What is the default value to be given for a std::thread reference in Thread_Guard class - multithreading

Question

I'm writing a class Thread_Guard as explained in the book Concurrency in Action at Listing 2.3 - Using RAII to wait for a thread to complete.
Below is the class
class Thread_Guard
{
thread& gThread;
public:
Thread_Guard() : gThread(NULL) {};
explicit Thread_Guard(thread& t1) : gThread(t1)
{
}
~Thread_Guard()
{
if (gThread.joinable())
gThread.join();
}
Thread_Guard(const Thread_Guard& gT1) = delete;
Thread_Guard& operator=(const Thread_Guard& gT1) = delete;
};
What is the value to be initialized for the reference to std::thread in the default constructor? I will be calling it from main like below. Where MyInt is a sample class I have written.
int main()
{
int a = 100;
MyInt customInt(a);
thread worker(customInt);
Thread_Guard(worker);
return 0;
}

Answer 1

The original version of thread_guard does not contain a default constructor; probably by design. It would be interesting to see the use-case motivating your question (perhaps there's an alternative?).
However, if you do want to do this, you could simply store a thread * in the class object, make the default constructor set it to nullptr, and have the destructor check this first:
class Thread_Guard
{
thread *gThread;
public:
Thread_Guard() : gThread(nullptr) {};
explicit Thread_Guard(thread& t1) : gThread(&t1)
{
}
~Thread_Guard()
{
if (gThread && gThread->joinable())
gThread->join();
}
Thread_Guard(const Thread_Guard& gT1) = delete;
Thread_Guard& operator=(const Thread_Guard& gT1) = delete;
};