std::try_lock

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

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

Return value

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

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

Compiler support
Freestanding and hosted
Language
Standard library
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Execution support library (C++26)
Technical specifications
Symbols index
External libraries

std::try_lock

Parameters

Return value

Example

See also

Navigation