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PaperBirdMaster
Asked: 2020-04-08 02:53:39 +0800 CST 2020-04-08 02:53:39 +0800 CST

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

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As a continuation of the question " Differences between Rvalue and Lvalue " and based on the question " What are rvalues, lvalues, xvalues, glvalues, and prvalues? ".

In C++03, an expression could be rvalueor lvalue.

As of C++11, three new categories were added, the full list of categories being:

  1. rvalue: Value of the right side ( right value).
  2. lvalue: Value of the left side ( left value).
  3. xvalue: Expiration value (e xpiring value).
  4. glvalue: Value of the generalized left-hand side ( generalized eft g) .lvalue
  5. prvalue: Value of the pure right hand side ( pure right value).

We have gone from two categories to five.

  • What are these new categories of expressions?
  • How are they related to the rvaluey lvaluethat existed before?
  • Are the rvaluey lvalue's in C++03 the same as in C++11?
  • Why are these categories necessary?

Is the standards committee just trying to confuse us?

c++

1 Answers

  1. Best Answer

    According to the C++ 1 standard (my translation):

    3.10 lvaluesandrvalues

    1. Expressions are categorized according to the following taxonomy:

    • An lvalue (so-called for historical reasons, since lvalues ​​can appear on the left side of an assignment expression) designates a function or an object. [ Example: if it Eis an expression of type pointer, then *Eit is an lvalue expression referring to the object or function it Epoints to. As a further example, the result of calling a function whose return type is a reference to lvalue is an lvalue. — end of example ]
    • An xvalue (an "expiring" value) refers to an object, usually near the end of its life cycle (that its resources are about to be moved, for example). An xvalue is the result of certain types of expressions involving rvalue references. [ Example: The result of calling a function whose return type is an rvalue reference is an xvalue. — end of example ]
    • A glvalue ("generalized" lvalue) is either an lvalue or an xvalue.
    • An rvalue (so called for historical reasons, since rvalues ​​can appear on the right hand side of an assignment expression) is an xvalue, temporary object, or subobject, also a value that is not associated with an object.
    • A prvalue ("pure" right-hand side value) is an rvalue that is not an xvalue. [ Example: The result of calling a function whose return type is not a reference is a prvalue. The value of a literal such as 12, 7.3e5, or trueis also a prvalue. — end of example ]

    This would answer us what these categories of expressions are and how they relate to each other. Regarding whether lvalueand rvalueare the same in C++03 and C++11, we can say that they are not quite:

    The lvalueones from C++03 keep the same category in C++11, while the rvalueones from C++03 are now the prvalueones from C++11. In general we can say that they rvaluehave evolved to take into account the semantics of movement , which also responds to why the new categorizations are necessary since without them, the semantics of movement would not be possible.

    1 That version of the standard was published at the end of March 2010.

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