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

From cppreference.com

Compiler support
Freestanding and hosted
Language
Standard library
Standard library headers
Named requirements
Feature test macros(C++20)
Language support library
Concepts library(C++20)
Diagnostics library
Memory management library
Metaprogramming library(C++11)
General utilities library
Containers library
Iterators library
Ranges library(C++20)
Algorithms library
Strings library
Text processing library
Numerics library
Date and time library
Input/output library
Filesystem library(C++17)
Concurrency support library(C++11)
Execution control library(C++26)
Technical specifications
Symbols index
External libraries

[edit]

Containers library

node-handle (C++17)
Sequence
array (C++11)
vector
vector<bool>
inplace_vector (C++26)
hive (C++26)
deque
forward_list (C++11)
list
Associative
set
multiset
map
multimap
Unordered associative
unordered_set (C++11)
unordered_multiset (C++11)
unordered_map (C++11)
unordered_multimap (C++11)
Adaptors
stack
queue
priority_queue
flat_set (C++23)
flat_multiset (C++23)
flat_map (C++23)
flat_multimap (C++23)
Views
span (C++20)
mdspan (C++23)
Tables
Iterator invalidation
Member function table
Non-member function table

[edit]

std::list

Member functions

list::list
list::~list
list::operator=
list::assign
list::assign_range (C++23)
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::size
list::empty
list::max_size

Modifiers
list::clear
list::insert
list::insert_range (C++23)
list::emplace (C++11)
list::erase
list::push_front
list::emplace_front (C++11)
list::prepend_range (C++23)
list::pop_front
list::push_back
list::emplace_back (C++11)
list::append_range (C++23)
list::pop_back
list::resize
list::swap
Operations
list::merge
list::splice
list::removelist::remove_if
list::reverse
list::unique
list::sort

Non-member functions

operator==operator<=> (C++20)
swap(std::list)
erase(std::list)erase_if(std::list) (C++20)(C++20)

operator!=operator<operator>operator<=operator>=

(until C++20)(until C++20)(until C++20)(until C++20)(until C++20)

Deduction guides(C++17)

[edit]

	(1)
void unique();			(until C++20)
size_type unique();			(since C++20) (constexpr since C++26)
		(2)
template<class BinaryPred> void unique( BinaryPred p);				(until C++20)
template<class BinaryPred> size_type unique( BinaryPred p);				(since C++20) (constexpr since C++26)

Removes allconsecutive duplicate elements from the container. Only the first element in each group of equal elements is left.

1) Equivalent tounique(std::equal_to<T>())(until C++14)unique(std::equal_to<>())(since C++14).

2) Usesp to compare the elements.

Ifp does not establish an equivalence relation, the behavior is undefined.

Invalidates only the iterators and references to the removed elements.

[edit]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 haveconst&, the function must not modify the objects passed to it and must be able to accept all values of type (possibly const)Type1 andType2 regardless ofvalue category (thus,Type1& is not allowed, nor isType1 unless forType1 a move is equivalent to a copy(since C++11)).
The typesType1 andType2 must be such that an object of typelist<T,Allocator>::const_iterator can be dereferenced and then implicitly converted to both of them.

Type requirements

BinaryPred must meet the requirements ofBinaryPredicate.

[edit]Return value

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

[edit]Complexity

Ifempty() istrue, no comparison is performed.

Otherwise, given\(\scriptsize N\)N asstd::distance(begin(), end()):

1) Exactly\(\scriptsize N-1\)N-1 comparisons usingoperator==.

2) Exactly\(\scriptsize N-1\)N-1 applications of the predicatep.

[edit]Notes

Feature-test macro	Value	Std	Feature
`__cpp_lib_list_remove_return_type`	`201806L`	(C++20)	Change the return type

[edit]Example

Run this code

#include <iostream>#include <list> std::ostream& operator<<(std::ostream& os,std::list<int>const& container){for(int val: container)        os<< val<<' ';return os<<'\n';} int main(){std::list<int> c{1,2,2,3,3,2,1,1,2};std::cout<<"Before unique(): "<< c;constauto count1= c.unique();std::cout<<"After unique():  "<< c<< count1<<" elements were removed\n";     c={1,2,12,23,3,2,51,1,2,2};std::cout<<"\nBefore unique(pred): "<< c; constauto count2= c.unique([mod=10](int x,int y){return(x% mod)==(y% mod);}); std::cout<<"After unique(pred):  "<< c<< count2<<" elements were removed\n";}

Output:

Before unique(): 1 2 2 3 3 2 1 1 2After unique():  1 2 3 2 1 23 elements were removed Before unique(pred): 1 2 12 23 3 2 51 1 2 2After unique(pred):  1 2 23 2 51 24 elements were removed