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: 
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;
}

```
When you detach thread it means that you don't have to `join()` it before exiting `main()`.

*Thread library will actually wait for each such thread **below-main**, but you should not care about it.*

`detach()` is mainly useful when you have a task that has to be done in background, but you don't care about its execution. This is usually a case for some libraries. They may silently create a background worker thread and detach it so you won't even notice it.