Pointer to class data member

Question

I came across this strange code snippet which compiles fine   class Car       public      int speed      int main         int Car   pSpeed    amp Car  speed      return 0      Why does C   have this pointer to a non-static data member of a class  What is the use of this strange pointer in real code

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You can later access this member  on any instance   int main           int Car   pSpeed    amp Car  speed        Car myCar    Car yourCar     int mySpeed   myCar  pSpeed    int yourSpeed   yourCar  pSpeed     assert mySpeed  gt  yourSpeed       -     return 0      Note that you do need an instance to call it on  so it does not work like a delegate  It is used rarely  I ve needed it maybe once or twice in all my years   Normally using an interface  i e  a pure base class in C    is the better design choice

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IBM has some more documentation on how to use this  Briefly  you re using the pointer as an offset into the class  You can t use these pointers apart from the class they refer to  so     int Car   pSpeed    amp Car  speed    Car mycar    mycar  pSpeed   65    It seems a little obscure  but one possible application is if you re trying to write code for deserializing generic data into many different object types  and your code needs to handle object types that it knows absolutely nothing about  for example  your code is in a library  and the objects into which you deserialize were created by a user of your library   The member pointers give you a generic  semi-legible way of referring to the individual data member offsets  without having to resort to typeless void   tricks the way you might for C structs

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I think you d only want to do this if the member data was pretty large  e g   an object of another pretty hefty class   and you have some external routine which only works on references to objects of that class  You don t want to copy the member object  so this lets you pass it around

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It s a  pointer to member  - the following code illustrates its use    include  lt iostream gt  using namespace std   class Car       public      int speed      int main         int Car   pSpeed    amp Car  speed       Car c1      c1 speed   1           direct access     cout  lt  lt   speed is    lt  lt  c1 speed  lt  lt  endl      c1  pSpeed   2         access via pointer to member     cout  lt  lt   speed is    lt  lt  c1 speed  lt  lt  endl      return 0      As to why you would want to do that  well it gives you another level of indirection that can solve some tricky problems  But to be honest  I ve never had to use them in my own code   Edit  I can t think off-hand of a convincing use for pointers to member data  Pointer to member functions can be used  in pluggable architectures  but once again producing an example in a small space defeats me  The following is my best  untested  try - an Apply function that would do some pre  amp post processing before applying a user-selected member function to an object   void Apply  SomeClass   c  void  SomeClass   func               do hefty pre-call processing      c- gt  func         call user specified function        do hefty post-call processing     The parentheses around c- gt  func are necessary because the - gt   operator has lower precedence than the function call operator

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A realworld example of a pointer-to-member could be a more narrow aliasing constructor for std  shared ptr  template  lt typename T gt  template  lt typename U gt  shared ptr lt T gt   shared ptr const shared ptr lt U gt   T U   member    What that constructor would be good for assume you have a struct foo  struct foo       int ival      float fval      If you have given a shared ptr to a foo  you could then retrieve shared ptr s to its members ival or fval using that constructor  auto foo shared   std  make shared lt foo gt     auto ival shared   std  shared ptr lt int gt  foo shared   amp foo  ival    This would be useful if want to pass the pointer foo shared- gt ival to some function which expects a shared ptr https   en cppreference com w cpp memory shared ptr shared ptr

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Just to add some use cases for  anon s  amp   Oktalist s answer  here s a great reading material about pointer-to-member-function and pointer-to-member-data   https   www dre vanderbilt edu  schmidt PDF C  -ptmf4 pdf

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Here is an example where pointer to data members could be useful    include  lt iostream gt   include  lt list gt   include  lt string gt   template  lt typename Container  typename T  typename DataPtr gt  typename Container  value type searchByDataMember  const Container amp  container  const T amp  t  DataPtr ptr        for  const typename Container  value type amp  x   container            if  x- gt  ptr    t              return x            return typename Container  value type       struct Object       int ID  value      std  string name      Object  int i  int v  const std  string amp  n    ID i   value v   name n         std  list lt Object  gt  objects   new Object 5 6  Sam    new Object 11 7  Mark    new Object 9 12  Rob        new Object 2 11  Tom    new Object 15 16  John       int main         const Object  object   searchByDataMember  objects  11   amp Object  value       std  cout  lt  lt  object- gt name  lt  lt    n       Tom

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Pointers to classes are not real pointers  a class is a logical construct and has no physical existence in memory  however  when you construct a pointer to a member of a class it gives an offset into an object of the member s class where the member can be found  This gives an important conclusion  Since static members are not associated with any object so a pointer to a member CANNOT point to a static member data or functions  whatsoever Consider the following   class x   public      int val      x int i    val   i        int get val     return val        int d val int i   return i i        int main         int  x    data     amp x  val                  pointer to data member     int  x    func  int     amp x  d val           pointer to function member      x ob1 1   ob2 2        cout  lt  lt ob1  data      cout  lt  lt ob2  data       cout  lt  lt  ob1  func  ob1  data       cout  lt  lt  ob2  func  ob2  data         return 0      Source  The Complete Reference C   - Herbert Schildt 4th Edition

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Another application are intrusive lists  The element type can tell the list what its next prev pointers are  So the list does not use hard-coded names but can still use existing pointers      say this is some existing structure  And we want to use    a list  We can tell it that the next pointer    is apple  next  struct apple       int data      apple   next         simple example of a minimal intrusive list  Could specify the    member pointer as template argument too  if we wanted     template lt typename E  E  E   next ptr gt  template lt typename E gt  struct List       List E  E   next ptr  head 0   next ptr next ptr           void add E  amp e               access its next pointer by the member pointer         e  next ptr   head          head    amp e             E   head      E  E   next ptr      int main         List lt apple gt  lst  amp apple  next        apple a      lst add a

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You can use an array of pointer to  homogeneous  member data to enable a dual  named-member  i e  x data  and array-subscript  i e  x idx   interface    include  lt cassert gt   include  lt cstddef gt   struct vector3       float x      float y      float z       float amp  operator   std  size t idx            static float vector3   component 3                   amp vector3  x   amp vector3  y   amp vector3  z                    return this- gt  component idx             int main         vector3 v     0 0f  1 0f  2 0f         assert  amp v 0      amp v x       assert  amp v 1      amp v y       assert  amp v 2      amp v z        for  std  size t i   0  i  lt  3    i            v i     1 0f             assert v x    1 0f       assert v y    2 0f       assert v z    3 0f        return 0

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Pointer to members are C   s type safe equivalent for C s offsetof    which is defined in stddef h  Both return the information  where a certain field is located within a class or struct  While offsetof   may be used with certain simple enough classes also in C    it fails miserably for the general case  especially with virtual base classes  So pointer to members were added to the standard  They also provide easier syntax to reference an actual field  struct C   int a  int b    c  int C    intptr    amp C  a           or  amp C  b  depending on the field wanted c  intptr    1   is much easier than  struct C   int a  int b    c  int intoffset   offsetof struct C  a      int       char     void     amp c    intoffset     1   As to why one wants to use offsetof    or pointer to members   there are good answers elsewhere on stackoverflow  One example is here  How does the C offsetof macro work

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Suppose you have a structure  Inside of that structure are   some sort of name   two variables of the same type but with different meaning  struct foo       std  string a      std  string b       Okay  now let s say you have a bunch of foos in a container      key  some sort of name  value  a foo instance std  map lt std  string  foo gt  container    Okay  now suppose you load the data from separate sources  but the data is presented in the same fashion  eg  you need the same parsing method    You could do something like this   void readDataFromText std  istream  amp  input  std  map lt std  string  foo gt   amp  container  std  string foo   storage        std  string line  name  value          while lines are successfully retrieved     while  std  getline input  line             std  stringstream linestr line           if   line empty                   continue                        retrieve name and value         linestr  gt  gt  name  gt  gt  value              store value into correct storage  whichever one is correct         container name   storage   value           std  map lt std  string  foo gt  readValues         std  map lt std  string  foo gt  foos       std  ifstream a  input-a        readDataFromText a  foos   amp foo  a       std  ifstream b  input-b        readDataFromText b  foos   amp foo  b       return foos      At this point  calling readValues   will return a container with a unison of  input-a  and  input-b   all keys will be present  and foos with have either a or b or both

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One way I ve used it is if I have two implementations of how to do something in a class and I want to choose one at run-time without having to continually go through an if statement i e   class Algorithm   public      Algorithm     m impFn   amp Algorithm  implementationA          void frequentlyCalled                    Avoid if   using A   else if   using B   type of thing          this- gt  m impFn           private      void implementationA                   void implementationB                    typedef void   Algorithm   IMP FN           IMP FN m impFn       Obviously this is only practically useful if you feel the code is being hammered enough that the if statement is slowing things done eg  deep in the guts of some intensive algorithm somewhere  I still think it s more elegant than the if statement even in situations where it has no practical use but that s just my opnion

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It makes it possible to bind member variables and functions in the uniform manner  The following is example with your Car class  More common usage would be binding std  pair  first and   second when using in STL algorithms and Boost on a map    include  lt list gt   include  lt algorithm gt   include  lt iostream gt   include  lt iterator gt   include  lt boost lambda lambda hpp gt   include  lt boost lambda bind hpp gt    class Car   public      Car int s   speed s         void drive             std  cout  lt  lt   Driving at    lt  lt  speed  lt  lt    km h   lt  lt  std  endl            int speed      int main          using namespace std      using namespace boost  lambda       list lt Car gt  l      l push back Car 10        l push back Car 140        l push back Car 130        l push back Car 60            Speeding cars     list lt Car gt  s          Binding a value to a member variable         Find all cars with speed over 60 km h      remove copy if l begin    l end                       back inserter s                      bind  amp Car  speed   1   lt   60           Binding a value to a member function         Call a function on each car      for each s begin    s end    bind  amp Car  drive   1         return 0

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Here s a real-world example I am working on right now  from signal processing   control systems   Suppose you have some structure that represents the data you are collecting   struct Sample       time t time      double value1      double value2      double value3       Now suppose that you stuff them into a vector   std  vector lt Sample gt  samples      fill the vector       Now suppose that you want to calculate some function  say the mean  of one of the variables over a range of samples  and you want to factor this mean calculation into a function   The pointer-to-member makes it easy   double Mean std  vector lt Sample gt   const iterator begin       std  vector lt Sample gt   const iterator end      double Sample    var        float mean   0      int samples   0      for   begin    end  begin              const Sample amp  s    begin          mean    s  var          samples              mean    samples      return mean         double mean   Mean samples begin    samples end     amp Sample  value2     Note Edited 2016 08 05 for a more concise template-function approach  And  of course  you can template it to compute a mean for any forward-iterator and any value type that supports addition with itself and division by size t   template lt typename Titer  typename S gt  S mean Titer begin  const Titer amp  end  S std  iterator traits lt Titer gt   value type    var        using T   typename std  iterator traits lt Titer gt   value type      S sum   0      size t samples   0      for    begin    end     begin             const T amp  s    begin          sum    s  var          samples              return sum   samples     struct Sample       double x     std  vector lt Sample gt  samples    1 0    2 0    3 0     double m   mean samples begin    samples end     amp Sample  x     EDIT - The above code has performance implications  You should note  as I soon discovered  that the code above has some serious performance implications   The summary is that if you re calculating a summary statistic on a time series  or calculating an FFT etc  then you should store the values for each variable contiguously in memory   Otherwise  iterating over the series will cause a cache miss for every value retrieved   Consider the performance of this code   struct Sample     float w  x  y  z      std  vector lt Sample gt  series         float sum   0  int samples   0  for auto it   series begin    it    series end    it        sum     it x    samples      float mean   sum   samples    On many architectures  one instance of Sample will fill a cache line   So on each iteration of the loop  one sample will be pulled from memory into the cache   4 bytes from the cache line will be used and the rest thrown away  and the next iteration will result in another cache miss  memory access and so on   Much better to do this   struct Samples     std  vector lt float gt  w  x  y  z      Samples series         float sum   0  float samples   0  for auto it   series x begin    it    series x end    it        sum     it    samples      float mean   sum   samples    Now when the first x value is loaded from memory  the next three will also be loaded into the cache  supposing suitable alignment   meaning you don t need any values loaded for the next three iterations   The above algorithm can be improved somewhat further through the use of SIMD instructions on eg SSE2 architectures   However  these work much better if the values are all contiguous in memory and you can use a single instruction to load four samples together  more in later SSE versions    YMMV - design your data structures to suit your algorithm

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This is the simplest example I can think of that conveys the rare cases where this feature is pertinent     include  lt iostream gt   class bowl   public      int apples      int oranges      int count fruit bowl   begin  bowl   end  int bowl   fruit        int count   0      for  bowl   iterator   begin  iterator    end     iterator          count    iterator- gt  fruit      return count     int main         bowl bowls 2                1  2              3  5              std  cout  lt  lt   I have    lt  lt  count fruit bowls  bowls   2   amp  bowl  apples   lt  lt    apples n       std  cout  lt  lt   I have    lt  lt  count fruit bowls  bowls   2   amp  bowl  oranges   lt  lt    oranges n       return 0      The thing to note here is the pointer passed in to count fruit  This saves you having to write separate count apples and count oranges functions

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