Using custom std set comparator

Question

I am trying to change the default order of the items in a set of integers to be lexicographic instead of numeric  and I can t get the following to compile with g     file cpp   bool lex compare const int64 t  amp a  const int64 t  amp b         stringstream s1 s2      s1  lt  lt  a      s2  lt  lt  b      return s1 str    lt  s2 str       void foo         set lt int64 t  lex compare gt  s      s insert 1               I get the following error   error  type value mismatch at argument 2 in template parameter list for    template lt class  Key  class  Compare  class  Alloc gt  class std  set    error    expected a type  got    lex compare      what am I doing wrong

User · Answer

You can use a function comparator without wrapping it like so:

bool comparator(const MyType &lhs, const MyType &rhs)
{
    return [...];
}

std::set<MyType, bool(*)(const MyType&, const MyType&)> mySet(&comparator);

which is irritating to type out every time you need a set of that type, and can cause issues if you don't create all sets with the same comparator.

User · Answer

You are using a function where as you should use a functor  a class that overloads the    operator so it can be called like a function    struct lex compare       bool operator    const int64 t amp  lhs  const int64 t amp  rhs  const           stringstream s1  s2          s1  lt  lt  lhs          s2  lt  lt  rhs          return s1 str    lt  s2 str               You then use the class name as the type parameter  set lt int64 t  lex compare gt  s    If you want to avoid the functor boilerplate code you can also use a function pointer  assuming lex compare is a function    set lt int64 t  bool    const int64 t amp  lhs  const int64 t amp  rhs  gt  s  amp lex compare

User · Answer

1  Modern C  20 solution  auto cmp      int a  int b    return        std  set lt int  decltype cmp  gt  s    We use lambda function as comparator  As usual  comparator should return boolean value  indicating whether the element passed as first argument is considered to go before the second in the specific strict weak ordering it defines   Online demo  2  Modern C  11 solution  auto cmp      int a  int b    return        std  set lt int  decltype cmp  gt  s cmp     Before C  20 we need to pass lambda as argument to set constructor  Online demo  3  Similar to first solution  but with function instead of lambda  Make comparator as usual boolean function  bool cmp int a  int b        return          Then use it  either this way   std  set lt int  decltype cmp   gt  s cmp     Online demo  or this way   std  set lt int  decltype  amp cmp  gt  s  amp cmp     Online demo  4  Old solution using struct with    operator  struct cmp       bool operator    int a  int b  const           return                         later std  set lt int  cmp gt  s    Online demo  5  Alternative solution  create struct from boolean function  Take boolean function  bool cmp int a  int b        return          And make struct from it using std  integral constant   include  lt type traits gt  using Cmp   std  integral constant lt decltype  amp cmp    amp cmp gt     Finally  use the struct as comparator  std  set lt X  Cmp gt  set    Online demo

User · Answer

Yacoby s answer inspires me to write an adaptor for encapsulating the functor boilerplate   template lt  class T  bool   comp   T const  amp   T const  amp     gt  class set funcomp       struct ftor           bool operator    T const  amp l  T const  amp r                 return comp  l  r             public      typedef std  set lt  T  ftor  gt  t         usage  bool my comparison  foo const  amp l  foo const  amp r    set funcomp lt  foo  my comparison  gt   t boo     just the way you want it    Wow  I think that was worth the trouble

User · Answer

std  less lt  gt  when using custom classes with operator lt   If you are dealing with a set of your custom class that has operator lt  defined  then you can just use std  less lt  gt    As mentioned at http   en cppreference com w cpp container set find C  14 has added two new find APIs   template lt  class K  gt  iterator find  const K amp  x    template lt  class K  gt  const iterator find  const K amp  x   const    which allow you to do   main cpp   include  lt cassert gt   include  lt set gt   class Point       public             Note that there is  no  conversion constructor             everything is done at the template level without            intermediate object creation            Point int x    x x             Point int x  int y    x x   y y             int x          int y     bool operator lt  const Point amp  c  int x    return c x  lt  x    bool operator lt  int x  const Point amp  c    return x  lt  c x    bool operator lt  const Point amp  c  const Point amp  d        return c x  lt  d     int main         std  set lt Point  std  less lt  gt  gt  s      s insert Point 1  -1        s insert Point 2  -2        s insert Point 0   0        s insert Point 3  -3        assert s find 0 - gt y     0       assert s find 1 - gt y    -1       assert s find 2 - gt y    -2       assert s find 3 - gt y    -3          Ignore 1234  find 1      assert s find Point 1  1234  - gt y    -1       Compile and run   g   -std c  14 -Wall -Wextra -pedantic -o main out main cpp   main out   More info about std  less lt  gt  can be found at  What are transparent comparators   Tested on Ubuntu 16 10  g   6 2 0

[c++] Using custom std::set comparator

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