Value Categories

General

each expression is characterized by two independent properties: a type and a value category
each expression has some non-reference type, and each expression belongs to exactly one of the three primary value categories: prvalue, xvalue, and lvalue
there are also mixed categories which aggregate the primary ones:
- lvalues and xvalues compromise glvalues (Generalized Lvalue)
- xvalues and prvalues compromise rvalues

address of an lvalue may be taken by built-in address-of operator (&)
modifiable lvalue may be used as the left-hand operand of the built-in assignment and compound assignment operators
even if the variable type is rvalue reference, the expression consisting it's name is lvalue
function call which return type is lvalue reference, is lvalue

function call which return type is rvalue reference to object (like std::move), is xvalue
any expression that designates a temporary object, after temporary materialization (since C++17)

glvalue may be implicitly converted to a prvalue with lvalue-to-rvalue, array-to-pointer, or function-to-pointer implicit conversion
may be polymorphic or have incomplete type

address of an rvalue cannot be taken by built-in address-of operator
rvalue can't be used as the left-hand operand of the built-in assignment operator
rvalue may be used to initialize an rvalue reference, in which case the lifetime of the object identified by the rvalue is extended until the scope of the reference ends
when used as a function argument and when two overloads of the function are available, one taking rvalue reference parameter and the other taking lvalue reference to const parameter, an rvalue binds to the rvalue reference overload