std::unique_lock<std::mutex> or std::lock_guard<std::mutex>?

chmike_75517 0
Viewed 211862

I have two use cases.

A. I want to synchronise access to a queue for two threads.

B. I want to synchronise access to a queue for two threads and use a condition variable because one of the threads will wait on content to be stored into the queue by the other thread.

For use case A I see code example using std::lock_guard<>. For use case B I see code example using std::unique_lock<>.

What is the difference between the two and which one should I use in which use case?

stdmutex mutual-exclusion c++11 multithreading c++
7 Answers

One missing difference is: std::unique_lock can be moved but std::lock_guard can't be moved.

Note: Both cant be copied.

user_35003150

They are not really same mutexes, lock_guard<muType> has nearly the same as std::mutex, with a difference that it's lifetime ends at the end of the scope (D-tor called) so a clear definition about these two mutexes :

lock_guard<muType> has a mechanism for owning a mutex for the duration of a scoped block.

And

unique_lock<muType> is a wrapper allowing deferred locking, time-constrained attempts at locking, recursive locking, transfer of lock ownership, and use with condition variables.

Here is an example implemetation :

#include <iostream>
#include <thread>
#include <mutex>
#include <condition_variable>
#include <functional>
#include <chrono>

using namespace std::chrono;

class Product{

   public:

       Product(int data):mdata(data){
       }

       virtual~Product(){
       }

       bool isReady(){
       return flag;
       }

       void showData(){

        std::cout<<mdata<<std::endl;
       }

       void read(){

         std::this_thread::sleep_for(milliseconds(2000));

         std::lock_guard<std::mutex> guard(mmutex);

         flag = true;

         std::cout<<"Data is ready"<<std::endl;

         cvar.notify_one();

       }

       void task(){

       std::unique_lock<std::mutex> lock(mmutex);

       cvar.wait(lock, [&, this]() mutable throw() -> bool{ return this->isReady(); });

       mdata+=1;

       }

   protected:

    std::condition_variable cvar;
    std::mutex mmutex;
    int mdata;
    bool flag = false;

};

int main(){

     int a = 0;
     Product product(a);

     std::thread reading(product.read, &product);
     std::thread setting(product.task, &product);

     reading.join();
     setting.join();


     product.showData();
    return 0;
}

In this example, i used the unique_lock<muType> with condition variable

user_96941340
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