initialize a const array in a class initializer in C

Question

I have the following class in C     class a       const int b 2          other stuff follows         and here s the constructor     a void       The question is  how do I initialize b in the initialization list  given that I can t initialize it inside the body of the function of the constructor  because b is const   This doesn t work   a  a void         b  2 3             other initialization stuff     Edit   The case in point is when I can have different values for b for different instances  but the values are known to be constant for the lifetime of the instance

User · Accepted Answer

Like the others said  ISO C   doesn t support that  But you can workaround it  Just use std  vector instead   int  a   new int N      fill a  class C     const std  vector lt int gt  v  public    C   v a  a N

User · Answer

It is not possible in the current standard  I believe you ll be able to do this in C  0x using initializer lists  see A Brief Look at C  0x  by Bjarne Stroustrup  for more information about initializer lists and other nice C  0x features

User · Answer

How about emulating a const array via an accessor function   It s non-static  as you requested   and it doesn t require stl or any other library   class a       int privateB 2   public      a int b0 b1    privateB 0  b0  privateB 1  b1        int b const int idx    return privateB idx         Because a  privateB is private  it is effectively constant outside a    and you can access it similar to an array  e g   a aobj 2 3         initialize  constant array  b   n   aobj b 1       read b 1   write impossible from here    If you are willing to use a pair of classes  you could additionally protect privateB from member functions   This could be done by inheriting a  but I think I prefer John Harrison s comp lang c   post using a const class

User · Answer

You can t do that from the initialization list    Have a look at this   http   www cprogramming com tutorial initialization-lists-c   html

User · Answer

std  vector uses the heap   Geez  what a waste that would be just for the sake of a const sanity-check   The point of std  vector is dynamic growth at run-time  not any old syntax checking that should be done at compile-time   If you re not going to grow then create a class to wrap a normal array    include  lt stdio h gt    template  lt class Type  size t MaxLength gt  class ConstFixedSizeArrayFiller   private      size t length   public      ConstFixedSizeArrayFiller     length 0               virtual  ConstFixedSizeArrayFiller                virtual void Fill Type  array    0   protected      void add element Type  array  const Type  amp  element                if length  gt   MaxLength                   todo  throw more appropriate out-of-bounds exception             throw 0                    array length    element          length               template  lt class Type  size t Length gt  class ConstFixedSizeArray   private      Type array Length    public      explicit ConstFixedSizeArray          ConstFixedSizeArrayFiller lt Type  Length gt   amp  filler                 filler Fill array              const Type  Array   const           return array             size t ArrayLength   const           return Length             class a   private      class b filler   public ConstFixedSizeArrayFiller lt int  2 gt        public          virtual  b filler                        virtual void Fill int  array                add element array  87               add element array  96                         const ConstFixedSizeArray lt int  2 gt  b   public      a void    b b filler                 void print items             size t i          for i   0  i  lt  b ArrayLength    i                          printf   d n   b Array   i                         int main         a x      x print items        return 0      ConstFixedSizeArrayFiller and ConstFixedSizeArray are reusable   The first allows run-time bounds checking while initializing the array  same as a vector might   which can later become const after this initialization   The second allows the array to be allocated inside another object  which could be on the heap or simply the stack if that s where the object is   There s no waste of time allocating from the heap   It also performs compile-time const checking on the array   b filler is a tiny private class to provide the initialization values   The size of the array is checked at compile-time with the template arguments  so there s no chance of going out of bounds   I m sure there are more exotic ways to modify this   This is an initial stab   I think you can pretty much make up for any of the compiler s shortcoming with classes

User · Answer

With C  11 the answer to this question has now changed and you can in fact do   struct a       const int b 2          other bits follow         and here s the constructor     a        a  a         b 2 3            other constructor work    int main      a a

User · Answer

interestingly  in C  you have the keyword const that translates to C   s static const  as opposed to readonly which can be only set at constructors and initializations  even by non-constants  ex   readonly DateTime a   DateTime Now    I agree  if you have a const pre-defined array you might as well make it static  At that point you can use this interesting syntax     in header file class a      static const int SIZE      static const char array   10        in cpp file  const int a  SIZE   5  const char array SIZE  10      hello    cruel   world   goodbye           however  I did not find a way around the constant  10   The reason is clear though  it needs it to know how to perform accessing to the array  A possible alternative is to use  define  but I dislike that method and I  undef at the end of the header  with a comment to edit there at CPP as well in case if a change

User · Answer

ISO standard C   doesn t let you do this   If it did  the syntax would probably be   a  a void    b  2 3            other initialization stuff     Or something along those lines   From your question it actually sounds like what you want is a constant class  aka static  member that is the array   C   does let you do this   Like so    include  lt iostream gt   class A    public      A        static const int a 2       const int A  a 2     0  1    A  A        int main  int argc  char   const argv           std  cout  lt  lt   A  a   gt     lt  lt  A  a 0   lt  lt        lt  lt  A  a 1   lt  lt    n       return 0      The output being   A  a   gt  0  1   Now of course since this is a static class member it is the same for every instance of class A   If that is not what you want  ie you want each instance of A to have different element values in the array a then you re making the mistake of trying to make the array const to begin with   You should just be doing this    include  lt iostream gt   class A    public      A        int a 2       A  A         a 0    9     or some calculation     a 1    10     or some calculation    int main  int argc  char   const argv           A v      std  cout  lt  lt   v a   gt     lt  lt  v a 0   lt  lt        lt  lt  v a 1   lt  lt    n       return 0

User · Answer

A solution without using the heap with std  vector is to use boost  array  though you can t initialize array members directly in the constructor    include  lt boost array hpp gt   const boost  array lt int  2 gt  aa     2  3      class A       const boost  array lt int  2 gt  b      A   b aa

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Where I ve a constant array  it s always been done as static   If you can accept that  this code should compile and run    include  lt stdio h gt   include  lt stdlib h gt   class a           static const int b 2   public          a void                    for int i   0  i  lt  2  i                              printf  b  d      d  n   i  b i                                    const int a  b 2      4  2     int main int argc  char   argv            a foo          return 0

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