Appending a vector to a vector

Question

Assuming I have 2 standard vectors   vector lt int gt  a  vector lt int gt  b    Let s also say the both have around 30 elements    How do I add the vector b to the end of vector a    The dirty way would be iterating through b and adding each element via vector lt int gt   push back    though I wouldn t like to do that

User · Answer

std  copy  b begin    b end    std  back inserter a      This can be used in case the items in vector a have no assignment operator  e g  const member    In all other cases this solution is ineffiecent compared to the above insert solution

User · Answer

a insert a end    b begin    b end       or  a insert std  end a   std  begin b   std  end b      The second variant is a more generically applicable solution  as b could also be an array  However  it requires C  11   If you want to work with user-defined types  use ADL   using std  begin  std  end  a insert end a   begin b   end b

User · Answer

While saying  the compiler can reserve   why rely on it   And what about automatic detection of move semantics   And what about all that repeating of the container name with the begins and ends   Wouldn t you want something  you know  simpler    Scroll down to main for the punchline    include  lt type traits gt   include  lt vector gt   include  lt iterator gt   include  lt iostream gt   template lt typename C typename void gt  struct can reserve  std  false type      template lt typename T  typename A gt  struct can reserve lt std  vector lt T A gt  void gt       std  true type      template lt int n gt  struct secret enum   enum class type        template lt int n gt  using SecretEnum   typename secret enum lt n gt   type   template lt bool b  int override num 1 gt  using EnableFuncIf   typename std  enable if lt  b  SecretEnum lt override num gt   gt   type  template lt bool b  int override num 1 gt  using DisableFuncIf   EnableFuncIf lt   b  -override num  gt    template lt typename C  EnableFuncIf lt  can reserve lt C gt   value  gt      gt  void try reserve  C amp  c  std  size t n       c reserve n     template lt typename C  DisableFuncIf lt  can reserve lt C gt   value  gt      gt  void try reserve  C amp  c  std  size t          do nothing  template lt typename C typename void gt  struct has size method std  false type     template lt typename C gt  struct has size method lt C  typename std  enable if lt std  is same lt    decltype  std  declval lt C gt    size        decltype  std  declval lt C gt    size      gt   value gt   type gt  std  true type      namespace adl aux     using std  begin  using std  end    template lt typename C gt    auto adl begin C amp  amp c - gt decltype  begin std  forward lt C gt  c        template lt typename C gt    auto adl end C amp  amp c - gt decltype  end std  forward lt C gt  c        template lt typename C gt  struct iterable traits       typedef decltype  adl aux  adl begin std  declval lt C amp  gt       iterator      typedef decltype  adl aux  adl begin std  declval lt C const amp  gt       const iterator     template lt typename C gt  using Iterator   typename iterable traits lt C gt   iterator  template lt typename C gt  using ConstIterator   typename iterable traits lt C gt   const iterator  template lt typename I gt  using IteratorCategory   typename std  iterator traits lt I gt   iterator category   template lt typename C  EnableFuncIf lt  has size method lt C gt   value  1 gt      gt  std  size t size at least  C amp  amp  c         return c size       template lt typename C  EnableFuncIf lt   has size method lt C gt   value  amp  amp    std  is base of lt  std  random access iterator tag  IteratorCategory lt Iterator lt C gt  gt   gt   value  2 gt      gt  std  size t size at least  C amp  amp  c         using std  begin  using std  end    return end c -begin c      template lt typename C  EnableFuncIf lt   has size method lt C gt   value  amp  amp     std  is base of lt  std  random access iterator tag  IteratorCategory lt Iterator lt C gt  gt   gt   value  3 gt      gt  std  size t size at least  C amp  amp  c       return 0      template  lt  typename It  gt  auto try make move iterator It i  std  true type  - gt  decltype make move iterator i         return make move iterator i     template  lt  typename It  gt  It try make move iterator It i             return i       include  lt iostream gt  template lt typename C1  typename C2 gt  C1 amp  amp  append containers  C1 amp  amp  c1  C2 amp  amp  c2       using std  begin  using std  end    try reserve  c1  size at least c1    size at least c2        using is rvref   std  is rvalue reference lt C2 amp  amp  gt     c1 insert  end c1                try make move iterator begin c2   is rvref                  try make move iterator end c2   is rvref          return std  forward lt C1 gt  c1      struct append infix op    append  template lt typename LHS gt  struct append on right op     LHS lhs    template lt typename RHS gt    LHS amp  amp  operator   RHS amp  amp  rhs         return append containers  std  forward lt LHS gt  lhs   std  forward lt RHS gt  rhs             template lt typename LHS gt  append on right op lt LHS gt  operator   LHS amp  amp  lhs  append infix op       return   std  forward lt LHS gt  lhs       template lt typename LHS typename RHS gt  typename std  remove reference lt LHS gt   type operator   append on right op lt LHS gt  amp  amp  lhs  RHS amp  amp  rhs       typename std  decay lt LHS gt   type retval   std  forward lt LHS gt  lhs lhs     return append containers  std  move retval   std  forward lt RHS gt  rhs        template lt typename C gt  void print container  C amp  amp  c       for  auto amp  amp  x c       std  cout  lt  lt  x  lt  lt         std  cout  lt  lt    n       int main       std  vector lt int gt  a    0 1 2     std  vector lt int gt  b    3 4 5     print container a     print container b     a  append  b    const int arr      6 7 8     a  append  arr    print container a     print container b     std  vector lt double gt  d     std  vector lt double gt  -3 14  -2  -1   append  a      print container d     std  vector lt double gt  c   std  move d   append  a    print container c     print container d     std  vector lt double gt  e   c  append  std  move a     print container e     print container a       hehe   Now with move-data-from-rhs  append-array-to-container  append forward list-to-container  move-container-from-lhs  thanks to  DyP s help   Note that the above does not compile in clang thanks to the EnableFunctionIf lt  gt     technique   In clang this workaround works

User · Answer

If you would like to add vector to itself both popular solutions will fail   std  vector lt std  string gt  v  orig   orig push back  first    orig push back  second        BAD  v   orig  v insert v end    v begin    v end        Now v contains     first    second                BAD  v   orig  std  copy v begin    v end    std  back inserter v       std  bad alloc exception is generated     GOOD  but I can t guarantee it will work with any STL  v   orig  v reserve v size   2   v insert v end    v begin    v end        Now v contains     first    second    first    second        GOOD  but I can t guarantee it will work with any STL  v   orig  v reserve v size   2   std  copy v begin    v end    std  back inserter v       Now v contains     first    second    first    second        GOOD  best   v   orig  v insert v end    orig begin    orig end        note  we use different vectors here    Now v contains     first    second    first    second

[c++] Appending a vector to a vector

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