RAII Locking with std::lock_guard<std::mutex>(m_mutex); instead of std::lock_guard<std::mutex> lk(m_mutex);

Question

It's probably a stupid question, but I discovered a few occurences of a lock_guard without variable.

void func() {
  std::lock_guard<std::mutex>(m_mutex);
  m_value = "2";
}

Instead of the following with lk variable.

void func() {
  std::lock_guard<std::mutex> lk(m_mutex);
  m_value = "2";
}

The first version is probably bullshit and causes the access to be in some kind of soft sync mode, since the lock is held for a short moment, so no longer running function holds the lock currently, right?

Or did I learn something here and lk variable is actually not required?

Answer 1

The following code shows you why it is wrong :

#include <iostream>

struct raii_class
{
    raii_class()
    {
        std::cout << "raii construct\n";
    }
    ~raii_class()
    {
        std::cout << "raii destruct\n";
    }
};

int main()
{
    raii_class{};
    std::cout << "body\n";
    return 0;
}

Outputs :

raii construct
raii destruct
body