unique_lock和lock_guard都是可以自动解锁的类，但是lock_guard更加高效体现在永远在析构函数中解锁，而unique_lock更加灵活，但执行效率会比lock_guard低一些，它们都是有不同的应用场景。
unique_lock是通用互斥包装器，允许延迟锁定、尝试锁定、递归锁定、所有权转移和条件变量一同使用；它可以移动mutex的所有权，但不可复制；
1.unique_lock的行为，通过第二个参数控制：
a)std::adopt_lock
在unique_lock创建对象，有adopt_lock参数时，对象不能执行lock成员函数，因为它的使用场景是mutex已经加了锁。
//测试使用adopt_lock参数创建的对象调用lock函数
#include <iostream>
#include <thread>
#include <mutex>
using namespace std;
int main()
{
mutex mutex_m;
unique_lock<mutex> lock(mutex_m, std::adopt_lock);
lock.lock();
return 0;
}
//打印结果
terminate called after throwing an instance of 'std::system_error'
what(): Resource deadlock avoided
Aborted (core dumped)
b)std::try_to_lock
在unique_lock创建对象，有try_to_lock参数时,对象会尝试去加锁，如果不成功，则会返回false，而不用继续等待。
#include <iostream>
#include <thread>
#include <mutex>
using namespace std;
class Test
{
public:
Test():age_m(0)
{
cout << "Test构造函数" << endl;
}
~Test()
{
cout << "Test析构函数" << endl;
}
void set(const int age)
{
std::lock_guard<std::mutex> lockgd(mutex_m);
std::this_thread::sleep_for(std::chrono::seconds(2));
cout << "set ,thread id:" << std::this_thread::get_id()<< endl;
age_m = age;
}
void get()
{
std::unique_lock<std::mutex> unilock(mutex_m, std::try_to_lock);
if (unilock.owns_lock())
{
cout << "get lock, thread id:" << std::this_thread::get_id()<< endl;
cout << "age:" << age_m << endl;
}
else
{
cout << "Cant get lock, thread id:" << std::this_thread::get_id()<< endl;
}
}
private:
int age_m;
static std::mutex mutex_m;
};
std::mutex Test::mutex_m;
int main()
{
Test test;
std::thread th[20];
// std::thread first(&Test::set, &test, 20);
// first.join();
for (int i=0; i<20; i++)
{
if (i%20 == 0)
th[i] = std::thread(&Test::set, &test, i);
else
th[i] = std::thread(&Test::get, &test);
}
for (int i=0; i<20; i++)
{
if (th[i].joinable())
{
th[i].join();
}
}
return 0;
}
//打印结果
root@epc:/home/share/eclipse_workspace/exercise/src# ./exercise
Test构造函数
get lock, thread id:139643157436160
age:0
Cant get lock, thread id:139643149043456
Cant get lock, thread id:139643140650752
Cant get lock, thread id:139643165828864
get lock, thread id:139643132258048
age:0
Cant get lock, thread id:139643123865344
Cant get lock, thread id:139643115472640
get lock, thread id:139643174221568
age:0
Cant get lock, thread id:139643107079936
get lock, thread id:139643098687232
age:0
get lock, thread id:139643090294528
age:0
get lock, thread id:139643081901824
age:0
Cant get lock, thread id:139643182614272
get lock, thread id:139643073509120
age:0
get lock, thread id:139643065116416
age:0
get lock, thread id:139643056723712
age:0
get lock, thread id:139643048331008
age:0
get lock, thread id:139643191006976
age:0
get lock, thread id:139643199399680
age:0
set ,thread id:139643207792384
Test析构函数
c)std::defer_lock
在unique_lock创建对象，有defer_lock参数时，需要调用成员函数lock进行加锁；
class Test
{
public:
Test():age_m(0)
{
cout << "Test构造函数" << endl;
}
~Test()
{
cout << "Test析构函数" << endl;
}
void set(const int age)
{
std::lock_guard<std::mutex> lockgd(mutex_m);
// std::this_thread::sleep_for(std::chrono::seconds(1));
cout << "set age= " << age << " ,thread id:" << std::this_thread::get_id()<< endl;
age_m = age;
}
void get()
{
std::unique_lock<std::mutex> unilock(mutex_m, std::defer_lock);
if (unilock.try_lock())
{
cout << "get age=" << age_m << ", get lock, thread id:" << std::this_thread::get_id()<< endl;
}
else
{
cout << "Cant get lock, thread id:" << std::this_thread::get_id()<< endl;
}
}
private:
int age_m;
static std::mutex mutex_m;
};
std::mutex Test::mutex_m;
2.unique_lock成员函数：
a)lock
在unique_lock创建对象，有defer_lock参数时，需要调用成员函数lock进行加锁；
b)unlock
可以利用unlock成员函数来控制加锁代码的粒度；
c)try_lock
在unique_lock创建对象，有defer_lock参数时，可以利用成员函数try_lock尝试加锁，成功则返回true，不成功则返回false；
d)release
执行完release后，将mutex的所有权释放，并返回mutex的对象指针；
举例：
std::mutex* mutex_p = unilock.release();
if (mutex_p != nullptr) mutex_p->unlock();
3.unique的所有权传递
unique_lock不支持拷贝和赋值，
举例：
std::unique_lock<std::mutex> templock(mutex_m, std::defer_lock);
std::unique_lock<std::mutex> unilock(move(templock));
