ValueCategories
Each C++ expression (an operator with its operands, a literal, a variable name, etc.) 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.
a glvalue (“generalized” lvalue) is an expression whose evaluation determines the identity of an object or function;
a prvalue (“pure” rvalue) is an expression whose evaluation computes the value of an operand of a built-in operator (such prvalue has no result object), or initializes an object (such prvalue is said to have a result object).The result object may be a variable, an object created by new-expression, a temporary created by temporary materialization, or a member thereof. Note that non-void discarded expressions have a result object (the materialized temporary). Also, every class and array prvalue has a result object except when it is the operand of decltype;
an xvalue (an “eXpiring” value) is a glvalue that denotes an object whose resources can be reused;
an lvalue is a glvalue that is not an xvalue;