Why is it OK to return a vector from a function

Question

Please consider this code  I have seen this type of code several times  words is a local vector  How is it possible to return it from a function   Can we guarantee it will not die    std  vector lt std  string gt  read file const std  string amp  path         std  ifstream file  E   names txt         if   file is open                  std  cerr  lt  lt   Unable to open file   lt  lt    n           std  exit -1              std  vector lt string gt  words   this vector will be returned     std  string token       while  std  getline file  token                      words push back token              return words

User · Answer

This is actually a failure of design  You shouldn t be using a return value for anything not a primitive for anything that is not relatively trivial   The ideal solution should be implemented through a return parameter with a decision on reference pointer and the proper use of a  const  y  ness  as a descriptor   On top of this  you should realise that the label on an array in C and C   is effectively a pointer and its subscription are effectively an offset or an addition symbol   So the label or ptr array ptr     array label thus returning foo offset  is really saying return element at memory pointer location foo   offset of type return type

User · Answer

I do not agree and do not recommend to return a vector   vector  lt double gt  vectorial vector  lt double gt  a  vector  lt double gt  b        vector  lt double gt  c  a 1    b 2  - b 1    a 2   -a 0    b 2    b 0    a 2   a 0    b 1  - b 0    a 1         return c      This is much faster   void vectorial vector  lt double gt  a  vector  lt double gt  b  vector  lt double gt   amp c        c 0    a 1    b 2  - b 1    a 2   c 1    -a 0    b 2    b 0    a 2   c 2    a 0    b 1  - b 0    a 1       I tested on Visual nbsp Studio nbsp 2017 with the following results in release mode   8 01 MOPs by reference  5 09 MOPs returning vector  In debug mode  things are much worse   0 053 MOPS by reference  0 034 MOPs by return vector

User · Answer

Can we guarantee it will not die    As long there is no reference returned  it s perfectly fine to do so  words will be moved to the variable receiving the result   The local variable will go out of scope  after it was moved  or copied

User · Answer

I think you are referring to the problem in C  and C    that returning an array from a function isn t allowed  or at least won t work as expected  - this is because the array return will  if you write it in the simple form  return a pointer to the actual array on the stack  which is then promptly removed when the function returns    But in this case  it works  because the std  vector is a class  and classes  like structs  can  and will  be copied to the callers context   Actually  most compilers will optimise out this particular type of copy using something called  Return Value Optimisation   specifically introduced to avoid copying large objects when they are returned from a function  but that s an optimisation  and from a programmers perspective  it will behave as if the assignment constructor was called for the object   As long as you don t return a pointer or a reference to something that is within the function returning  you are fine

User · Answer

To well understand the behaviour  you can run this code    include  lt iostream gt   class MyClass     public      MyClass     std  cout  lt  lt   run constructor MyClass  MyClass     lt  lt  std  endl         MyClass     std  cout  lt  lt   run destructor MyClass   MyClass     lt  lt  std  endl        MyClass const MyClass amp  x    std  cout  lt  lt   run copy constructor MyClass  MyClass const MyClass amp     lt  lt  std  endl        MyClass amp  operator    const MyClass amp  x    std  cout  lt  lt   run assignation MyClass  operator  const MyClass amp     lt  lt  std  endl        MyClass my function       std  cout  lt  lt   run my function     lt  lt  std  endl    MyClass a    std  cout  lt  lt   my function is going to return a      lt  lt  std  endl    return a     int main int argc  char   argv      MyClass b   my function       MyClass c    c   my function       return 0      The output is the following   run my function   run constructor MyClass  MyClass   my function is going to return a    run constructor MyClass  MyClass   run my function   run constructor MyClass  MyClass   my function is going to return a    run assignation MyClass  operator  const MyClass amp   run destructor MyClass   MyClass   run destructor MyClass   MyClass   run destructor MyClass   MyClass     Note that this example was provided in C  03 context  it could be improved for C      11

User · Answer

Pre C  11   The function will not return the local variable  but rather a copy of it  Your compiler might however perform an optimization where no actual copy action is made   See this question  amp  answer for further details   C  11   The function will move the value  See this answer for further details

[c++] Why is it OK to return a 'vector' from a function?

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