Question

Looking over the new threading stuff in C++11 to see how easily it maps to pthreads, I notice the curious section in the thread constructor area:

thread();
Effects: Constructs a thread object that does not represent a thread of execution.
Postcondition: get_id() == id()
Throws: Nothing.

In other words, the default constructor for a thread doesn’t actually seem to create a thread. Obviously, it creates a thread object, but how exactly is that useful if there’s no backing code for it? Is there some other way that a “thread of execution” can be attached to that object, like thrd.start() or something similar?

Answer 1

Is there some other way that a “thread of execution” can be attached to that object, like thrd.start() or something similar?

// deferred start
std::thread thread;

// ...

// let's start now
thread = std::thread(functor, arg0, arg1);

std::thread is a MoveConstructible and MoveAssignable type. So that means that in code like std::thread zombie(some_functor); std::thread steal(std::move(zombie)); zombie will be left in a special, but valid, state associated with no thread of execution. The default constructor comes free in a sense since all it has to do is put the object into that exact state. It also allows arrays of std::thread and operations like std::vector<std::thread>::resize.