What are rvalues lvalues xvalues glvalues and prvalues

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In C  03  an expression is either an rvalue or an lvalue   In C  11  an expression can be an         rvalue lvalue xvalue glvalue prvalue   Two categories have become five categories    What are these new categories of expressions  How do these new categories relate to the existing rvalue and lvalue categories    Are the rvalue and lvalue categories in C  0x the same as they are in C  03  Why are these new categories needed   Are the WG21 gods just trying to confuse us mere mortals

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Why are these new categories needed  Are the WG21 gods just trying to confuse us mere mortals    I don t feel that the other answers  good though many of them are  really capture the answer to this particular question  Yes  these categories and such exist to allow move semantics  but the complexity exists for one reason  This is the one inviolate rule of moving stuff in C  11   Thou shalt move only when it is unquestionably safe to do so   That is why these categories exist  to be able to talk about values where it is safe to move from them  and to talk about values where it is not   In the earliest version of r-value references  movement happened easily  Too easily  Easily enough that there was a lot of potential for implicitly moving things when the user didn t really mean to   Here are the circumstances under which it is safe to move something    When it s a temporary or subobject thereof   prvalue  When the user has explicitly said to move it    If you do this   SomeType  amp  amp Func            SomeType  amp  amp val   Func    SomeType otherVal val     What does this do  In older versions of the spec  before the 5 values came in  this would provoke a move  Of course it does  You passed an rvalue reference to the constructor  and thus it binds to the constructor that takes an rvalue reference  That s obvious   There s just one problem with this  you didn t ask to move it  Oh  you might say that the  amp  amp  should have been a clue  but that doesn t change the fact that it broke the rule  val isn t a temporary because temporaries don t have names  You may have extended the lifetime of the temporary  but that means it isn t temporary  it s just like any other stack variable   If it s not a temporary  and you didn t ask to move it  then moving is wrong   The obvious solution is to make val an lvalue  This means that you can t move from it  OK  fine  it s named  so its an lvalue   Once you do that  you can no longer say that SomeType amp  amp  means the same thing everwhere  You ve now made a distinction between named rvalue references and unnamed rvalue references  Well  named rvalue references are lvalues  that was our solution above  So what do we call unnamed rvalue references  the return value from Func above    It s not an lvalue  because you can t move from an lvalue  And we need to be able to move by returning a  amp  amp   how else could you explicitly say to move something  That is what std  move returns  after all  It s not an rvalue  old-style   because it can be on the left side of an equation  things are actually a bit more complicated  see this question and the comments below   It is neither an lvalue nor an rvalue  it s a new kind of thing   What we have is a value that you can treat as an lvalue  except that it is implicitly moveable from  We call it an xvalue   Note that xvalues are what makes us gain the other two categories of values     A prvalue is really just the new name for the previous type of rvalue  i e  they re the rvalues that aren t xvalues   Glvalues are the union of xvalues and lvalues in one group  because they do share a lot of properties in common    So really  it all comes down to xvalues and the need to restrict movement to exactly and only certain places  Those places are defined by the rvalue category  prvalues are the implicit moves  and xvalues are the explicit moves  std  move returns an xvalue

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INTRODUCTION  ISOC  11  officially ISO IEC 14882 2011  is the most recent version of the standard of the C   programming language  It contains some new features  and concepts  for example    rvalue references xvalue  glvalue  prvalue expression value categories move semantics   If we would like to understand the concepts of the new expression value categories we have to be aware of that there are rvalue and lvalue references  It is better to know rvalues can be passed to non-const rvalue references   int amp  r i 7     compile error int amp  amp  rr i 7     OK   We can gain some intuition of the concepts of value categories if we quote the subsection titled Lvalues and rvalues from the working draft N3337  the most similar draft to the published ISOC  11 standard       3 10 Lvalues and rvalues  basic lval       1 Expressions are categorized according to the taxonomy in Figure 1          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     end example     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  8 3 2     Example  The result of calling   a function whose return type is an rvalue reference is an xvalue     end   example     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  12 2  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 prvalue  The value of a literal such as 12  7 3e5  or   true is also a prvalue     end example           Every expression belongs to exactly one of the fundamental   classifications in this taxonomy  lvalue  xvalue  or prvalue  This   property of an expression is called its value category    But I am not quite sure about that this subsection is enough to understand the concepts clearly  because  usually  is not really general   near the end of its lifetime  is not really concrete   involving rvalue references  is not really clear  and  Example  The result of calling a function whose return type is an rvalue reference is an xvalue   sounds like a snake is biting its tail   PRIMARY VALUE CATEGORIES  Every expression belongs to exactly one primary value category  These value categories are lvalue  xvalue and prvalue categories   lvalues  The expression E belongs to the lvalue category if and only if E refers to an entity that ALREADY has had an identity  address  name or alias  that makes it accessible outside of E    include  lt iostream gt   int i 7   const int amp  f        return i     int main         std  cout lt  lt  amp  www  lt  lt std  endl     The expression  www  in this row is an lvalue expression  because string literals are arrays and every array has an address         i     The expression i in this row is an lvalue expression  because it refers to the same entity         i         as the entity the expression i in this row refers to       int  p i new int 7        p i     The expression  p i in this row is an lvalue expression  because it refers to the same entity          p i         as the entity the expression  p i in this row refers to       const int amp  r I 7      r I     The expression r I in this row is an lvalue expression  because it refers to the same entity         r I         as the entity the expression r I in this row refers to       f       The expression f   in this row is an lvalue expression  because it refers to the same entity         i         as the entity the expression f   in this row refers to       return 0      xvalues  The expression E belongs to the xvalue category if and only if it is      the result of calling a function  whether implicitly or explicitly  whose return type is an rvalue reference to the type of object being returned  or  int amp  amp  f        return 3     int main         f       The expression f   belongs to the xvalue category  because f   return type is an rvalue reference to object type       return 0          a cast to an rvalue reference to object type  or  int main         static cast lt int amp  amp  gt  7      The expression static cast lt int amp  amp  gt  7  belongs to the xvalue category  because it is a cast to an rvalue reference to object type      std  move 7      std  move 7  is equivalent to static cast lt int amp  amp  gt  7        return 0          a class member access expression designating a non-static data member of non-reference type in which the object expression is an xvalue  or  struct As       int i      As amp  amp  f        return As       int main         f   i     The expression f   i belongs to the xvalue category  because As  i is a non-static data member of non-reference type  and the subexpression f   belongs to the xvlaue category       return 0          a pointer-to-member expression in which the first operand is an xvalue and the second operand is a pointer to data member   Note that the effect of the rules above is that named rvalue references to objects are treated as lvalues and unnamed rvalue references to objects are treated as xvalues  rvalue references to functions are treated as lvalues whether named or not    include  lt functional gt   struct As       int i      As amp  amp  f        return As       int main         f       The expression f   belongs to the xvalue category  because it refers to an unnamed rvalue reference to object      As amp  amp  rr a As        rr a     The expression rr a belongs to the lvalue category  because it refers to a named rvalue reference to object      std  ref f      The expression std  ref f  belongs to the lvalue category  because it refers to an rvalue reference to function       return 0      prvalues  The expression E belongs to the prvalue category if and only if E belongs neither to the lvalue nor to the xvalue category   struct As       void f            this     The expression this is a prvalue expression  Note  that the expression this is not a variable            As f        return As       int main         f       The expression f   belongs to the prvalue category  because it belongs neither to the lvalue nor to the xvalue category       return 0      MIXED VALUE CATEGORIES  There are two further important mixed value categories  These value categories are rvalue and glvalue categories   rvalues  The expression E belongs to the rvalue category if and only if E belongs to the xvalue category  or to the prvalue category   Note that this definition means that the expression E belongs to the rvalue category if and only if E refers to an entity that has not had any identity that makes it accessible outside of E YET   glvalues  The expression E belongs to the glvalue category if and only if E belongs to the lvalue category  or to the xvalue category   A PRACTICAL RULE  Scott Meyer has published a very useful rule of thumb to distinguish rvalues from lvalues         If you can take the address of an expression  the expression is an lvalue    If the type of an expression is an lvalue reference  e g   T amp  or const T amp   etc    that expression is an lvalue    Otherwise  the expression is an rvalue   Conceptually  and typically also in fact   rvalues correspond to temporary objects  such   as those returned from functions or created through implicit type   conversions  Most literal values  e g   10 and 5 3  are also rvalues

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I have struggled with this for a long time  until I came across the cppreference com explanation of the value categories  It is actually rather simple  but I find that it is often explained in a way that s hard to memorize  Here it is explained very schematically  I ll quote some parts of the page   Primary categories The primary value categories correspond to two properties of expressions   has identity  it s possible to determine whether the expression refers to the same entity as another expression  such as by comparing addresses of the objects or the functions they identify  obtained directly or indirectly    can be moved from  move constructor  move assignment operator  or another function overload that implements move semantics can bind to the expression    Expressions that   have identity and cannot be moved from are called lvalue expressions  have identity and can be moved from are called xvalue expressions  do not have identity and can be moved from are called prvalue expressions  do not have identity and cannot be moved from are not used   lvalue An lvalue   quot left value quot   expression is an expression that has identity and cannot be moved from  rvalue  until C  11   prvalue  since C  11  A prvalue   quot pure rvalue quot   expression is an expression that does not have identity and can be moved from  xvalue An xvalue   quot expiring value quot   expression is an expression that has identity and can be moved from  glvalue A glvalue   quot generalized lvalue quot   expression is an expression that is either an lvalue or an xvalue  It has identity  It may or may not be moved from  rvalue  since C  11  An rvalue   quot right value quot   expression is an expression that is either a prvalue or an xvalue  It can be moved from  It may or may not have identity

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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 prvalue    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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One addendum to the excellent answers above  on a point that confused me even after I had read Stroustrup and thought I understood the rvalue lvalue distinction  When you see   int amp  amp  a   3   it s very tempting to read the int amp  amp  as a type and conclude that a is an rvalue  It s not    int amp  amp  a   3  int amp  amp  c   a    error  cannot bind  int  lvalue to  int amp  amp   int amp  b   a    compiles   a has a name and is ipso facto an lvalue  Don t think of the  amp  amp  as part of the type of a  it s just something telling you what a is allowed to bind to    This matters particularly for T amp  amp  type arguments in constructors  If you write   Foo  Foo T amp  amp   t    t  t      you will copy  t into t  You need   Foo  Foo T amp  amp   t    t std  move  t      if you want to move  Would that my compiler warned me when I left out the move

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What are these new categories of expressions   The FCD  n3092  has an excellent description       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     end example       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  8 3 2     Example  The result of calling a function whose return type is an rvalue reference is an xvalue     end example       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 expressions  is an xvalue  a temporary object  12 2  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 prvalue  The value of a literal such as 12  7 3e5  or true is also a prvalue     end example   Every expression belongs to exactly one of the fundamental classifications in this taxonomy  lvalue  xvalue  or prvalue  This property of an expression is called its value category    Note  The discussion of each built-in operator in Clause 5 indicates the category of the value it yields and the value categories of the operands it expects  For example  the built-in assignment operators expect that the left operand is an lvalue and that the right operand is a prvalue and yield an lvalue as the result  User-defined operators are functions  and the categories of values they expect and yield are determined by their parameter and return types     end note  I suggest you read the entire section 3 10 Lvalues and rvalues though   How do these new categories relate to the existing rvalue and lvalue categories   Again    Are the rvalue and lvalue categories in C  0x the same as they are in C  03   The semantics of rvalues has evolved particularly with the introduction of move semantics   Why are these new categories needed   So that move construction assignment could be defined and supported

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IMHO  the best explanation about its meaning gave us Stroustrup   take into account examples of D  niel S  ndor and Mohan   Stroustrup      Now I was seriously worried  Clearly we were headed for an impasse or   a mess or both  I spent the lunchtime doing an analysis to see which   of the properties  of values  were independent  There were only two   independent properties          has identity     i e  and address  a pointer  the user can    determine whether two copies are identical  etc     can be moved from     i e  we are allowed to leave to source of a  copy  in some indeterminate  but valid state         This led me to the conclusion that there are exactly three kinds of   values  using the regex notational trick of using a capital letter to   indicate a negative     I was in a hurry           iM  has identity and cannot be moved from   im  has identity and can be moved from  e g  the result of casting an lvalue to a rvalue reference    Im  does not have identity and can be moved from         The fourth possibility  IM   doesn   t have identity and cannot be moved  is not   useful in C    or  I think  in any other language          In addition to these three fundamental classifications of values  we   have two obvious generalizations that correspond to the two   independent properties          i  has identity   m  can be moved from         This led me to put this diagram on the board          Naming      I observed that we had only limited freedom to name  The two points to   the left  labeled iM and i  are what people with more or less   formality have called lvalues and the two points on the right    labeled  m and Im  are what people with more or less formality   have called rvalues  This must be reflected in our naming  That is    the left  leg  of the W should have names related to lvalue and the   right  leg  of the W should have names related to rvalue  I note   that this whole discussion problem arise from the  introduction of   rvalue references and move semantics  These notions simply don   t exist   in Strachey   s world consisting of just rvalues and lvalues  Someone   observed that the ideas that         Every value is either an lvalue or an rvalue   An lvalue is not an rvalue and an rvalue is not an lvalue          are deeply embedded in our consciousness  very useful properties  and   traces of this dichotomy can be found all over the draft standard  We   all agreed that we ought to preserve those properties  and make them   precise   This further constrained our naming choices  I observed that   the standard library wording uses rvalue to mean m  the   generalization   so that to preserve the expectation and text of the   standard library the right-hand bottom point of the W should be named   rvalue       This led to a focused discussion of naming  First  we needed to decide   on lvalue  Should lvalue mean iM or the generalization i  Led   by Doug Gregor  we listed the places in the core language wording   where the word lvalue was qualified to mean the one or the other  A   list was made and in most cases  and in the most tricky brittle text   lvalue currently means iM  This is the classical meaning of lvalue   because  in the old days  nothing was moved  move is a novel notion   in C  0x  Also  naming the topleft point of the W lvalue gives us   the property that every value is an lvalue or an rvalue  but not both       So  the top left point of the W is lvalue and the bottom right point   is rvalue  What does that make the bottom left and top right points    The bottom left point is a generalization of the classical lvalue    allowing for move  So it is a generalized lvalue  We named it   glvalue  You can quibble about the abbreviation  but  I think  not   with the logic  We assumed that in serious use generalized lvalue   would somehow be abbreviated anyway  so we had better do it   immediately  or risk confusion   The top right point of the W is less   general than the bottom right  now  as ever  called rvalue   That   point represent the original pure notion of an object you can move   from because it cannot be referred to again  except by a destructor     I liked the phrase specialized rvalue in contrast to generalized   lvalue but pure rvalue abbreviated to prvalue won out  and   probably rightly so   So  the left leg of the W is lvalue and   glvalue and the right leg is prvalue and rvalue  Incidentally    every value is either a glvalue or a prvalue  but not both       This leaves the top middle of the W  im  that is  values that have   identity and can be moved  We really don   t have anything that guides   us to a good name for those esoteric beasts  They are important to   people working with the  draft  standard text  but are unlikely to   become a household name  We didn   t find any  real constraints on the   naming to guide us  so we picked    x    for the center  the unknown  the   strange  the xpert only  or even x-rated

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C  03 s categories are too restricted to capture the introduction of rvalue references correctly into expression attributes    With the introduction of them  it was said that an unnamed rvalue reference evaluates to an rvalue  such that overload resolution would prefer rvalue reference bindings  which would make it select move constructors over copy constructors  But it was found that this causes problems all around  for example with Dynamic Types and with qualifications    To show this  consider  int const amp  amp  f     int main       int  amp  amp i   f       disgusting      On pre-xvalue drafts  this was allowed  because in C  03  rvalues of non-class types are never cv-qualified  But it is intended that const applies in the rvalue-reference case  because here we do refer to objects    memory    and dropping const from non-class rvalues is mainly for the reason that there is no object around    The issue for dynamic types is of similar nature  In C  03  rvalues of class type have a known dynamic type - it s the static type of that expression  Because to have it another way  you need references or dereferences  which evaluate to an lvalue  That isn t true with unnamed rvalue references  yet they can show polymorphic behavior  So to solve it     unnamed rvalue references become xvalues  They can be qualified and potentially have their dynamic type different  They do  like intended  prefer rvalue references during overloading  and won t bind to non-const lvalue references   What previously was an rvalue  literals  objects created by casts to non-reference types  now becomes an prvalue  They have the same preference as xvalues during overloading   What previously was an lvalue stays an lvalue     And two groupings are done to capture those that can be qualified and can have different dynamic types  glvalues  and those where overloading prefers rvalue reference binding  rvalues

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I ll start with your last question      Why are these new categories needed     The C   standard contains many rules that deal with the value category of an expression  Some rules make a distinction between lvalue and rvalue  For example  when it comes to overload resolution  Other rules make a distinction between glvalue and prvalue  For example  you can have a glvalue with an incomplete or abstract type but there is no prvalue with an incomplete or abstract type  Before we had this terminology the rules that actually need to distinguish between glvalue prvalue referred to lvalue rvalue and they were either unintentionally wrong or contained lots of explaining and exceptions to the rule a la     unless the rvalue is due to unnamed rvalue reference      So  it seems like a good idea to just give the concepts of glvalues and prvalues their own name      What are these new categories of expressions    How do these new categories relate to the existing rvalue and lvalue categories    We still have the terms lvalue and rvalue that are compatible with C  98  We just divided the rvalues into two subgroups  xvalues and prvalues  and we refer to lvalues and xvalues as glvalues  Xvalues are a new kind of value category for unnamed rvalue references  Every expression is one of these three  lvalue  xvalue  prvalue  A Venn diagram would look like this                               X                                 l    x    pr                                  X               gl    r   Examples with functions   int   prvalue    int amp   lvalue    int amp  amp  xvalue      But also don t forget that named rvalue references are lvalues   void foo int amp  amp  t         t is initialized with an rvalue expression      but is actually an lvalue expression itself

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As the previous answers exhaustively covered the theory behind the value categories  there is just another thing I d like to add  you can actually play with it and test it   For some hands-on experimentation with the value categories  you can make use of the decltype specifier  Its behavior explicitly distinguishes between the three primary value categories  xvalue  lvalue  and prvalue    Using the preprocessor saves us some typing      Primary categories    define IS XVALUE X  std  is rvalue reference lt decltype  X   gt   value  define IS LVALUE X  std  is lvalue reference lt decltype  X   gt   value  define IS PRVALUE X   std  is reference lt decltype  X   gt   value   Mixed categories    define IS GLVALUE X   IS LVALUE X     IS XVALUE X    define IS RVALUE X   IS PRVALUE X     IS XVALUE X     Now we can reproduce  almost  all the examples from cppreference on value category   Here are some examples with C  17  for terse static assert    void doesNothing     struct S       int x 0      int x   1  int y   2  S s   static assert IS LVALUE x    static assert IS LVALUE x  y    static assert IS LVALUE  Hello world      static assert IS LVALUE   x     static assert IS PRVALUE 1    static assert IS PRVALUE x      static assert IS PRVALUE static cast lt double gt  x     static assert IS PRVALUE std  string      static assert IS PRVALUE throw std  exception      static assert IS PRVALUE doesNothing       static assert IS XVALUE std  move s        The next one doesn t work in gcc 8 2 but in gcc 9 1  Clang 7 0 0 and msvc 19 16 are doing fine  static assert IS XVALUE S   x       The mixed categories are kind of boring once you figured out the primary category   For some more examples  and experimentation   check out the following link on compiler explorer  Don t bother reading the assembly  though  I added a lot of compilers just to make sure it works across all the common compilers

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How do these new categories relate to the existing rvalue and lvalue categories    A C  03 lvalue is still a C  11 lvalue  whereas a C  03 rvalue is called a prvalue in C  11

[c++] What are rvalues, lvalues, xvalues, glvalues, and prvalues?

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