How to return an array from a function

Question

How can I return an array from a method  and how must I declare it   int   test void

User · Answer

It is not possible to return an array from a C++ function. 8.3.5[dcl.fct]/6:

Functions shall not have a return type of type array or function[...]

Most commonly chosen alternatives are to return a value of class type where that class contains an array, e.g.

struct ArrayHolder
{
    int array[10];
};

ArrayHolder test();

Or to return a pointer to the first element of a statically or dynamically allocated array, the documentation must indicate to the user whether he needs to (and if so how he should) deallocate the array that the returned pointer points to.

E.g.

int* test2()
{
    return new int[10];
}

int* test3()
{
    static int array[10];
    return array;
}

While it is possible to return a reference or a pointer to an array, it's exceedingly rare as it is a more complex syntax with no practical advantage over any of the above methods.

int (&test4())[10]
{
        static int array[10];
        return array;
}

int (*test5())[10]
{
        static int array[10];
        return &array;
}

User · Answer

how can i return a array in a c   method and how must i declare it    int   test void         template  lt class X gt    class Array     X      m data    int    m size  public         there constructor  destructor  some methods     int Get X   amp  null pointer                if   null pointer                         null pointer   new X  m size               memcpy  null pointer  m data  m size   sizeof X                return m size                   return 0              just for int  class IntArray     int    m data    int    m size  public         there constructor  destructor  some methods     int Get int   amp  null pointer                if   null pointer                         null pointer   new int  m size               memcpy  null pointer  m data  m size   sizeof int                return m size                   return 0              example  Array lt float gt  array  float   n data   NULL  int     data size  if data size   array Get n data            work with array       delete    n data    example for int  IntArray   array  int        n data   NULL  int        data size  if data size   array Get n data         work with array     delete    n data

User · Answer

int  test     but it would be  more C    to use vectors   std  vector lt  int  gt  test     EDIT I ll clarify some point  Since you mentioned C    I ll go with new   and delete   operators  but it s the same with malloc free   In the first case  you ll write something like   int  test         return new int size needed       but it s not a nice idea because your function s client doesn t really know the size of the array you are returning  although the client can safely deallocate it with a call to delete     int  theArray   test    for  size t i  i  lt         i       I don t know what is the array size               delete   theArray     ok    A better signature would be this one   int  test size t amp  arraySize        array size   10      return new int array size       And your client code would now be   size t theSize   0  int  theArray   test theSize   for  size t i  i  lt  theSize    i       now I can safely iterate the array              delete   theArray     still ok    Since this is C    std  vector lt T gt  is a widely-used solution   std  vector lt int gt  test         std  vector lt int gt  vector 10       return vector      Now you don t have to call delete    since it will be handled by the object  and you can safely iterate it with   std  vector lt int gt  v   test    std  vector lt int gt   iterator it   v begin    for    it    v end      it          do your things     which is easier and safer

User · Answer

how can i return a array in a c   method and how must i declare it  int   test void        This sounds like a simple question  but in C   you have quite a few options   Firstly  you should prefer      std  vector lt  gt   which grows dynamically to however many elements you encounter at runtime  or std  array lt  gt   introduced with C  11   which always stores a number of elements specified at compile time       as they manage memory for you  ensuring correct behaviour and simplifying things considerably   std  vector lt int gt  fn         std  vector lt int gt  x      x push back 10       return x     std  array lt int  2 gt  fn2       C  11       return  3  4      void caller         std  vector lt int gt  a   fn        const std  vector lt int gt  amp  b   fn       extend lifetime but read-only                                          b valid until scope exit return      std  array lt int  2 gt  c   fn2        const std  array lt int  2 gt  amp  d   fn2        The practice of creating a const reference to the returned data can sometimes avoid a copy  but normally you can just rely on Return Value Optimisation  or - for vector but not array - move semantics  introduced with C  11    If you really want to use an inbuilt array  as distinct from the Standard library class called array mentioned above   one way is for the caller to reserve space and tell the function to use it   void fn int x    int n        for  int i   0  i  lt  n    i          x i    n     void caller            local space on the stack - destroyed when caller   returns     int x 10       fn x  sizeof x   sizeof x 0            or  use the heap  lives until delete   p  called        int  p   new int 10       fn p  10       Another option is to wrap the array in a structure  which - unlike raw arrays - are legal to return by value from a function   struct X       int x 10       X fn         X x      x x 0    10                 return x     void caller         X x   fn        Starting with the above  if you re stuck using C  03 you might want to generalise it into something closer to the C  11 std  array   template  lt typename T  size t N gt  struct array       T amp  operator   size t n    return x n         const T amp  operator   size t n  const   return x n         size t size   const   return N           iterators  constructors etc       private      T x N        Another option is to have the called function allocate memory on the heap   int  fn         int  p   new int 2       p 0    0      p 1    1      return p     void caller         int  p   fn           use p        delete   p      To help simplify the management of heap objects  many C   programmers use  smart pointers  that ensure deletion when the pointer s  to the object leave their scopes   With C  11   std  shared ptr lt int gt  p new int 2      int  p    delete   p       std  unique ptr lt int   gt  p new int 3      If you re stuck on C  03  the best option is to see if the boost library is available on your machine  it provides boost  shared array   Yet another option is to have some static memory reserved by fn    though this is NOT THREAD SAFE  and means each call to fn   overwrites the data seen by anyone keeping pointers from previous calls   That said  it can be convenient  and fast  for simple single-threaded code   int  fn int n        static int x 2       clobbered by each call to fn       x 0    n      x 1    n   1      return x      every call to fn   returns a pointer to the same static x memory    void caller         int  p   fn 3          use p  hoping no other thread calls fn   meanwhile and clobbers the values           no clean up necessary

User · Answer

Well if you want to return your array from a function you must make sure that the values are not stored on the stack as they will be gone when you leave the function   So either make your array static or allocate the memory  or pass it in but your initial attempt is with a void parameter   For your method I would define it like this   int  gnabber      static int foo      1 2 3    return foo

[c++] How to return an array from a function?

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