what are the use cases for std::unique_lock::release?

Question

In what situations would one use the release method of std::unique_lock ? I made the mistake of using the release method instead of the unlock method and it took a while to understand why the following code wasn't working.

#include <mutex>
#include <iostream>
#include <vector>
#include <thread>
#include <chrono>

std::mutex mtx;

void foo()
{    
    std::unique_lock<std::mutex> lock(mtx);
    std::cout << "in critical section\n";
    std::this_thread::sleep_for(std::chrono::seconds(1));
    lock.release();
}

int main()
{
    std::vector<std::thread> threads;
    for (int i = 0; i < 5; ++i)
        threads.push_back(std::thread(foo));

    for (std::thread& t : threads)
        t.join();
}

Answer 1

There's a good use for it in this answer where ownership of the locked state is explicitly transferred from a function-local unique_lock to an external entity (a by-reference Lockable parameter).

This concrete example is typical of the use: To transfer ownership of the locked state from one object (or even type) to another.