C constructor initializer for arrays

Question

I m having a brain cramp    how do I initialize an array of objects properly in C     non-array example   struct Foo   Foo int x                     struct Bar         Foo foo        Bar     foo 4          array example   struct Foo   Foo int x                     struct Baz         Foo foo 3                I know the following syntax is wrong  but what s correct       Baz     foo 0  4   foo 1  5   foo 2  6          edit  Wild  amp  crazy workaround ideas are appreciated  but they won t help me in my case  I m working on an embedded processor where std  vector and other STL constructs are not available  and the obvious workaround is to make a default constructor and have an explicit init   method that can be called after construction-time  so that I don t have to use initializers at all   This is one of those cases where I ve gotten spoiled by Java s final keyword   flexibility with constructors

User · Answer

Just to update this question for C  11  this is now both possible to do and very natural   struct Foo   Foo int x                     struct Baz         Foo foo 3         Baz     foo  4    5    6            Those braces can also be elided for an even more concise   struct Baz         Foo foo 3         Baz     foo 4  5  6           Which can easily be extended to multi-dimensional arrays too   struct Baz       Foo foo 3  2        Baz     foo 1  2  3  4  5  6

User · Answer

There is no array-construction syntax that ca be used in this context  at least not directly   You can accomplish what you re trying to accomplish by something along the lines of   Bar  Bar         static const int inits       4 5 6       static const size t numInits   sizeof inits  sizeof inits 0        std  copy  amp inits 0   amp inits numInits  foo       be careful that there are enough slots in foo        but you ll need to give Foo a default constructor

User · Answer

in visual studio 2012 or above  you can do like this  struct Foo   Foo int x                     struct Baz         Foo foo 3         Baz     foo

User · Answer

You can use C  0x auto keyword together with template specialization on for example a function named boost  make array    similar to make pair     For the case of where N is either 1 or 2 arguments we can then write variant A as  namespace boost       Construct Array from  p a     template  lt typename T gt  boost  array lt T 1 gt  make array const T  amp  a        return boost  array lt T 2 gt      a            Construct Array from  p a   p b     template  lt typename T gt  boost  array lt T 2 gt  make array const T  amp  a  const T  amp  b        return boost  array lt T 2 gt      a  b            and variant B as  namespace boost       Construct Array from  p a     template  lt typename T gt  boost  array lt T 1 gt  make array const T  amp  a        boost  array lt T 1 gt  x      x 0    a      return x        Construct Array from  p a   p b     template  lt typename T gt  boost  array lt T 2 gt  make array const T  amp  a  const T  amp  b        boost  array lt T 2 gt  x      x 0    a      x 1    b      return x        GCC-4 6 with -std gnu  0x and -O3 generates the exact same binary code for  auto x   boost  make array 1 2     using both A and B as it does for  boost  array lt int  2 gt  x     1 2      For user defined types  UDT   though  variant B results in an extra copy constructor  which usually slow things down  and should therefore be avoided   Note that boost  make array errors when calling it with explicit char array literals as in the following case  auto x   boost  make array  a   b      I believe this is a good thing as const char  literals can be deceptive in their use   Variadic templates  available in GCC since 4 5  can further be used reduce all template specialization boiler-plate code for each N into a single template definition of boost  make array   defined as      Construct Array from  p a   p b     template  lt typename T  typename     R gt  boost  array lt T 1 sizeof    R  gt  make array T a  const R  amp      b        return boost  array lt T 1 sizeof    R  gt     a  b             This works pretty much as we expect  The first argument determines boost  array template argument T and all other arguments gets converted into T  For some cases this may undesirable  but I m not sure how if this is possible to specify using variadic templates   Perhaps boost  make array   should go into the Boost Libraries

User · Answer

You can do it  but it s not pretty    include  lt iostream gt   class A       int mvalue  public      A int value    mvalue value         int value     return mvalue        class B          TODO  hack that respects alignment of A   maybe C  14 s alignof      char  hack sizeof A 3         A  marr  public      B     marr reinterpret cast lt A  gt   hack             new   amp marr 0   A 5           new   amp marr 1   A 6           new   amp marr 2   A 7              A  arr     return marr        int main int argc  char   argv        B b      A  arr   b arr        std  cout  lt  lt  arr 0  value    lt  lt       lt  lt  arr 1  value    lt  lt       lt  lt  arr 2  value    lt  lt    n       return 0      If you put this in your code  I hope you have a VERY good reason

User · Answer

In the specific case when the array is a data member of the class you can t initialize it in the current version of the language  There s no syntax for that  Either provide a default constructor for array elements or use std  vector   A standalone array can be initialized with aggregate initializer  Foo foo 3      4  5  6      but unfortunately there s no corresponding syntax for the constructor initializer list

User · Answer

class C      static const int myARRAY 10       only declaration         public     C           const int C  myARRAY 10   0 1 2 3 4 5 6 7 8 9       here is definition  int main void       C myObj

User · Answer

This seems to work  but I m not convinced it s right    include  lt iostream gt   struct Foo   int x  Foo int x   x x          struct Baz         Foo foo 3        static int bar 3           Hmm         Baz     foo bar         int Baz  bar 3     4  5  6    int main         Baz z      std  cout  lt  lt  z foo 1  x  lt  lt    n       Output     make arrayinit -B CXXFLAGS -pedantic  amp  amp    arrayinit g   -pedantic    arrayinit cpp   -o arrayinit 5   Caveat emptor   Edit  nope  Comeau rejects it   Another edit  This is kind of cheating  it just pushes the member-by-member array initialization to a different place  So it still requires Foo to have a default constructor  but if you don t have std  vector then you can implement for yourself the absolute bare minimum you need    include  lt iostream gt   struct Foo        int x       Foo int x   x x            Foo            very stripped-down replacement for vector struct Three        Foo data 3        Three int d0  int d1  int d2            data 0    d0          data 1    d1          data 2    d2            Foo  amp operator   int idx    return data idx         const Foo  amp operator   int idx  const   return data idx         struct Baz        Three foo       static Three bar         construct foo using the copy ctor of Three with bar as parameter      Baz     foo bar            or get rid of  bar  entirely and do this     Baz bool    foo 4 5 6         Three Baz  bar 4 5 6    int main         Baz z      std  cout  lt  lt  z foo 1  x  lt  lt    n       z foo isn t actually an array  but it looks about as much like one as a vector does  Adding begin   and end   functions to Three is trivial

User · Answer

Unfortunately there is no way to initialize array members till C  0x   You could use a std  vector and push back the Foo instances in the constructor body   You could give Foo a default constructor  might be private and making Baz a friend    You could use an array object that is copyable  boost or std  tr1  and initialize from a static array    include  lt boost array hpp gt   struct Baz        boost  array lt Foo  3 gt  foo      static boost  array lt Foo  3 gt  initFoo      Baz     foo initFoo                   boost  array lt Foo  3 gt  Baz  initFoo     4  5  6

User · Answer

There is no way   You need a default constructor for array members and it will be called  afterwards  you can do any initialization you want in the constructor

User · Answer

Right now  you can t use the initializer list for array members   You re stuck doing it the hard way   class Baz       Foo foo 3        Baz             foo 0    Foo 4           foo 1    Foo 5           foo 2    Foo 6              In C  0x you can write   class Baz       Foo foo 3        Baz     foo  4  5  6

User · Answer

Only the default constructor can be called when creating objects in an array

User · Answer

This is my solution for your reference   struct Foo       Foo      used to make compiler happy      Foo int x               struct Bar       Foo foo 3        Bar                   initialize foo array here          for int i 0 i lt 3   i                        foo i  Foo 4 i

User · Answer

Ideas from a twisted mind    class mytwistedclass  static std  vector lt int gt  initVector  mytwistedclass           initialise with initVector 0  and then delete it  -          now set this initVector to something u want to before u instantiate an object  Then your objects are initialized with your parameters

[c++] C++: constructor initializer for arrays

Examples related to c++

Examples related to constructor

Examples related to initializer