std::try_lock(3)

✨ NAME ✨

std::try_lock — okay bestie, this is the std::try_lock(3) page and the name is literally std::try_lock. We’re in C++ Standard Library territory, and this one is all about trying to lock a bunch of Lockable objects without committing to a whole dramatic blocking situation. 💅

🦋 Synopsis 🦋

Defined in header <mutex>
template< class Lockable1, class Lockable2, class... LockableN >
int try_lock( Lockable1& lock1, Lockable2& lock2, LockableN&... lockn
(since C++11)
);

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

If a call to try_lock fails, no further call to try_lock is performed, unlock is called for any locked objects and a 0-based index of the object that failed to lock is returned. 😭

If a call to try_lock results in an exception, unlock is called for any locked objects before rethrowing. 🚨

💅 Parameters 💅

• lock1, lock2, ..., lockn — the Lockable objects to lock. That’s the whole squad getting the try-lock treatment, in order. ✨

🎀 Return value 🎀

• -1 on success, or 0-based index value of the object that failed to lock. So -1 is the slay result, and any other number is like “this one said no bestie.” 💖

🔥 Example 🔥

The following example uses std::try_lock to periodically tally and reset counters running in separate threads.

// Run this code

#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';
}

🌈 Possible output 🌈

bar: 1
foo: 1
foo: 2
bar: 2
foo: 3
overall: 5
bar: 1
foo: 1
bar: 2
foo: 2
bar: 3
overall: 10
bar: 1
foo: 1
bar: 2
foo: 2
overall: 14
bar: 1
foo: 1
bar: 2
overall: 17
foo: 1
bar: 1
foo: 2
overall: 20
Done processing
foo: 0
bar: 0
overall: 20

👑 See also 👑

• lock — locks specified mutexes, blocks if any are unavailable (C++11) (function template) 🦋

💖 source/version/date: http://cppreference.com, 2024.06.10 — mwah, bye bestie 💋