Prefer `gsl::joining_thread` over `std::thread`

Reason

A joining_thread is a thread that joins at the end of its scope. Detached threads are hard to monitor. It is harder to ensure absence of errors in detached threads (and potentially detached threads).

Example, bad

void f() { std::cout << "Hello "; }

struct F {
    void operator()() const { std::cout << "parallel world "; }
};

int main()
{
    std::thread t1{f};      // f() executes in separate thread
    std::thread t2{F()};    // F()() executes in separate thread
}  // spot the bugs

Example

void f() { std::cout << "Hello "; }

struct F {
    void operator()() const { std::cout << "parallel world "; }
};

int main()
{
    std::thread t1{f};      // f() executes in separate thread
    std::thread t2{F()};    // F()() executes in separate thread

    t1.join();
    t2.join();
}  // one bad bug left

Note

Make "immortal threads" globals, put them in an enclosing scope, or put them on the free store rather than detach(). Don't detach.

Note

Because of old code and third party libraries using std::thread, this rule can be hard to introduce.

Enforcement

Flag uses of std::thread:

• Suggest use of gsl::joining_thread or C++20 std::jthread.
• Suggest "exporting ownership" to an enclosing scope if it detaches.
• Warn if it is not obvious whether a thread joins or detaches.