std::try_lock

Since C++11

Header: <mutex>

Tries to lock each of the given Lockable objects lock1, lock2, …, lockn by calling try_lock in order beginning with the first.

# Declarations

template< class Lockable1, class Lockable2, class... LockableN >
int try_lock( Lockable1& lock1, Lockable2& lock2, LockableN&... lockn );

(since C++11)

# Parameters

lock1, lock2, ..., lockn: the Lockable objects to lock

# Return value

-1 on success, or 0-based index value of the object that failed to lock.

# Example

#include <chrono>
#include <functional>
#include <iostream>
#include <mutex>
#include <thread>
#include <vector>
 
int main()
{
    int foo_count = 0;
    std::mutex foo_count_mutex;
    int bar_count = 0;
    std::mutex bar_count_mutex;
    int overall_count = 0;
    bool done = false;
    std::mutex done_mutex;
 
    auto increment = [](int& counter, std::mutex& m, const char* desc)
    {
        for (int i = 0; i < 10; ++i)
        {
            std::unique_lock<std::mutex> lock(m);
            ++counter;
            std::cout << desc << ": " << counter << '\n';
            lock.unlock();
            std::this_thread::sleep_for(std::chrono::seconds(1));
        }
    };
 
    std::thread increment_foo(increment, std::ref(foo_count), 
        std::ref(foo_count_mutex), "foo");
    std::thread increment_bar(increment, std::ref(bar_count), 
        std::ref(bar_count_mutex), "bar");
 
    std::thread update_overall([&]()
    {
        done_mutex.lock();
        while (!done)
        {
            done_mutex.unlock();
            int result = std::try_lock(foo_count_mutex, bar_count_mutex);
            if (result == -1)
            {
                overall_count += foo_count + bar_count;
                foo_count = 0;
                bar_count = 0;
                std::cout << "overall: " << overall_count << '\n';
                foo_count_mutex.unlock();
                bar_count_mutex.unlock();
            }
            std::this_thread::sleep_for(std::chrono::seconds(2));
            done_mutex.lock();
        }
        done_mutex.unlock();
    });
 
    increment_foo.join();
    increment_bar.join();
    done_mutex.lock();
    done = true;
    done_mutex.unlock();
    update_overall.join();
 
    std::cout << "Done processing\n"
              << "foo: " << foo_count << '\n'
              << "bar: " << bar_count << '\n'
              << "overall: " << overall_count << '\n';
}

# See also

lock