& 返回左值或右值是什么？

Question

As per textbook.

The address-of operator (§ 2.3.2, p. 52) requires an lvalue operand and returns a pointer to its operand as an rvalue.

int a = 6,c=9;
int *x = &a; //Here x is a lvalue.
x = &c;

As per my knowledge if we can assign, it is lvalue.Then why does textbook says address of return rvalue.Can anyone explain me this in layman language

Answer 1

Text book says &a is an rvalue. Ie value cannot be assigned to &a . Compilation error will occur if we try to compile below code.

int a = 6,c=9;
int *x = &a; //Here x is a lvalue.
&a = x; // Compilation error, as '&a' is an rvalue

Answer 2

As per my knowledge if we can assign

This is not necessarily true in case of class types. But it does always hold for pointers, which you use in the example.

Then why does textbook says address of return rvalue

Well, you cannot assign to it:

&a = &c; // ill formed because &a is rvalue
int *x = &a; // OK. Not an assignment
x = &c;  // OK because x is lvalue