Question: Sometime I have to use `std::thread` to speed up my application. I also know `join()` waits until a thread completes. This is easy to understand, but what's the difference between calling `detach()` and not calling it?

I thought that without `detach()`, the thread's method will work using a thread independently.

Not detaching:

```
void Someclass::Somefunction() {
    //...

    std::thread t([ ] {
        printf("thread called without detach");
    });

    //some code here
}

```

Calling with detaching:

```
void Someclass::Somefunction() {
    //...

    std::thread t([ ] {
        printf("thread called with detach");
    });

    t.detach();

    //some code here
}

```

Answer: 
In the destructor of `std::thread`, `std::terminate` is called if:

* the thread was not joined (with `t.join()`)
* and was not detached either (with `t.detach()`)

Thus, you should always either `join` or `detach` a thread before the flows of execution reaches the destructor.

---

When a program terminates (ie, `main` returns) the remaining detached threads executing in the background are not waited upon; instead their execution is suspended and their thread-local objects destructed.

Crucially, this means that *the stack of those threads is not unwound* and thus some destructors are not executed. Depending on the actions those destructors were supposed to undertake, this might be as bad a situation as if the program had crashed or had been killed. Hopefully the OS will release the locks on files, etc... but you could have corrupted shared memory, half-written files, and the like.

---

So, should you use `join` or `detach` ?

* Use `join`
* Unless you need to have more flexibility AND are willing to provide a synchronization mechanism to wait for the thread completion *on your own*, in which case you may use `detach`
This answer is aimed at answering question in the title, rather than explaining the difference between `join` and `detach`. So when should `std::thread::detach` be used?

In properly maintained C++ code `std::thread::detach` should not be used at all. Programmer must ensure that all the created threads gracefully exit releasing all the acquired resources and performing other necessary cleanup actions. This implies that giving up ownership of threads by invoking `detach` is not an option and therefore `join` should be used in all scenarios.

However some applications rely on old and often not well designed and supported APIs that may contain indefinitely blocking functions. Moving invocations of these functions into a dedicated thread to avoid blocking other stuff is a common practice. There is no way to make such a thread to exit gracefully so use of `join` will just lead to primary thread blocking. That's a situation when using `detach` would be a less evil alternative to, say, allocating `thread` object with dynamic storage duration and then purposely leaking it.

```
#include <LegacyApi.hpp>
#include <thread>

auto LegacyApiThreadEntry(void)
{
    auto result{NastyBlockingFunction()};
    // do something...
}

int main()
{
    ::std::thread legacy_api_thread{&LegacyApiThreadEntry};
    // do something...
    legacy_api_thread.detach();
    return 0;
}

```