std::defer_lock, std::try_to_lock, std::adopt_lock, std::defer_lock_t, std::try_to_lock_t, std::adopt_lock_t
来自cppreference.com
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| 在标头 <mutex> 定义
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struct defer_lock_t { explicit defer_lock_t() = default; }; |
(1) | (C++11 起) |
constexpr std::defer_lock_t defer_lock {}; |
(2) | (C++11 起) (C++17 起 inline) |
struct try_to_lock_t { explicit try_to_lock_t() = default; }; |
(3) | (C++11 起) |
constexpr std::try_to_lock_t try_to_lock {}; |
(4) | (C++11 起) (C++17 起 inline) |
struct adopt_lock_t { explicit adopt_lock_t() = default; }; |
(5) | (C++11 起) |
constexpr std::adopt_lock_t adopt_lock {}; |
(6) | (C++11 起) (C++17 起 inline) |
1,3,5) 空类标签类型
std::defer_lock_t、std::try_to_lock_t 和 std::adopt_lock_t 可在 std::unique_lock 和 std::shared_lock 的构造函数形参列表中使用，以指定锁定策略。2,4,6) (1,3,5) 的相应实例
std::defer_lock、std::try_to_lock 和 std::adopt_lock 可以用于传递给构造函数以标明锁定策略的类型。类模板 std::lock_guard 仅有一个构造函数接受标签 std::adopt_lock。
| 类型 | 效果 |
defer_lock_t
|
不获得互斥体的所有权 |
try_to_lock_t
|
尝试获得互斥体的所有权而不阻塞 |
adopt_lock_t
|
假设调用方线程已拥有互斥体的所有权 |
示例
运行此代码
#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) // 避免自我转账情况中的死锁
return;
// 锁定两个互斥体而不死锁
std::lock(from.m, to.m);
// 保证两个已锁定互斥体在作用域结尾解锁
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();
std::cout << "my_account.balance = " << my_account.balance << "\n"
"your_account.balance = " << your_account.balance << '\n';
}
输出：
my_account.balance = 95
your_account.balance = 55
参阅
构造 lock_guard，可选地锁定给定的互斥体 ( std::lock_guard<Mutex> 的公开成员函数)
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构造 unique_lock，可选地锁定提供的互斥体 ( std::unique_lock<Mutex> 的公开成员函数)
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