NAME¶

std::try_lock - std::try_lock

Synopsis¶

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

See also¶

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