push back vs emplace back

Question

I m a bit confused regarding the difference between push back and emplace back   void emplace back Type amp  amp   Val   void push back const Type amp   Val   void push back Type amp  amp   Val     As there is a push back overload taking a rvalue reference I don t quite see what the purpose of emplace back becomes

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In addition to what visitor said    The function void emplace back Type amp  amp   Val  provided by MSCV10 is non conforming and redundant  because as you noted it is strictly equivalent to push back Type amp  amp   Val    But the real C  0x form of emplace back is really useful  void emplace back Args amp  amp        Instead of taking a value type it takes a variadic list of arguments  so that means that you can now perfectly forward the arguments and construct directly an object into a container without a temporary at all    That s useful because no matter how much cleverness RVO and move semantic bring to the table there is still complicated cases where a push back is likely to make unnecessary copies  or move   For example  with the traditional insert   function of a std  map  you have to create a temporary  which will then be copied into a std  pair lt Key  Value gt   which will then be copied into the map     std  map lt int  Complicated gt  m  int anInt   4  double aDouble   5 0  std  string aString    C         cross your finger so that the optimizer is really good m insert std  make pair 4  Complicated anInt  aDouble  aString          should be easier for the optimizer m emplace 4  anInt  aDouble  aString     So why didn t they implement the right version of emplace back in MSVC  Actually  it bugged me too a while ago  so I asked the same question on the Visual C   blog  Here is the answer from Stephan T Lavavej  the official maintainer of the Visual C   standard library implementation at Microsoft      Q  Are beta 2 emplace functions just some kind of placeholder right now       A  As you may know  variadic templates   aren t implemented in VC10  We   simulate them with preprocessor   machinery for things like   make shared lt T gt     tuple  and the new   things in  lt functional gt   This   preprocessor machinery is relatively   difficult to use and maintain  Also    it significantly affects compilation   speed  as we have to repeatedly   include subheaders  Due to a   combination of our time constraints   and compilation speed concerns  we   haven t simulated variadic templates   in our emplace functions       When variadic templates are   implemented in the compiler  you can   expect that we ll take advantage of   them in the libraries  including in   our emplace functions  We take   conformance very seriously  but   unfortunately  we can t do everything   all at once    It s an understandable decision  Everyone who tried just once to emulate variadic template with preprocessor horrible tricks knows how disgusting this stuff gets

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A nice code for the push back and emplace back is shown here   http   en cppreference com w cpp container vector emplace back  You can see the move operation on push back and not on emplace back

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Optimization for emplace back can be demonstrated in next example   For emplace back constructor A  int x arg  will be called  And for  push back A  int x arg  is called first and move A  A  amp  amp rhs  is called afterwards   Of course  the constructor has to be marked as explicit  but for current example is good to remove explicitness    include  lt iostream gt   include  lt vector gt  class A   public    A  int x arg    x  x arg    std  cout  lt  lt   A  x arg  n       A      x   0  std  cout  lt  lt   A    n       A  const A  amp rhs  noexcept   x   rhs x  std  cout  lt  lt   A  A  amp   n       A  A  amp  amp rhs  noexcept   x   rhs x  std  cout  lt  lt   A  A  amp  amp   n      private    int x      int main              std  vector lt A gt  a      std  cout  lt  lt   call emplace back  n       a emplace back  0               std  vector lt A gt  a      std  cout  lt  lt   call push back  n       a push back  1         return 0      output   call emplace back  A  x arg   call push back  A  x arg  A  A  amp  amp

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emplace back conforming implementation will forward arguments to the vector lt Object gt   value typeconstructor when added to the vector  I recall Visual Studio didn t support variadic templates  but with variadic templates will be supported in Visual Studio 2013 RC  so I guess a conforming signature will be added   With emplace back  if you forward the arguments directly to vector lt Object gt   value type constructor  you don t need a type to be movable or copyable for emplace back function  strictly speaking  In the vector lt NonCopyableNonMovableObject gt  case  this is not useful  since vector lt Object gt   value type  needs a copyable or movable type to grow    But note that this could be useful for std  map lt Key  NonCopyableNonMovableObject gt   since once you allocate an entry in the map  it doesn t need to be moved or copied ever anymore  unlike with vector  meaning that you can use std  map effectively with a mapped type that is neither copyable nor movable

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emplace back shouldn t take an argument of type vector  value type  but instead variadic arguments that are forwarded to the constructor of the appended item   template  lt class    Args gt  void emplace back Args amp  amp     args      It is possible to pass a value type which will be forwarded to the copy constructor   Because it forwards the arguments  this means that if you don t have rvalue  this still means that the container will store a  copied  copy  not a moved copy    std  vector lt std  string gt  vec   vec emplace back std  string  Hello        moves  std  string s   vec emplace back s     copies   But the above should be identical to what push back does  It is probably rather meant for use cases like    std  vector lt std  pair lt std  string  std  string gt   gt  vec   vec emplace back std  string  Hello    std  string  world          should end up invoking this constructor     template lt class U  class V gt  pair U amp  amp  x  V amp  amp  y      without making any copies of the strings

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One more example for lists     constructs the elements in place                                                  emplace back  quot element quot        creates a new object and then copies  or moves  that object  push back ExplicitDataType  quot element quot

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Specific use case for emplace back  If you need to create a temporary object which will then be pushed into a container  use emplace back instead of push back  It will create the object in-place within the container    Notes    push back in the above case will create a temporary object and move it into the container  However  in-place construction used for emplace back would be more performant than constructing and then moving the object  which generally involves some copying   In general  you can use emplace back instead of push back in all the cases without much issue   See exceptions

[c++] push_back vs emplace_back

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