Is it correct to say that xvalues have identity and are movable?

Question

According to this document:

http://www.stroustrup.com/terminology.pdf

1. l-values have identity and are not movable.
2. pr-values are movable but don't have identity.
3. x-values have identity and are movable.

I have a couple of questions around these.

a. What's the example of an x-value having identity? The following isn't legal:

Foo f;
&std::move(f);

b. I could overload the &-operator of class Foo and make it return this so that the following becomes legal:

&Foo(5);

But pr-values like Foo(5) cannot have identity. Or is there a subtler interpretation of identity?

Answer 1

Or is there a subtler interpretation of identity?

The document you linked contains these two definitions:

• “has identity” – i.e. and address, a pointer, the user can determine whether two copies are identical, etc.
• “can be moved from” – i.e. we are allowed to leave to source of a “copy” in some indeterminate, but valid state

Taking a look at the Standard we can gather the definition of an object (as per §1.8/1) is:

An object is a region of storage.

that as per, §1.7/1:

Every byte has a unique address.

has an address. This means that the definition is awfully broad and includes pretty much every object of any type of value (rvalue, xvalue, prvalue, lvalue, etc..).

---

What's the example of an x-value having identity?

The standard itself gives a list of expressions that generates xvalues (at §5/7):

An expression is an xvalue if it is:

• the result of calling a function, whether implicitly or explicitly, whose return type is an rvalue reference to object type,
• a cast to an rvalue reference to object type,
• a class member access expression designating a non-static data member of non-reference type in which the object expression is an xvalue, or
• a .* pointer-to-member expression in which the first operand is an xvalue and the second operand is a pointer to data member.

In general, the effect of this rule is that named rvalue references are treated as lvalues and unnamed rvalue references to objects are treated as xvalues; rvalue references to functions are treated as lvalues whether namedornot.

And as a follow up few examples are given. Here's an extension to them:

struct A { int m; };

A&& operator+(A, A);
A a;
A b;
a + b;   // xvalue

A&& f();
f();     // xvalue
f().m;   // xvalue

A a;
static_cast<A&&>(a);  // xvalue
std::move(a);         // xvalue

Answer 2

The following from Stroutrup's The C++ Programming Language might clear this question up:

void f(vector<string>& vs)
{
vector<string>& v2 = std::move(vs);// move vs to v2
// ...
}

Here, std::move(vs) is an xvalue: it
clearly has identity (we can refer to
it as vs), but we have explicitly given
permission for it to be moved from by
calling std::move() (§3.3.2, §35.5.1).
For practical programming, thinking in
terms of rvalue and lvalue is usually 
sufficient.