How can I create my own comparator for a map

Question

typedef map lt string  string gt  myMap    When inserting a new pair to myMap  it will use the key string to compare by its own string comparator  Is it possible to override that comparator  For example  I d like to compare the key string by its length  not by the alphabet  Or is there any other way to sort the map

User · Accepted Answer

std  map takes up to four template type arguments  the third one being a comparator  E g    struct cmpByStringLength       bool operator   const std  string amp  a  const std  string amp  b  const           return a length    lt  b length                     std  map lt std  string  std  string  cmpByStringLength gt  myMap    Alternatively you could also pass a comparator to maps constructor   Note however that when comparing by length you can only have one string of each length in the map as a key

User · Answer

Yes  the 3rd template parameter on map specifies the comparator  which is a binary predicate   Example   struct ByLength   public std  binary function lt string  string  bool gt        bool operator   const string amp  lhs  const string amp  rhs  const               return lhs length    lt  rhs length              int main         typedef map lt string  string  ByLength gt  lenmap      lenmap mymap       mymap  one      one       mymap  a      a       mymap  fewbahr      foobar        for  lenmap  const iterator it   mymap begin    end   mymap end    it    end    it           cout  lt  lt  it- gt first  lt  lt    n

User · Answer

Specify the type of the pointer to your comparison function as the 3rd type into the map  and provide the function pointer to the map constructor  map lt keyType  valueType  typeOfPointerToFunction gt  mapName pointerToComparisonFunction    Take a look at the example below for providing a comparison function to a map  with vector iterator as key and int as value    include  headers h   bool int vector iter comp const vector lt int gt   iterator iter1  const vector lt int gt   iterator iter2        return  iter1  lt   iter2     int main            Without providing custom comparison function     map lt vector lt int gt   iterator  int gt  default comparison          Providing custom comparison function        Basic version     map lt vector lt int gt   iterator  int          bool     const vector lt int gt   iterator iter1  const vector lt int gt   iterator iter2  gt          basic int vector iter comp           use decltype     map lt vector lt int gt   iterator  int  decltype int vector iter comp   gt  with decltype  amp int vector iter comp           Use type alias or using     typedef bool my predicate const vector lt int gt   iterator iter1  const vector lt int gt   iterator iter2       map lt vector lt int gt   iterator  int  my predicate  gt  with typedef  amp int vector iter comp        using my predicate pointer type   bool     const vector lt int gt   iterator iter1  const vector lt int gt   iterator iter2       map lt vector lt int gt   iterator  int  my predicate pointer type gt  with using  amp int vector iter comp            Testing      vector lt int gt  v    1  2  3        default comparison insert pair lt vector lt int gt   iterator  int gt   v end    0         default comparison insert pair lt vector lt int gt   iterator  int gt   v begin    0         default comparison insert pair lt vector lt int gt   iterator  int gt   v begin    1         default comparison insert pair lt vector lt int gt   iterator  int gt   v begin     1  1          cout  lt  lt   size     lt  lt  default comparison size    lt  lt  endl      for  auto amp  p   default comparison            cout  lt  lt    p first   lt  lt        lt  lt  p second  lt  lt  endl             basic insert pair lt vector lt int gt   iterator  int gt   v end    0         basic insert pair lt vector lt int gt   iterator  int gt   v begin    0         basic insert pair lt vector lt int gt   iterator  int gt   v begin    1         basic insert pair lt vector lt int gt   iterator  int gt   v begin     1  1          cout  lt  lt   size     lt  lt  basic size    lt  lt  endl      for  auto amp  p   basic            cout  lt  lt    p first   lt  lt        lt  lt  p second  lt  lt  endl             with decltype insert pair lt vector lt int gt   iterator  int gt   v end    0         with decltype insert pair lt vector lt int gt   iterator  int gt   v begin    0         with decltype insert pair lt vector lt int gt   iterator  int gt   v begin    1         with decltype insert pair lt vector lt int gt   iterator  int gt   v begin     1  1          cout  lt  lt   size     lt  lt  with decltype size    lt  lt  endl      for  auto amp  p   with decltype            cout  lt  lt    p first   lt  lt        lt  lt  p second  lt  lt  endl             with typedef insert pair lt vector lt int gt   iterator  int gt   v end    0         with typedef insert pair lt vector lt int gt   iterator  int gt   v begin    0         with typedef insert pair lt vector lt int gt   iterator  int gt   v begin    1         with typedef insert pair lt vector lt int gt   iterator  int gt   v begin     1  1          cout  lt  lt   size     lt  lt  with typedef size    lt  lt  endl      for  auto amp  p   with typedef            cout  lt  lt    p first   lt  lt        lt  lt  p second  lt  lt  endl

User · Answer

Since C  11  you can also use a lambda expression instead of defining a comparator struct   auto comp      const string amp  a  const string amp  b    return a length    lt  b length       map lt string  string  decltype comp  gt  my map comp    my map  1           a   my map  three       b   my map  two         c   my map  fouuur      d    for auto const  amp kv   my map      cout  lt  lt  kv first  lt  lt  endl    Output      1   two   three   fouuur   I d like to repeat the final note of Georg s answer  When comparing by length you can only have one string of each length in the map as a key   Code on Ideone

[c++] How can I create my own comparator for a map?

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