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

class template

<mutex>

std::lock_guard

template <class Mutex> class lock_guard;

Lock guard

A lock guard is an object that manages a mutex object by keeping it always locked.

On construction, the mutex object is locked by the calling thread, and on destruction, the mutex is unlocked. It is the simplest lock, and is specially useful as an object with automatic duration that lasts until the end of its context. In this way, it guarantees the mutex object is properly unlocked in case an exception is thrown.

Note though that the lock_guard object does not manage the lifetime of the mutex object in any way: the duration of the mutex object shall extend at least until the destruction of the lock_guard that locks it.

Template parameters

Mutex: A mutex-like type.
It shall be a basic lockable type, such as mutex (see BasicLockable for requirements).

Member types member type definition description mutex_type The template parameter (Mutex) The managed mutex object type
Member functions

(constructor): Construct lock_guard (public member function)

(destructor): Destroy lock_guard (unlocking mutex) (public member function)

Example

// lock_guard example
#include <iostream>       // std::cout
#include <thread>         // std::thread
#include <mutex>          // std::mutex, std::lock_guard
#include <stdexcept>      // std::logic_error

std::mutex mtx;

void print_even (int x) {
  if (x%2==0) std::cout << x << " is even\n";
  else throw (std::logic_error("not even"));
}

void print_thread_id (int id) {
  try {
    // using a local lock_guard to lock mtx guarantees unlocking on destruction / exception:
    std::lock_guard<std::mutex> lck (mtx);
    print_even(id);
  }
  catch (std::logic_error&) {
    std::cout << "[exception caught]\n";
  }
}

int main ()
{
  std::thread threads[10];
  // spawn 10 threads:
  for (int i=0; i<10; ++i)
    threads[i] = std::thread(print_thread_id,i+1);

  for (auto& th : threads) th.join();

  return 0;
}

Possible output:

[exception caught]
2 is even
[exception caught]
4 is even
[exception caught]
6 is even
[exception caught]
8 is even
[exception caught]
10 is even

The order of the lines printed may be different.