Recommended way to insert elements into map

Question

Possible Duplicate    In STL maps  is it better to use map  insert than           I was wondering  when I insert element into map  what is the recommended way  Should I  map key    value    or  map insert std  pair lt key type  value type gt  key  value      I did the following quick test    include  lt map gt   include  lt string gt   include  lt iostream gt   class Food   public      Food const std  string amp  name    name name    std  cout  lt  lt   constructor with string parameter   lt  lt  std  endl        Food const Food amp  f    name f name    std  cout  lt  lt   copy   lt  lt  std  endl        Food amp  operator  const Food amp  f    name   f name  std  cout  lt  lt       lt  lt  std  endl  return  this         Food     std  cout  lt  lt   default   lt  lt  std  endl        std  string name      int main         std  map lt std  string  Food gt  m0      1  constructor with string parameter 2  copy 3  copy 4  copy        m0 insert std  pair lt std  string  Food gt   Key   Food  Ice Cream          1  constructor with string parameter 2  default 3  copy 4  copy 5              If we do not provide default constructor         C2512   Food  Food    no appropriate default constructor available     m0  Key     Food  Ice Cream         I realize by using member function insert  less value s function call will be involved  So  is using insert a recommended way  Why default constructor is needed  when map key    value way is being used    I know that insert doesn t overwrite existence key value pair  but map key    value does  However  is this the only factor I take into consideration  when try to choose among the both   How about   Performance Availability of value s default constructor

User · Answer

Use insert if you want to insert a new element   insert will not  overwrite an existing element  and you can verify that there was no previously exising element   if    myMap insert  std  make pair  key  value     second             Element already present        Use    if you want to overwrite a possibly existing element   myMap  key     value  assert  myMap find  key  - gt second    value       post-condition   This form will overwrite any existing entry

User · Answer

insert is not a recommended way - it is one of the ways to insert into map  The difference with operator   is that the insert can tell whether the element is inserted into the map  Also  if your class has no default constructor  you are forced to use insert      operator   needs the default constructor because the map checks if the element exists  If it doesn t then it creates one using default constructor and returns a reference  or const reference to it     Because map containers do not allow for duplicate key values  the insertion operation checks for each element inserted whether another element exists already in the container with the same key value  if so  the element is not inserted and its mapped value is not changed in any way

User · Answer

To quote      Because map containers do not allow for duplicate key values  the insertion operation checks for each element inserted whether another element exists already in the container with the same key value  if so  the element is not inserted and its mapped value is not changed in any way    So insert will not change the value if the key already exists  the    operator will   EDIT   This reminds me of another recent question - why use at   instead of the    operator to retrieve values from a vector  Apparently at   throws an exception if the index is out of bounds whereas    operator doesn t  In these situations it s always best to look up the documentation of the functions as they will give you all the details  But in general  there aren t  or at least shouldn t be  two functions operators that do the exact same thing   My guess is that  internally  insert   will first check for the entry and afterwards itself use the    operator

User · Answer

map key    value is provided for easier syntax  It is easier to read and write   The reason for which you need to have default constructor is that map key  is evaluated before assignment  If key wasn t present in map  new one is created  with default constructor  and reference to it is returned from operator

[c++] Recommended way to insert elements into map

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