Efficient way to return a std vector in c

Question

How much data is copied  when returning a std  vector in a function and how big an optimization will it be to place the std  vector in free-store  on the heap  and return a pointer instead i e  is   std  vector  f       std  vector  result   new std  vector             Insert elements into result        return result       more efficient than   std  vector f       std  vector result           Insert elements into result        return result

User · Answer

As nice as  return by value  might be  it s the kind of code that can lead one into error   Consider the following program        include  lt string gt       include  lt vector gt       include  lt iostream gt      using namespace std      static std  vector lt std  string gt  strings      std  vector lt std  string gt  vecFunc void    return strings         int main int argc  char   argv              set up the vector of strings to hold however          many strings the user provides on the command line       for int idx 1   idx lt argc     idx            strings push back argv idx                      now  iterate the strings and print them using the vector function          as accessor       for std  vector lt std  string gt   interator idx vecFunc   begin     idx  vecFunc   end       idx            cout  lt  lt   Addr     lt  lt  idx- gt c str    lt  lt  std  endl           cout  lt  lt   Val      lt  lt   idx  lt  lt  std  endl              return 0            Q  What will happen when the above is executed   A   A coredump   Q  Why didn t the compiler catch the mistake   A  Because the program is syntactically  although not semantically  correct  Q  What happens if you modify vecFunc   to return a reference  A  The program runs to completion and produces the expected result  Q  What is the difference   A   The compiler does not have to create and manage anonymous objects   The programmer has instructed the compiler to use exactly one object for the iterator and for endpoint determination  rather than two different objects as the broken example does    The above erroneous program will indicate no errors even if one uses the GNU g   reporting options -Wall -Wextra -Weffc    If you must produce a value  then the following would work in place of calling vecFunc   twice      std  vector lt std  string gt  lclvec vecFunc        for std  vector lt std  string gt   iterator idx lclvec begin     idx  lclvec end       idx       The above also produces no anonymous objects during iteration of the loop  but requires a possible copy operation  which  as some note  might be optimized away under some circumstances   But the reference method guarantees that no copy will be produced   Believing the compiler will perform RVO is no substitute for trying to build the most efficient code you can   If you can moot the need for the compiler to do RVO  you are ahead of the game

User · Answer

vector lt string gt  getseq char   db file    And if you want to print it on main   you should do it in a loop   int main          vector lt string gt  str vec   getseq argv 1         for vector lt string gt   iterator it   str vec begin    it    str vec end    it               cout  lt  lt   it  lt  lt  endl

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It s time I post an answer about RVO  me too     If you return an object by value  the compiler often optimizes this so it doesn t get constructed twice  since it s superfluous to construct it in the function as a temporary and then copy it  This is called return value optimization  the created object will be moved instead of being copied

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A common pre-C  11 idiom is to pass a reference to the object being filled   Then there is no copying of the vector   void f  std  vector  amp  result              Insert elements into result

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In C  11  this is the preferred way   std  vector lt X gt  f      That is  return by value    With C  11  std  vector has move-semantics  which means the local vector declared in your function will be moved on return and in some cases even the move can be elided by the compiler

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You should return by value   The standard has a specific feature to improve the efficiency of returning by value  It s called  copy elision   and more specifically in this case the  named return value optimization  NRVO     Compilers don t have to implement it  but then again compilers don t have to implement function inlining  or perform any optimization at all   But the performance of the standard libraries can be pretty poor if compilers don t optimize  and all serious compilers implement inlining and NRVO  and other optimizations    When NRVO is applied  there will be no copying in the following code   std  vector lt int gt  f         std  vector lt int gt  result          populate the vector         return result     std  vector lt int gt  myvec   f      But the user might want to do this   std  vector lt int gt  myvec      some time later     myvec   f      Copy elision does not prevent a copy here because it s an assignment rather than an initialization  However  you should still return by value  In C  11  the assignment is optimized by something different  called  move semantics   In C  03  the above code does cause a copy  and although in theory an optimizer might be able to avoid it  in practice its too difficult  So instead of myvec   f    in C  03 you should write this   std  vector lt int gt  myvec      some time later     f   swap myvec     There is another option  which is to offer a more flexible interface to the user   template  lt typename OutputIterator gt  void f OutputIterator it            write elements to the iterator like this          it     0       it     1      You can then also support the existing vector-based interface on top of that   std  vector lt int gt  f         std  vector lt int gt  result      f std  back inserter result        return result      This might be less efficient than your existing code  if your existing code uses reserve   in a way more complex than just a fixed amount up front  But if your existing code basically calls push back on the vector repeatedly  then this template-based code ought to be as good

User · Answer

If the compiler supports Named Return Value Optimization  http   msdn microsoft com en-us library ms364057 v vs 80  aspx   you can directly return the vector provide that there is no    Different paths returning different named objects Multiple return paths  even if the same named object is returned on all paths  with EH  states introduced  The named object returned is referenced in an inline asm block    NRVO optimizes out the redundant copy constructor and destructor calls and thus improves overall performance    There should be no real diff in your example

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vector lt string gt  func1   const            vector lt string gt  parts        return vector lt string gt  parts begin   parts end

[c++] Efficient way to return a std::vector in c++

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