C correct way to return pointer to array from function

Question

I am fairly new to C   and have been avoiding pointers  From what I ve read online I cannot return an array but I can return a pointer to it  I made a small code to test it and was wondering if this was the normal   correct way to do this    include  lt iostream gt  using namespace std   int  test  int in 5         int  out   in      return out     int main         int arr 5     1  2  3  4  5       int  pArr   test arr       for  int i   0  i  lt  5  i    cout lt  lt pArr i  lt  lt endl      cout lt  lt endl      return 0        Edit  This seems to be no good  How should I rewrite it   int  test  int a 5   int b 5         int c 5       for  int i   0  i  lt  5  i    c i    a i  b i       int  out   c      return out

User · Answer

you can  sort of  return an array  instead of  int m1 5     1  2  3  4  5   int m2 5     6  7  8  9  10   int  m3   test m1  m2     write  struct mystruct     int arr 5        int m1 5     1  2  3  4  5   int m2 5     6  7  8  9  10   mystruct m3   test m1 m2     where test looks like  struct mystruct test int m1 5   int m2 5       struct mystruct s    for  int i   0  i  lt  5    i   s arr i  m1 i  m2 i     return s      not very efficient since one is copying it delivers a copy of the array

User · Answer

Your code  which looks ok  doesn t return a pointer to an array  It returns a pointer to the first element of an array   In fact that s usually what you want to do  Most manipulation of arrays are done via pointers to individual elements  not via pointers to the array as a whole   You can define a pointer to an array  for example this   double   p  42     defines p as a pointer to a 42-element array of doubles   A big problem with that is that you have to specify the number of elements in the array as part of the type -- and that number has to be a compile-time constant   Most programs that deal with arrays need to deal with arrays of varying sizes  a given array s size won t vary after it s been created  but its initial size isn t necessarily known at compile time  and different array objects can have different sizes   A pointer to the first element of an array lets you use either pointer arithmetic or the indexing operator    to traverse the elements of the array  But the pointer doesn t tell you how many elements the array has  you generally have to keep track of that yourself   If a function needs to create an array and return a pointer to its first element  you have to manage the storage for that array yourself  in one of several ways  You can have the caller pass in a pointer to  the first element of  an array object  probably along with another argument specifying its size -- which means the caller has to know how big the array needs to be  Or the function can return a pointer to  the first element of  a static array defined inside the function -- which means the size of the array is fixed  and the same array will be clobbered by a second call to the function  Or the function can allocate the array on the heap -- which makes the caller responsible for deallocating it later   Everything I ve written so far is common to C and C    and in fact it s much more in the style of C than C    Section 6 of the comp lang c FAQ discusses the behavior of arrays and pointers in C   But if you re writing in C    you re probably better off using C   idioms  For example  the C   standard library provides a number of headers defining container classes such as  lt vector gt  and  lt array gt   which will take care of most of this stuff for you  Unless you have a particular reason to use raw arrays and pointers  you re probably better off just using C   containers instead   EDIT   I think you edited your question as I was typing this answer  The new code at the end of your question is  as you observer  no good  it returns a pointer to an object that ceases to exist as soon as the function returns  I think I ve covered the alternatives

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New answer to new question   You cannot return pointer to automatic variable  int c 5   from the function  Automatic variable ends its lifetime with return enclosing block  function in this case  - so you are returning pointer to not existing array   Either make your variable dynamic   int  test  int a 5   int b 5         int  c   new int 5       for  int i   0  i  lt  5  i    c i    a i  b i       return c      Or change your implementation to use std  array    std  array lt int 5 gt  test  const std  array lt int 5 gt  amp  a  const std  array lt int 5 gt  amp  b         std  array lt int 5 gt  c      for  int i   0  i  lt  5  i    c i    a i  b i       return c      In case your compiler does not provide std  array you can replace it with simple struct containing an array   struct array int 5       int data 5      int amp  operator    int i    return data i         int operator const    int i    return data i             Old answer to old question   Your code is correct  and     hmm  well      useless  Since arrays can be assigned to pointers without extra function  note that you are already using this in your function    int arr 5     1  2  3  4  5     int  pArr   test arr   int  pArr   arr    Morever signature of your function   int  test  int in 5     Is equivalent to   int  test  int  in    So you see it makes no sense   However this signature takes an array  not pointer   int  test  int   amp in  5

User · Answer

In a real app  the way you returned the array is called using an out parameter  Of course you don t actually have to return a pointer to the array  because the caller already has it  you just need to fill in the array  It s also common to pass another argument specifying the size of the array so as to not overflow it   Using an out parameter has the disadvantage that the caller may not know how large the array needs to be to store the result  In that case  you can return a std  vector or similar array class instance

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Your code as it stands is correct but I am having a hard time figuring out how it could would be used in a real world scenario  With that said  please be aware of a few caveats when returning pointers from functions    When you create an array with syntax int arr 5    it s allocated on the stack and is local to the function  C   allows you to return a pointer to this array  but it is undefined behavior to use the memory pointed to by this pointer outside of its local scope  Read this great answer using a real world analogy to get a much clear understanding than what I could ever explain  You can still use the array outside the scope if you can guarantee that memory of the array has not be purged  In your case this is true when you pass arr to test    If you want to pass around pointers to a dynamically allocated array without worrying about memory leaks  you should do some reading on std  unique ptr std  shared ptr lt  gt     Edit - to answer the use-case of matrix multiplication  You have two options  The naive way is to use std  unique ptr std  shared ptr lt  gt   The Modern C   way is to have a Matrix class where you overload operator   and you absolutely must use the new rvalue references if you want to avoid copying the result of the multiplication to get it out of the function  In addition to having your copy constructor  operator   and destructor  you also need to have move constructor and move assignment operator  Go through the questions and answers of this search to gain more insight on how to achieve this   Edit 2 - answer to appended question  int  test  int a 5   int b 5         int  c   new int 5       for  int i   0  i  lt  5  i    c i    a i  b i       return c      If you are using this as int  res   test a b    then sometime later in your code  you should call delete   res to free the memory allocated in the test   function  You see now the problem is it is extremely hard to manually keep track of when to make the call to delete  Hence the approaches on how to deal with it where outlined in the answer

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A variable referencing an array is basically a pointer to its first element  so yes  you can legitimately return a pointer to an array  because thery re essentially the same thing  Check this out yourself    include  lt assert h gt   int main       int a      1  2  3  4  5       int  pArr   a    int  pFirstElem    amp  a 0       assert a    pArr     assert a    pFirstElem      return 0      This also means that passing an array to a function should be done via pointer  and not via int in 5    and possibly along with the length of the array   int  test int  in  int len        int  out   in      return out      That said  you re right that using pointers  without fully understanding them  is pretty dangerous  For example  referencing an array that was allocated on the stack and went out of scope yields undefined behavior    include  lt iostream gt   using namespace std   int main       int  pArr   0          int a      1  2  3  4  5       pArr   a     or test a  if you wish          a   went out of scope here  but pArr holds a pointer to it       all bets are off  this can output  1   output 1st chapter      of  Romeo and Juliet   crash the program or destroy the      universe   cout  lt  lt  pArr 0   lt  lt  endl     WRONG     return 0      So if you don t feel competent enough  just use std  vector    answer to the updated question   The correct way to write your test function is either this   void test int  a  int  b  int  c  int len      for  int i   0  i  lt  len    i  c i    a i    b i         int main        int a 5           b 5           c 5           test a  b  c  5         c now holds the result     Or this  using std  vector     include  lt vector gt   vector lt int gt  test const vector lt int gt  amp  a  const vector lt int gt  amp  b      vector lt int gt  result a size       for  int i   0  i  lt  a size      i        result i    a i    b i         return result     copy will be elided

User · Answer

Your code is OK  Note though that if you return a pointer to an array  and that array goes out of scope  you should not use that pointer anymore  Example   int  test  void        int out 5       return out      The above will never work  because out does not exist anymore when test   returns  The returned pointer must not be used anymore  If you do use it  you will be reading writing to memory you shouldn t   In your original code  the arr array goes out of scope when main   returns  Obviously that s no problem  since returning from main   also means that your program is terminating   If you want something that will stick around and cannot go out of scope  you should allocate it with new   int  test  void        int  out   new int 5       return out      The returned pointer will always be valid  Remember do delete it again when you re done with it though  using delete     int  array   test              Done with the array  delete   array    Deleting it is the only way to reclaim the memory it uses

[c++] C++ correct way to return pointer to array from function

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