c++ - C++11 rvalue reference vs const reference

Question

This may be obvious but I think it is something difficult to me. Given this:

void test(std::string&&) { }

std::string x{"test"};
test(std::move(x)); // ok

This code calls test() with a rvalue reference as parameter so the program compiles as I expect.

Now look at this:

void other_test(const std::string&) { }

std::string x{"test"};
other_test(std::move(x)); // ok???

And here I'm tilted. Why does this version compile? The std::move returns a && type; why then I don't get an error in the second method where I use const&?

---

I know that

int&& s = 5;
const int& s = 5;

is valid because in both cases I provide something that has not an lvalue, it has no addresses. Are && and const& equivalent? If no, are there differences?

Answer 1

std::move doesn't actually move anything out of it's own. It's just a fancy name for a cast to a T&&. Calling test like this test(std::move(x)); only shows that a T&& is implicitly convertible to a const T&. The compiler sees that test only accepts const T& so it converts the T&& returned from std::move to a const T&, that's all there is to it.