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Showing content from https://cplusplus.com/reference/mutex/lock/ below:

function template

<mutex>

std::lock 
template <class Mutex1, class Mutex2, class... Mutexes>  void lock (Mutex1& a, Mutex2& b, Mutexes&... cde);

Lock multiple mutexes

Locks all the objects passed as arguments, blocking the calling thread if necessary.

The function locks the objects using an unspecified sequence of calls to their members lock, try_lock and unlock that ensures that all arguments are locked on return (without producing any deadlocks).

If the function cannot lock all objects (such as because one of its internal calls threw an exception), the function first unlocks all objects it successfully locked (if any) before failing.



Parameters
a, b, cde
Objects to locked.
Mutex1, Mutex2 and Mutexes shall be lockable types.

Return Value none

Example 
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// std::lock example
#include <iostream>       // std::cout
#include <thread>         // std::thread
#include <mutex>          // std::mutex, std::lock

std::mutex foo,bar;

void task_a () {
  // foo.lock(); bar.lock(); // replaced by:
  std::lock (foo,bar);
  std::cout << "task a\n";
  foo.unlock();
  bar.unlock();
}

void task_b () {
  // bar.lock(); foo.lock(); // replaced by:
  std::lock (bar,foo);
  std::cout << "task b\n";
  bar.unlock();
  foo.unlock();
}

int main ()
{
  std::thread th1 (task_a);
  std::thread th2 (task_b);

  th1.join();
  th2.join();

  return 0;
}
Note that before replacing the individual locks by the call to std::lock, if task_a locked foo while task_b locked bar, neither could ever get the second lock, causing a deadlock.

Possible output (order of lines may vary):




Data races The arguments are modified.

Exception safety Provides the same level of guarantees as the operation performed on the arguments.

See also
try_lock
Try to lock multiple mutexes (function template)

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