为什么我必须在右值引用上调用 move ？

Question

In the code below, why doesn't the first call mkme = mvme_rv dispatch to T& operator=(const T&&) ?

#include <iostream>
#include <string>
#include <vector>

using namespace std;
using T = vector<int>;

int main()
{
  T mvme(10, 1), mkme;
  T&& mvme_rv = move(mvme); // rvalue ref?
  mkme = mvme_rv;           // calls T& operator=(const T&)?
  cout << mvme.empty();     // 0
  mkme = move(mvme_rv);     // calls T& operator=(const T&&)?
  cout << mvme.empty();     // 1
}

Answer 1

As skypjack correctly comments, accessing an object through its name always results in an lvalue reference.

This is a safety feature and if you think it through you will realise that you are glad of it.

As you know, std::move simply casts an l-value reference to an r-value reference. If we use the returned r-value reference immediately (ie un-named) then it remains an r-value reference.

This means that the use of the r-value can only be at the point in the code where move(x) is mentioned. From a code-reader's perspective, it's now easy to see where x's state became undefined.

so:

 1: auto x = make_x();
 2: auto&& r = std::move(x);
 3: // lots of other stuff
35: // ...
54: // ...
55: take_my_x(r);

does not work. If it did, someone maintaining the code would have a hard time seeing (and remembering) that x (defined on line 1) enters an undefined state on line 55 through a reference taken on line 2.

This is a good deal more explicit:

 1: auto x = make_x();
 2: //
 3: // lots of other stuff
35: // ...
54: // ...
55: take_my_x(std::move(x));

Answer 2

This line of code:

mkme = mvme_rv;

is a copy and will thus use a copy assignment ( T& operator=(const T&) ). The key thing about this is BOTH objects can be used afterwards and should - if implemented correctly - provide two identical objects.

By contrast, this line of code:

mkme = move(mvme_rv);

is a move assignment ( T& operator=(const T&&) ). By convention, this will trash the mvme_rv object (or at least clear it) and make mkme basically what mvme_rv was previously.

Effectively T&& means a temporary object (aka xvalue) - something that will not last. The std::move method basically casts the object to a temporary (credit to @richard-Hodges for that wording). This can then be used in the move assignment method.

So finally to answer you question of why doesn't mkme = mvme_rv dispatch to T& operator=(const T&&) : it's because mvme_rv isn't a temporary object (aka xavalue).

---

More about xvalues: http://en.cppreference.com/w/cpp/language/value_category