std::lock_guard or std::scoped_lock?

Question

C++17 introduced a new lock class called std::scoped_lock.

Judging from the documentation it looks similar to the already existing std::lock_guard clas

Answer 1

Here is a sample and quote from C++ Concurrency in Action:

friend void swap(X& lhs, X& rhs)
{
    if (&lhs == & rhs)
        return;
    std::lock(lhs.m, rhs.m);
    std::lock_guard lock_a(lhs.m, std::adopt_lock);
    std::lock_guard lock_b(rhs.m, std::adopt_lock);
    swap(lhs.some_detail, rhs.some_detail);
}

vs.

friend void swap(X& lhs, X& rhs)
{
    if (&lhs == &rhs)
        return;
    std::scoped_lock guard(lhs.m, rhs.m);
    swap(lhs.some_detail, rhs.some_detail);
}

The existence of std::scoped_lock means that most of the cases where you would have used std::lock prior to c++17 can now be written using std::scoped_lock, with less potential for mistakes, which can only be a good thing!