Why are arrays of references illegal

Question

The following code does not compile   int a   1  b   2  c   3  int amp  arr      a b c 8     What does the C   standard say about this   I know I could declare a class that contains a reference  then create an array of that class  as shown below   But I really want to know why the code above doesn t compile   struct cintref       cintref const int  amp  ref    ref ref         operator const int  amp      return ref    private      const int  amp  ref      void operator  const cintref  amp        int main        int a 1 b 2 c 3      typedef const int  amp   cintref    cintref arr      a b c 8       It is possible to use struct cintref instead of const int  amp  to simulate an array of references

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When you store something in an array   its size needs to be known  since array indexing relies on the size   Per the C   standard It is unspecified whether or not a reference requires storage  as a result indexing an array of references would not be possible

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Given int amp  arr      a b c 8    what is sizeof  arr     Everywhere else  a reference is treated as being simply the thing itself  so sizeof  arr  should simply be sizeof int   But this would make array pointer arithmetic on this array wrong  assuming that references are not the same widths is ints   To eliminate the ambiguity  it s forbidden

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A reference object has no size  If you write sizeof referenceVariable   it will give you the size of the object referenced by referenceVariable  not that of the reference itself  It has no size of its own  which is why the compiler can t calculate how much size the array would require

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Just to add to all the conversation  Since arrays requires consecutive memory locations to store the item  so if we create an array of references then it s not guaranteed that they will be at consecutive memory location so accessing will be a problem and hence we can t even apply all the mathematical operations on array

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Comment to your edit   Better solution is std  reference wrapper   Details  http   www cplusplus com reference functional reference wrapper   Example    include  lt iostream gt   include  lt functional gt  using namespace std   int main         int a 1 b 2 c 3 d 4      using intlink   std  reference wrapper lt int gt       intlink arr      a b c d       return 0

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Because like many have said here  references are not objects  they are simply aliases  True some compilers might implement them as pointers  but the standard does not force specify that  And because references are not objects  you cannot point to them  Storing elements in an array means there is some kind of index address  i e   pointing to elements at a certain index   and that is why you cannot have arrays of references  because you cannot point to them   Use boost  reference wrapper  or boost  tuple instead  or just pointers

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This is an interesting discussion  Clearly arrays of refs are outright illegal  but IMHO the reason why is not so simple as saying  they are not objects  or  they have no size   I d point out that arrays themselves are not full-fledged objects in C C   - if you object to that  try instantiating some stl template classes using an array as a  class  template parameter  and see what happens  You can t return them  assign them  pass them as parameters    an array param is treated as a pointer   But it is legal to make arrays of arrays  References do have a size that the compiler can and must calculate - you can t sizeof   a reference  but you can make a struct containing nothing but references  It will have a size sufficient to contain all the pointers which implement the references  You can t instantiate such a struct without initializing all the members   struct mys    int  amp  a   int  amp  b   int  amp  c         int ivar1  ivar2  arr 200   mys my refs     ivar1  ivar2  arr 12      my refs a    3        add 3 to ivar1   In fact you can add this line to the struct definition  struct mys         int  amp  operator    int i     return i  0 a   i  1  b   c            and now I have something which looks a LOT like an array of refs   int ivar1  ivar2  arr 200   mys my refs     ivar1  ivar2  arr 12      my refs 1    my refs 2         copy arr 12  to ivar2  amp my refs 0                    gives  amp my refs a     amp ivar1   Now  this is not a real array  it s an operator overload  it won t do things that arrays normally do like sizeof arr  sizeof arr 0    for instance  But it does exactly what I want an array of references to do  with perfectly legal C    Except  a  it s a pain to set up for more than 3 or 4 elements  and  b  it s doing a calculation using a bunch of    which could be done using indexing  not with normal C-pointer-calculation-semantics indexing  but indexing nonetheless   I d like to see a very limited  array of reference  type which can actually do this  I e  an array of references would not be treated as a general array of things which are references  but rather it would be a new  array-of-reference  thing which effectively maps to an internally generated class similar to the one above  but which you unfortunately can t make with templates    this would probably work  if you don t mind this kind of nasty  recast   this  as an array of int   s and return a reference made from one   not recommended  but it shows how the proper  array  would work     int  amp  operator    int i     return   reinterpret cast lt int   gt  this  i

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You can get fairly close with this template struct  However  you need to initialize with expressions that are pointers to T  rather than T  and so  though you can easily make a  fake constref array  similarly  you won t be able to bind that to rvalues as done in the OP s example   8      include  lt stdio h gt   template lt class T  int N gt   struct fake ref array      T   ptrs N     T  amp  operator      int i    return  ptrs i         int A B X 3    void func  int j  int k      fake ref array lt int 3 gt  refarr      amp A   amp B   amp X 1       refarr j    k       -         You could probably make the following work using an overload of   that returns       a proxy that overloads    Still not a real array though  so it would just be       stunt programming at that point          refarr   j    k      int main         func 1 7      B   7     func 2 8          X 1    8     printf  A  d B  d X     d  d  d  n   A B X 0  X 1  X 2            return 0      --  A 0 B 7 X    0 8 0

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An array is implicitly convertable to a pointer  and pointer-to-reference is illegal in C

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Actually  this is a mixture of C and C   syntax   You should either use pure C arrays  which cannot be of references  since reference are part of C   only  Or you go the C   way and use the std  vector or std  array class for your purpose   As for the edited part  Even though the struct is an element from C  you define a constructor and operator functions  which make it a C   class  Consequently  your struct would not compile in pure C

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Answering to your question about standard I can cite the C   Standard  sect 8 3 2 4      There shall be no references to references  no arrays of references  and no pointers to references

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I believe the answer is very simple and it has to do with semantic rules of references and how arrays are handled in C     In short  References can be thought of as structs which don t have a default constructor  so all the same rules apply   1  Semantically  references don t have a default value  References can only be created by referencing something  References don t have a value to represent the absence of a reference   2  When allocating an array of size X  program creates a collection of default-initialized objects  Since reference doesn t have a default value  creating such an array is semantically illegal   This rule also applies to structs classes which don t have a default constructor  The following code sample doesn t compile   struct Object       Object int value          Object objects 1      Error  no appropriate default constructor available

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Consider an array of pointers  A pointer is really an address  so when you initialize the array  you are analogously telling the computer   allocate this block of memory to hold these X numbers  which are addresses of other items    Then if you change one of the pointers  you are just changing what it points to  it is still a numerical address which is itself sitting in the same spot   A reference is analogous to an alias  If you were to declare an array of references  you would basically be telling the computer   allocate this amorphous blob of memory consisting of all these different items scattered around

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References are not objects  They don t have storage of their own  they just reference existing objects  For this reason it doesn t make sense to have arrays of references   If you want a light-weight object that references another object then you can use a pointer  You will only be able to use a struct with a reference member as objects in arrays if you provide explicit initialization for all the reference members for all struct instances  References cannot be default initalized   Edit  As jia3ep notes  in the standard section on declarations there is an explicit prohibition on arrays of references

[c++] Why are arrays of references illegal?

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