std::defer_lock_t, std::try_to_lock_t, std::adopt_lock_t

 

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mutex (C++11) recursive_mutex (C++11) shared_mutex (C++17)	timed_mutex (C++11) recursive_timed_mutex (C++11) shared_timed_mutex (C++14)
通用锁管理
lock_guard (C++11) scoped_lock (C++17) unique_lock (C++11) shared_lock (C++14) defer_lock_ttry_to_lock_tadopt_lock_t (C++11)(C++11)(C++11)	lock (C++11) try_lock (C++11) defer_locktry_to_lockadopt_lock (C++11)(C++11)(C++11) once_flag (C++11) call_once (C++11)
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定义于头文件 <mutex>
struct defer_lock_t { }; struct try_to_lock_t { }; struct adopt_lock_t { };		(C++11 起) (C++17 前)
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; };		(C++17 起)

std::defer_lock_t 、 std::try_to_lock_t 和 std::adopt_lock_t 是用于为 std::lock_guard 、 std::scoped_lock 、 std::unique_lock 和 std::shared_lock 指定锁定策略的空结构体标签类型。

类型	效果
defer_lock_t	不获得互斥的所有权
try_to_lock_t	尝试获得互斥的所有权而不阻塞
adopt_lock_t	假设调用方线程已拥有互斥的所有权

示例

运行此代码

#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)
{
    // 锁定两个互斥而不死锁
    std::lock(from.m, to.m);
    // 保证二个已锁定互斥在作用域结尾解锁
    std::lock_guard<std::mutex> lock1(from.m, std::adopt_lock);
    std::lock_guard<std::mutex> lock2(to.m, std::adopt_lock);
 
// 等价方法：
//    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();
}

参阅

defer_locktry_to_lockadopt_lock (C++11)(C++11)(C++11)	用于指定锁定策略的标签常量 (常量)
(构造函数)	构造 lock_guard ，可选地锁定给定的互斥 (std::lock_guard<Mutex> 的公开成员函数)
(构造函数)	构造 unique_lock ，可选地锁定提供的互斥 (std::unique_lock<Mutex> 的公开成员函数)