Lvalues and rvalues

Expressions can be categorized into one of the following value categories:

Lvalue

An expression can appear on the left side of an assignment expression if the expression is not const qualified.

Xvalue

An rvalue reference that is to expire.

(Prvalue) rvalue

A non-xvalue expression that appears only on the right side of an assignment expression.

Rvalues include both xvalues and prvalues. Lvalues and xvalues can be referred as glvalues.

Notes:

• Class (prvalue) rvalues can be cv-qualified, but non-class (prvalue) rvalues cannot be cv-qualified.
• Lvalues and xvalues can be of incomplete types, but (prvalue) rvalues must be of complete types or void types.

An object is a region of storage that can be examined and stored into. An lvalue or xvalueis an expression that refers to such an object. An lvalue does not necessarily permit modification of the object it designates. For example, a const object is an lvalue that cannot be modified. The term modifiable lvalue is used to emphasize that the lvalue allows the designated object to be changed as well as examined. The following object types are lvalues, but not modifiable lvalues:

• An array type
• An incomplete type
• A const-qualified type
• A structure or union type with one of its members qualified as a const type

Because these lvalues are not modifiable, they cannot appear on the left side of an assignment statement.

The term rvalue refers to a data value that is stored at some address in memory. An rvalue is an expression that cannot have a value assigned to it. Both a literal constant and a variable can serve as an rvalue. When an lvalue appears in a context that requires an rvalue, the lvalue is implicitly converted to an rvalue. The reverse, however, is not true: an rvalue cannot be converted to an lvalue. Rvalues always have complete types or the void type.

C defines a function designator as an expression that has function type. A function designator is distinct from an object type or an lvalue. It can be the name of a function or the result of dereferencing a function pointer. The C language also differentiates between its treatment of a function pointer and an object pointer.

On the other hand, in C++, a function call that returns a reference is an lvalue. Otherwise, a function call is an rvalue expression. In C++, every expression produces an lvalue, a xvalue, a (prvalue) rvalue, or no value.

For example, all assignment operators evaluate their right operand and assign that value to their left operand. The left operand must be a modifiable lvalue or a reference to a modifiable object.

The address operator (&) requires an lvalue as an operand while the increment (++) and the decrement (--) operators require a modifiable lvalue as an operand. The following example shows expressions and their corresponding lvalues.

Expression	Lvalue
x = 42	x
*ptr = newvalue	*ptr
a++	a
int& f()	The function call to f()

Beginning of C++11 only.

The following expressions are xvalues:

• The result of calling a function whose return type is of an rvalue reference type
• A cast to an rvalue reference
• A nonstatic data member of a non-reference type accessed through an xvalue expression
• A pointer to member access expression in which the first operand is an xvalue expression and the second operand is of a pointer to member type

See the following example:

int a;
int&& b= static_cast<int&&>(a);

struct str{
   int c;
};

int&& f(){
   int&& var =1;
   return var;
}

str&& g();
int&& rc = g().c;

In this example, The initializer for rvalue reference b is an xvalue because it is a result of a cast to an rvalue reference. A call to the function f() produces an xvalue because the return type of this function is of the int&& type. The initializer for rvalue reference rc is an xvalue because it is an expression that accesses a nonstatic non-reference data member c through an xvalue expression.

End of C++11 only.