std::unique_lock与std::lock_guard区别示例

std::lock_guard

std::lock_guard<std::mutex> lk(frame_mutex);

std::unique_lock<std::mutex> lk(frame_mutex);

std::lock_guard是RAII模板类的简单实现，功能简单。

它是与mutex配合使用，把锁放到lock_guard中时，mutex自动上锁，lock_guard析构时，同时把mutex解锁。

#include <thread> 
#include <mutex> 
int g_i = 0; 
std::mutex g_i_mutex; // protects g_i 
void safe_increment() 
{ 
std::lock_guard<std::mutex> lock(g_i_mutex); 
++g_i; 
// g_i_mutex is automatically released when lock 
// goes out of scope 
} 

int main() 
{ 
std::thread t1(safe_increment); 
std::thread t2(safe_increment); 
t1.join(); 
t2.join(); 
} 

unique_lock使用实例

#include <iostream>       // std::cout
#include <thread>         // std::thread
#include <mutex>          // std::mutex, std::unique_lock
#include <vector>

std::mutex mtx;           // mutex for critical section
std::once_flag flag;

void print_block (int n, char c) {
    //unique_lock有多组构造函数, 这里std::defer_lock不设置锁状态
    std::unique_lock<std::mutex> my_lock (mtx, std::defer_lock);
    //尝试加锁, 如果加锁成功则执行
    //(适合定时执行一个job的场景, 一个线程执行就可以, 可以用更新时间戳辅助)
    if(my_lock.try_lock()){
        for (int i=0; i<n; ++i)
            std::cout << c;
        std::cout << '
';
    }
}

void run_one(int &n){
    std::call_once(flag, [&n]{n=n+1;}); //只执行一次, 适合延迟加载; 多线程static变量情况
}

int main () {
    std::vector<std::thread> ver;
    int num = 0;
    for (auto i = 0; i < 10; ++i){
        ver.emplace_back(print_block,50,'*');
        ver.emplace_back(run_one, std::ref(num));
    }

    for (auto &t : ver){
        t.join();
    }
    std::cout << num << std::endl;
    return 0;
}

std::unique_lock<std::mutex> lk(frame_mutex);

可以自动解锁，也可以手动解锁

lk.unlock();

mutex基本的加解锁

#include <thread>
#include <mutex>
#include <vector>
#include <iostream>
#include <algorithm>

std::mutex my_lock;

void add(int &num, int &sum){
    while(true){
        std::lock_guard<std::mutex> lock(my_lock);  
        if (num < 100){ //运行条件
            num += 1;
            sum += num;
        }   
        else {  //退出条件
            break;
        }   
    }   
}

int main(){
    int sum = 0;
    int num = 0;
    std::vector<std::thread> ver;   //保存线程的vector
    for(int i = 0; i < 20; ++i){
        std::thread t = std::thread(add, std::ref(num), std::ref(sum));
        ver.emplace_back(std::move(t)); //保存线程
    }   

    std::for_each(ver.begin(), ver.end(), std::mem_fn(&std::thread::join)); //join
    std::cout << sum << std::endl;
}