C++ Value Categories

Each C++ expression is characterized by two independent properties: a type and a value category. Each Expression has some non-reference type, and each experssion belongs to exactly one of the three primary value categories: prvalue, xvalue, and lvalue, defined as follows:

• a glvalue ("generalized" lvalue) is an expression whose evaluation determines the identity of an object, bit-field, or function;
• a prvalue ("pure rvalue) is an expression whose evaluation either
  • computes the value of the operand of an operator (such prvalue has no result object), or
  • initializes an object or a bit-field (such prvalue is said to have a result object)
• an xvalue (an "eXpiring" value) is a glvalue that denotes an object or bit-field whose resources can be reused
• an lvalue is a gvalue that is not an xvalue
• an rvalue is a prvalue or an xvalue