std::try_lock - cppreference.com (original) (raw)

| | | | | ------------------------------------------------------------------------------------------------------------------------------------------- | | ------------- | | 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.

[edit] Parameters

[edit] Return value

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

[edit] Example

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

#include #include #include #include #include #include   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

[edit] See also

| | locks specified mutexes, blocks if any are unavailable (function template) [edit] | | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |