Is std vector copying the objects with a push back

Question

After a lot of investigations with valgrind  I ve made the conclusion that std  vector makes a copy of an object you want to push back   Is that really true   A vector cannot keep a reference or a pointer of an object without a copy     Thanks

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Not only does std  vector make a copy of whatever you re pushing back  but the definition of the collection states that it will do so  and that you may not use objects without the correct copy semantics within a vector   So  for example  you do not use auto ptr in a vector

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std  vector always makes a copy of whatever is being stored in the vector   If you are keeping a vector of pointers  then it will make a copy of the pointer  but not the instance being to which the pointer is pointing   If you are dealing with large objects  you can  and probably should  always use a vector of pointers   Often  using a vector of smart pointers of an appropriate type is good for safety purposes  since handling object lifetime and memory management can be tricky otherwise

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Relevant in C  11 is the emplace family of member functions  which allow you to transfer ownership of objects by moving them into containers   The idiom of usage would look like  std  vector lt Object gt  objs   Object l value obj      initialize          use object here     objs emplace back std  move l value obj      The move for the lvalue object is important as otherwise it would be forwarded as a reference or const reference and the move constructor would not be called

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From C  11 onwards  all the standard containers  std  vector  std  map  etc  support move semantics  meaning that you can now pass rvalues to standard containers and avoid a copy      Example object class  class object   private      int             m val1      std  string     m val2   public         Constructor for object class      object int val1  std  string  amp  amp val2            m val1 val1           m val2 std  move val2                    std  vector lt object gt  myList       1 Copy into the vector  object foo1 1   foo    myList push back foo1        2 Move into the vector  no copy   object foo2 1024   bar    myList push back std  move foo2         3 Move temporary into vector  no copy   myList push back object 453   baz          4 Create instance of object directly inside the vector  no copy  no move   myList emplace back 453   qux      Alternatively you can use various smart pointers to get mostly the same effect   std  unique ptr example  std  vector lt std  unique ptr lt object gt  gt  myPtrList       5a unique ptr can only ever be moved  auto pFoo   std  make unique lt object gt  1   foo    myPtrList push back std  move pFoo         5b unique ptr can only ever be moved  myPtrList push back std  make unique lt object gt  1   foo       std  shared ptr example  std  vector lt std  shared ptr lt object gt  gt  objectPtrList2       6 shared ptr can be used to retain a copy of the pointer and update both the vector    value and the local copy simultaneously  auto pFooShared   std  make shared lt object gt  1   foo    objectPtrList2 push back pFooShared      Pointer to object stored in the vector  but pFooShared is still valid

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Why did it take a lot of valgrind investigation to find this out  Just prove it to yourself with some simple code e g   std  vector lt std  string gt  vec           std  string obj  hello world          vec push pack obj      std  cout  lt  lt  vec 0   lt  lt  std  endl      If  hello world  is printed  the object must have been copied

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Yes  std  vector lt T gt   push back   creates a copy of the argument and stores it in the vector   If you want to store pointers to objects in your vector  create a std  vector lt whatever  gt  instead of std  vector lt whatever gt     However  you need to make sure that the objects referenced by the pointers remain valid while the vector holds a reference to them  smart pointers utilizing the RAII idiom solve the problem

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Yes  std  vector stores copies  How should vector know what the expected life-times of your objects are   If you want to transfer or share ownership of the objects use pointers  possibly smart pointers like shared ptr  found in Boost or TR1  to ease resource management

User · Answer

if you want not the copies  then the best way is to use a pointer vector or another structure that serves for the same goal   if you want the copies  use directly push back    you dont have any other choice

[c++] Is std::vector copying the objects with a push_back?

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