c custom compare function for std sort

Question

I want to create custom compare function for std  sort    to sort some key-value pairs std  pair  Here is my function   template  lt typename K  typename V gt   int comparePairs const void  left  const void  right           if    pair lt K V gt   left - gt first   lt      pair lt K V gt   right - gt first               return 1          else              return -1          Then  inside some class I have vector of pairs class member   vector lt pair lt K V gt  gt  items      And some method for sort this vector by keys  using std  sort    std  sort items begin    items end    comparePairs lt K V gt      I have compilation errors within   which said       cannot convert parameter number from  std  pair lt  Ty1  Ty2   to  const   void        What is a mistake

User · Answer

Look here  http   en cppreference com w cpp algorithm sort   It says   template lt  class RandomIt  class Compare  gt  void sort  RandomIt first  RandomIt last  Compare comp       comp - comparison function which returns  true if the first argument is less than the second  The signature of the comparison function should be equivalent to the following  bool cmp const Type1  amp a  const Type2  amp b     Also  here s an example of how you can use std  sort using a custom C  14 polymorphic lambda   std  sort std  begin container   std  end container                 const auto amp  lhs  const auto amp  rhs        return lhs first  lt  rhs first

User · Answer

Your comparison function is not even wrong   Its arguments should be the type stored in the range  i e  std  pair lt K V gt   not const void    It should return bool not a positive or negative value  Both  bool 1 and  bool -1 are true so your function says every object is ordered before every other object  which is clearly impossible   You need to model the less-than operator  not strcmp or memcmp style comparisons   See StrictWeakOrdering which describes the properties the function must meet

User · Answer

std  pair already has the required comparison operators  which perform lexicographical comparisons using both elements of each pair  To use this  you just have to provide the comparison operators for types for types K and V    Also bear in mind that std  sort requires a strict weak ordeing comparison  and  lt   does not satisfy that  You would need  for example  a less-than comparison  lt  for K and V  With that in place  all you need is  std  vector lt pair lt K V gt  gt  items   std  sort items begin    items end        If you really need to provide your own comparison function  then you need something along the lines of  template  lt typename K  typename V gt  bool comparePairs const std  pair lt K V gt  amp  lhs  const std  pair lt K V gt  amp  rhs      return lhs first  lt  rhs first

[c++] c++ custom compare function for std::sort()

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