std::list::unique

C++

Language

Headers

Freestanding and hosted implementations

Named requirements

Language support library

Concepts library (C++20)

Diagnostics library

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Filesystem library (C++17)

Technical Specifications

Containers library

array (C++11)

vector

deque

forward_list (C++11)

list

set

multiset

map

multimap

unordered_set (C++11)

unordered_multiset (C++11)

unordered_map (C++11)

unordered_multimap (C++11)

stack

queue

priority_queue

span (C++20)

std::list

Member functions

list::list

list::~list

list::operator=

list::assign

list::get_allocator

Element access

list::front

list::back

Iterators

list::beginlist::cbegin (C++11)

list::endlist::cend (C++11)

list::rbeginlist::crbegin (C++11)

list::rendlist::crend (C++11)

Capacity

list::empty

list::size

list::max_size

Modifiers

list::clear

list::insert

list::emplace (C++11)

list::erase

list::push_front

list::emplace_front (C++11)

list::pop_front

list::push_back

list::emplace_back (C++11)

list::pop_back

list::resize

list::swap

Operations

list::merge

list::splice

list::removelist::remove_if

list::reverse

list::unique

list::sort

Deduction guides(C++17)

	(1)
void unique();			(until C++20)
size_type unique();			(since C++20)
		(2)
template< class BinaryPredicate > void unique( BinaryPredicate p );				(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 first version uses operator== to compare the elements, the second version uses the given binary predicate p.

Parameters

p

-

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); The signature does not need to have const &, but the function must not modify the objects passed to it. The types Type1 and Type2 must be such that an object of type 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

Linear in the size of the container

Example

Run this code

#include <iostream>
#include <list>

int main()
{
  std::list<int> x = {1, 2, 2, 3, 3, 2, 1, 1, 2};

  std::cout << "contents before:";
  for (auto val : x)
    std::cout << ' ' << val;
  std::cout << '\n';

  x.unique();
  std::cout << "contents after unique():";
  for (auto val : x)
    std::cout << ' ' << val;
  std::cout << '\n';

  return 0;
}

Output:

contents before: 1 2 2 3 3 2 1 1 2
contents after unique(): 1 2 3 2 1 2

See also

unique

removes consecutive duplicate elements in a range (function template)