how std::thread constructor detects rvalue reference?

Question

Obviously it is possible to pass an rvalue reference to std::thread constructor. My problem is with definition of this constructor in cppreference. It says that

Answer 1

auto s = std::decay_copy(std::string("hello"));

Is equivalent to:

template<>
std::string std::decay_copy<std::string>(std::string&& src) {
    return std::string(std::move(src));
}

std::string s = decay_copy<std::string>(std::string("hello"));

Answer 2

It is common problem of the perfect forwarding. If you want to restore information about rvalue in the function, you have to use std::forward std::forward . If you are interested in the value type detection you may read this value_category . From the description you can find the information how the compiler recognizes rvalue, xvalue, lvalue, prvalue, gvalue on compile time.

Answer 3

The std::thread constructor knows the value category of its arguments, because it knows what Function and Args... are, which it uses to perfectly forward the its parameters to decay_copy (or equivalent).

The actual thread function doesn't know the value category. It's always invoked as an rvalue, with all rvalue arguments - which makes sense: the copies of f and args... are local to the thread, and won't be used anywhere else.