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std::forward_list<T,Allocator>::unique

	(1)
void unique();		(since C++11) (until C++20)
size_type unique();		(since C++20)
	(2)
template< class BinaryPredicate > void unique( BinaryPredicate p );		(since C++11) (until C++20)
template< class BinaryPredicate > size_type unique( BinaryPredicate p );		(since C++20)

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);

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	Value	Std	Comment
`__cpp_lib_list_remove_return_type`	201806L	(C++20)	Change the return type

Example

#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: