Initializing a member array in constructor initializer

Question

class C    public   C     arr  1 2 3     doesn t compile               C     arr 1 2 3    doesn t compile either           private   int arr 3        I believe the reason is that arrays can be initialized only with   syntax  that is   int arr 3     1 3 4     Questions   How can I do what I want to do  that is  initialize an array in a constructor  not assigning elements in the body    Is it even possible     Does the C  03 standard say anything special about initializing aggregates  including arrays  in ctor initializers  Or the invalidness of the above code is a corollary of some other rules    Do C  0x initializer lists solve the problem    P S  Please do not mention vectors  boost  arrays  and their superiority to arrays  which I am well aware of

User · Answer

Workaround   template lt class T  size t N gt  struct simple array      like std  array in C  0x    T arr N        class C   private simple array lt int  3 gt           static simple array lt int  3 gt  myarr                simple array lt int  3 gt  arr    1 2 3              return arr          public        C     simple array lt int  3 gt  myarr

User · Answer

How can I do what I want to do  that is  initialize an array in a constructor  not assigning elements in the body    Is it even possible        Yes  It s using a struct that contains an array  You say you already know about that  but then I don t understand the question  That way  you do initialize an array in the constructor  without assignments in the body  This is what boost  array does       Does the C  03 standard say anything special about initializing aggregates  including arrays  in ctor initializers  Or the invalidness of the above code is a corollary of some other rules     A mem-initializer uses direct initialization  And the rules of clause 8 forbid this kind of thing  I m not exactly sure about the following case  but some compilers do allow it    struct A     char foo 6     A   foo  hello          valid          See this GCC PR for further details       Do C  0x initializer lists solve the problem    Yes  they do  However your syntax is invalid  I think  You have to use braces directly to fire off list initialization  struct A     int foo 3     A   foo 1  2  3        A   foo  1  2  3          invalid

User · Answer

You want to init an array of ints in your constructor  Point it to a static array    class C    public      int  cArray       C  C       static int c init   1 2 3       cArray   c init

User · Answer

How about        C     arr   1 2 3                Compiles fine on g   4 8

User · Answer

No  unfortunately  You just can t in the way you want  as it s not allowed by the grammar  more below    You can only use ctor-like initialization  and  as you know  that s not available for initializing each item in arrays  I believe so  as they generalize initialization across the board in many useful ways   But I m not sure on the details    In C  03  aggregate initialization only applies with syntax similar as below  which must be a separate statement and doesn t fit in a ctor initializer   T var

User · Answer

C  98 doesn t provide a direct syntax for anything but zeroing  or for non-POD elements  value-initializing  the array  For that you just write C    arr        I thing Roger Pate is wrong about the alleged limitations of C  0x aggregate initialization  but I m too lazy to look it up or check it out  and it doesn t matter  does it  EDIT  Roger was talking about  C  03   I misread it as  C  0x   Sorry  Roger     A C  98 workaround for your current code is to wrap the array in a struct and initialize it from a static constant of that type  The data has to reside somewhere anyway  Off the cuff it can look like this   class C    public      C     arr  arrData       private       struct Arr  int elem 3           Arr arr       static Arr const arrData      C  Arr const C  arrData     1  2  3

[c++] Initializing a member array in constructor initializer

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