(since C++17)
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)
constexpr std::adopt_lock_t adopt_lock {};		(since C++11) (until C++17)
inline constexpr std::adopt_lock_t adopt_lock {};		(since C++17)

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, 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

Run this code

#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();
}

Language
Standard Library Headers
Freestanding and hosted implementations
Named requirements
Language support library
Concepts library (C++20)
Diagnostics library
Utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Input/output library
Localizations library
Regular expressions library (C++11)
Atomic operations library (C++11)
Thread support library (C++11)
Filesystem library (C++17)
Technical Specifications

defer_lock_ttry_to_lock_tadopt_lock_t (C++11)(C++11)(C++11)	tag type used to specify locking strategy (class)
(constructor)	constructs a lock_guard, optionally locking the given mutex (public member function of `std::lock_guard<Mutex>`)
(constructor)	constructs a `unique_lock`, optionally locking the supplied mutex (public member function of `std::unique_lock<Mutex>`)

std::defer_lock, std::try_to_lock, std::adopt_lock

Example

See also