What are rvalues, lvalues, xvalues, glvalues, and prvalues?

I guess this document might serve as a not so short introduction : n3055

The whole massacre began with the move semantics. Once we have expressions that can be moved and not copied, suddenly easy to grasp rules demanded distinction between expressions that can be moved, and in which direction.

From what I guess based on the draft, the r/l value distinction stays the same, only in the context of moving things get messy.

Are they needed? Probably not if we wish to forfeit the new features. But to allow better optimization we should probably embrace them.

Quoting n3055:

  • An lvalue (so-called, historically,
    because lvalues could appear on the
    left-hand side of an assignment
    expression) designates a function or
    an object. [Example: If E is an
    expression of pointer type, then *E
    is an lvalue expression referring to
    the object or function to which E
    points. As another example, the
    result of calling a function whose
    return type is an lvalue reference is
    an lvalue.]
  • An xvalue (an
    “eXpiring” value) also refers to an
    object, usually near the end of its
    lifetime (so that its resources may
    be moved, for example). An xvalue is
    the result of certain kinds of
    expressions involving rvalue
    references. [Example: The
    result of calling a function whose
    return type is an rvalue reference is
    an xvalue.]
  • A glvalue (“generalized” lvalue) is an lvalue
    or an xvalue.
  • An rvalue (so-called,
    historically, because rvalues could
    appear on the right-hand side of an
    assignment expression) is an xvalue,
    a temporary object or
    subobject thereof, or a value that is
    not associated with an object.
  • A
    prvalue (“pure” rvalue) is an rvalue
    that is not an xvalue. [Example: The
    result of calling a function whose
    return type is not a reference is a

The document in question is a great reference for this question, because it shows the exact changes in the standard that have happened as a result of the introduction of the new nomenclature.

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