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

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

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

Invalidates only the iterators and references to the removed elements.

1) Uses operator== to compare the elements.

2) Uses p to compare the elements.

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

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:

boolpred(constType1&a,constType2&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 list<T,Allocator>::const_iterator can be dereferenced and then implicitly converted to both of them.

Type requirements

-BinaryPredicate must meet the requirements of BinaryPredicate.

Return value

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

Complexity

If empty() is true, no comparison is performed.

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

1) Exactly \(\scriptsize N-1\)N-1 comparisons using operator==.

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

Notes

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

Example

import std;

int main()
{
    std::list<int> c{1, 2, 2, 3, 3, 2, 1, 1, 2};
    std::println("Before unique(): {}", c);

    const auto count1 = c.unique();
    std::println("After unique():  {} ({} elements removed)", c, count1);

    c = {1, 2, 12, 23, 3, 2, 51, 1, 2, 2};
    std::println("Before unique(pred): {}", c);

    auto pred = [mod = 10](int x, int y) { return (x % mod) == (y % mod); };
    const auto count2 = c.unique(pred);
    std::println("After unique(pred):  {} ({} elements removed)", c, count2);
}

Output:

Before unique(): [1, 2, 2, 3, 3, 2, 1, 1, 2]
After unique():  [1, 2, 3, 2, 1, 2] (3 elements 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 removed)

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 1207	C++98	it was unclear whether iterators and/or references will be invalidated	only invalidates iterators and references to the removed elements

unique	removes consecutive duplicate elements in a range (function template & algorithm function object)
ranges::unique (C++20)