std::forward_list::unique(3)

NAME
std::forward_list::unique - std::forward_list::unique

Synopsis
   void unique(); (since C++11)
                        (until C++20)
   size_type unique(); (since C++20)
   template< class BinaryPredicate > (1) (since C++11)
   void unique( BinaryPredicate p ); (2) (until C++20)
   template< class BinaryPredicate > (since C++20)
   size_type unique( BinaryPredicate p );
   Removes all consecutive duplicate elements from the container. Only the first
   element in each group of equal elements is left. The behavior is undefined if the
   selected comparator does not establish an equivalence relation.
   1) Uses operator== to compare the elements.
   2) Uses the given binary predicate p to compare the elements.

Parameters
       binary predicate which returns true if the elements should be treated as
       equal.
       The signature of the predicate function should be equivalent to the following:
       bool pred(const Type1 &a, const Type2 &b);
   p - While the signature does not need to have const &, the function must not modify
       the objects passed to it and must be able to accept all values of type (possibly
       const) Type1 and Type2 regardless of value category (thus, Type1 & is not
       allowed
       , nor is Type1 unless for Type1 a move is equivalent to a copy
       (since C++11)).
       The types Type1 and Type2 must be such that an object of type
       forward_list<T,Allocator>::const_iterator can be dereferenced and then
       implicitly converted to both of them.

Return value
   (none) (until C++20)
   The number of elements removed. (since C++20)

Complexity
   Exactly std::distance(begin(), end()) - 1 comparisons of the elements, if the
   container is not empty. Otherwise, no comparison is performed.

Notes
   Feature-test macro: __cpp_lib_list_remove_return_type

Example
// Run this code
 #include <iostream>
 #include <forward_list>
 auto print = [](auto remark, auto const& container) {
   std::cout << remark;
   for (auto const& val : container)
     std::cout << ' ' << val;
   std::cout << '\n';
 };
 int main()
 {
   std::forward_list<int> c = {1, 2, 2, 3, 3, 2, 1, 1, 2};
   print("Before unique():", c);
   const auto count1 = c.unique();
   print("After unique(): ", c);
   std::cout << count1 << " elements were removed\n";
   c = {1, 2, 12, 23, 3, 2, 51, 1, 2, 2};
   print("Before unique(pred):", c);
   const auto count2 = c.unique([mod=10](int x, int y) {
       return (x % mod) == (y % mod);
   });
   print("After unique(pred): ", c);
   std::cout << count2 << " elements were removed\n";
 }

Output:
 Before unique(): 1 2 2 3 3 2 1 1 2
 After unique(): 1 2 3 2 1 2
 3 elements were removed
 Before unique(pred): 1 2 12 23 3 2 51 1 2 2
 After unique(pred): 1 2 23 2 51 2
 4 elements were removed

See also
   unique removes consecutive duplicate elements in a range
          (function template)