What is the preferred idiomatic way to insert into a map

Question

I have identified four different ways of inserting elements into a std  map   std  map lt int  int gt  function   function 0    42  function insert std  map lt int  int gt   value type 0  42    function insert std  pair lt int  int gt  0  42    function insert std  make pair 0  42      Which of those is the preferred idiomatic way   And is there another way I have not thought of

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As of C  11  you have two major additional options  First  you can use insert   with list initialization syntax   function insert  0  42      This is functionally equivalent to  function insert std  map lt int  int gt   value type 0  42      but much more concise and readable  As other answers have noted  this has several advantages over the other forms    The operator   approach requires the mapped type to be assignable  which isn t always the case  The operator   approach can overwrite existing elements  and gives you no way to tell whether this has happened  The other forms of insert that you list involve an implicit type conversion  which may slow your code down    The major drawback is that this form used to require the key and value to be copyable  so it wouldn t work with e g  a map with unique ptr values  That has been fixed in the standard  but the fix may not have reached your standard library implementation yet   Second  you can use the emplace   method   function emplace 0  42     This is more concise than any of the forms of insert    works fine with move-only types like unique ptr  and theoretically may be slightly more efficient  although a decent compiler should optimize away the difference   The only major drawback is that it may surprise your readers a little  since emplace methods aren t usually used that way

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In short     operator is more efficient for updating values because it involves calling default constructor of the value type and then assigning it a new value  while insert   is more efficient for adding values    The quoted snippet from Effective STL  50 Specific Ways to Improve Your Use of the Standard Template Library by Scott Meyers  Item 24 might help    template lt typename MapType  typename KeyArgType  typename ValueArgType gt  typename MapType  iterator insertKeyAndValue MapType amp  m  const KeyArgType amp k  const ValueArgType amp  v        typename MapType  iterator lb   m lower bound k        if  lb    m end    amp  amp    m key comp   k  lb- gt first              lb- gt second   v          return lb        else           typedef typename MapType  value type MVT          return m insert lb  MVT k  v              You may decide to choose a generic-programming-free version of this  but the point is that I find this paradigm  differentiating  add  and  update   extremely useful

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First of all  operator   and insert member functions are not functionally equivalent     The operator   will search for the key  insert a default constructed value if not found  and return a reference to which you assign a value  Obviously  this can be inefficient if the mapped type can benefit from being directly initialized instead of default constructed and assigned  This method also makes it impossible to determine if an insertion has indeed taken place or if you have only overwritten the value for an previously inserted key The insert member function will have no effect if the key is already present in the map and  although it is often forgotten  returns an std  pair lt iterator  bool gt  which can be of interest  most notably to determine if insertion has actually been done     From all the listed possibilities to call insert  all three are almost equivalent  As a reminder  let s have look at insert signature in the standard    typedef pair lt const Key  T gt  value type                pair lt iterator  bool gt  insert const value type amp  x     So how are the three calls different     std  make pair relies on template argument deduction and could  and in this case will  produce something of a different type than the actual value type of the map  which will require an additional call to std  pair template constructor in order to convert to value type  ie   adding const to first type  std  pair lt int  int gt  will also require an additional call to the template constructor of std  pair in order to convert the parameter to value type  ie   adding const to first type  std  map lt int  int gt   value type leaves absolutely no place for doubt as it is directly the parameter type expected by the insert member function    In the end  I would avoid using operator   when the objective is to insert  unless there is no additional cost in default-constructing and assigning the mapped type  and that I don t care about determining if a new key has effectively inserted  When using insert  constructing a value type is probably the way to go

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I have been running some time comparisons between the abovementioned versions   function 0    42  function insert std  map lt int  int gt   value type 0  42    function insert std  pair lt int  int gt  0  42    function insert std  make pair 0  42      Turns out that time differences between the insert versions are tiny    include  lt map gt   include  lt vector gt   include  lt boost date time posix time posix time hpp gt  using namespace boost  posix time  class Widget   public      Widget             m vec resize 100           for unsigned long it   0  it  lt  100 it                  m vec it    1 0                      Widget double el              m vec resize 100           for unsigned long it   0  it  lt  100 it                  m vec it    el                  private      std  vector lt double gt  m vec       int main int argc  char  argv             std  map lt int Widget gt  map W      ptime t1   boost  posix time  microsec clock  local time            for int it   0  it  lt  10000 it              map W insert std  pair lt int Widget gt  it Widget 2 0               ptime t2   boost  posix time  microsec clock  local time        time duration diff   t2 - t1      std  cout  lt  lt  diff total milliseconds    lt  lt  std  endl       std  map lt int Widget gt  map W 2      ptime t1 2   boost  posix time  microsec clock  local time            for int it   0  it  lt  10000 it              map W 2 insert std  make pair it Widget 2 0               ptime t2 2   boost  posix time  microsec clock  local time        time duration diff 2   t2 2 - t1 2      std  cout  lt  lt  diff 2 total milliseconds    lt  lt  std  endl       std  map lt int Widget gt  map W 3      ptime t1 3   boost  posix time  microsec clock  local time            for int it   0  it  lt  10000 it              map W 3 it    Widget 2 0             ptime t2 3   boost  posix time  microsec clock  local time        time duration diff 3   t2 3 - t1 3      std  cout  lt  lt  diff 3 total milliseconds    lt  lt  std  endl       std  map lt int Widget gt  map W 0      ptime t1 0   boost  posix time  microsec clock  local time            for int it   0  it  lt  10000 it              map W 0 insert std  map lt int Widget gt   value type it Widget 2 0               ptime t2 0   boost  posix time  microsec clock  local time        time duration diff 0   t2 0 - t1 0      std  cout  lt  lt  diff 0 total milliseconds    lt  lt  std  endl       system  pause        This gives respectively for the versions  I ran the file 3 times  hence the 3 consecutive time differences for each    map W insert std  pair lt int Widget gt  it Widget 2 0       2198 ms  2078 ms  2072 ms  map W 2 insert std  make pair it Widget 2 0       2290 ms  2037 ms  2046 ms   map W 3 it    Widget 2 0     2592 ms  2278 ms  2296 ms    map W 0 insert std  map lt int Widget gt   value type it Widget 2 0       2234 ms  2031 ms  2027 ms  Hence  results between different insert versions can be neglected  didn t perform a hypothesis test though    The map W 3 it    Widget 2 0   version takes about 10-15   more time for this example due to an initialization with the default constructor for Widget

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The first version   function 0    42     version 1   may or may not insert the value 42 into the map  If the key 0 exists  then it will assign 42 to that key  overwriting whatever value that key had  Otherwise it inserts the key value pair   The insert functions   function insert std  map lt int  int gt   value type 0  42        version 2 function insert std  pair lt int  int gt  0  42                   version 3 function insert std  make pair 0  42                        version 4   on the other hand  don t do anything if the key 0 already exists in the map  If the key doesn t exist  it inserts the key value pair   The three insert functions are almost identical  std  map lt int  int gt   value type is the typedef for std  pair lt const int  int gt   and std  make pair   obviously produces a std  pair lt  gt  via template deduction magic  The end result  however  should be the same for versions 2  3  and 4   Which one would I use  I personally prefer version 1  it s concise and  natural   Of course  if its overwriting behavior is not desired  then I would prefer version 4  since it requires less typing than versions 2 and 3  I don t know if there is a single de facto way of inserting key value pairs into a std  map   Another way to insert values into a map via one of its constructors   std  map lt int  int gt  quadratic func   quadratic func 0    0  quadratic func 1    1  quadratic func 2    4  quadratic func 3    9   std  map lt int  int gt  my func quadratic func begin    quadratic func end

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I just change the problem a little bit  map of strings  to show another interest of insert   std  map lt int  std  string gt  rancking   rancking 0    42       lt  lt  some compilers  gcc  show no error  rancking insert std  pair lt int  std  string gt  0  42      always a compile error   the fact that compiler shows no error on  rancking 1    42   can have devastating impact

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If you want to overwrite the element with key 0  function 0    42    Otherwise   function insert std  make pair 0  42

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If you want to insert element in std  map - use insert   function  and if you want to find element  by key  and assign some to it - use operator     For simplify inserting use boost  assign library  like this   using namespace boost  assign      For inserting one element   insert  function    0  41        For inserting several elements   insert  function    0  41    0  42    0  43

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Since C  17 std  map offers two new insertion methods  insert or assign   and try emplace    as also mentioned in the comment by sp2danny   insert or assign    Basically  insert or assign   is an  improved  version of operator    In contrast to operator    insert or assign   doesn t require the map s value type to be default constructible  For example  the following code doesn t compile  because MyClass does not have a default constructor   class MyClass   public      MyClass int i    m i i          int m i      int main         std  map lt int  MyClass gt  myMap          VS2017   C2512   MyClass  MyClass    no appropriate default constructor available         Coliru   error  no matching function for call to  MyClass  MyClass        myMap 0    MyClass 1        return 0      However  if you replace myMap 0    MyClass 1   by the following line  then the code compiles and the insertion takes place as intended   myMap insert or assign 0  MyClass 1      Moreover  similar to insert    insert or assign   returns a pair lt iterator  bool gt   The Boolean value is true if an insertion occurred and false if an assignment was done  The iterator points to the element that was inserted or updated   try emplace    Similar to the above  try emplace   is an  improvement  of emplace    In contrast to emplace    try emplace   doesn t modify its arguments if insertion fails due to a key already existing in the map  For example  the following code attempts to emplace an element with a key that is already stored in the map  see      int main         std  map lt int  std  unique ptr lt MyClass gt  gt  myMap2      myMap2 emplace 0  std  make unique lt MyClass gt  1         auto pMyObj   std  make unique lt MyClass gt  2           auto  it  b    myMap2 emplace 0  std  move pMyObj               if   b          std  cout  lt  lt   pMyObj was not inserted   lt  lt  std  endl       if  pMyObj    nullptr          std  cout  lt  lt   pMyObj was modified anyway   lt  lt  std  endl      else         std  cout  lt  lt   pMyObj m i      lt  lt  pMyObj- gt m i  lt  lt   std  endl       return 0      Output  at least for VS2017 and Coliru       pMyObj was not inserted   pMyObj was modified anyway   As you can see  pMyObj no longer points to the original object  However  if you replace auto  it  b    myMap2 emplace 0  std  move pMyObj    by the the following code  then the output looks different  because pMyObj remains unchanged   auto  it  b    myMap2 try emplace 0  std  move pMyObj      Output      pMyObj was not inserted   pMyObj pMyObj m i   2   Code on Coliru  Please note  I tried to keep my explanations as short and simple as possible to fit them into this answer  For a more precise and comprehensive description  I recommend reading this article on Fluent C

[c++] What is the preferred/idiomatic way to insert into a map?

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