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

From cppreference.net

C++
Containers library
std::list
Member functions
Non-member functions
(until C++20) (until C++20) (until C++20) (until C++20) (until C++20)
Deduction guides (C++17)
(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 all consecutive duplicate elements from the container. Only the first element in each group of equal elements is left.

1) Equivalent to unique ( std:: equal_to < T > ( ) ) (until C++14) unique ( std:: equal_to <> ( ) ) (since C++14) .
2) Uses p to compare the elements.
If p does not establish an equivalence relation, the behavior is undefined.

Invalidates only the iterators and references to the removed elements.

Contents

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 ) ;

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
-
BinaryPred must meet the requirements of BinaryPredicate .

Return value

(none)

(until C++20)

The number of elements removed.

(since C++20)

Complexity

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

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

1) Exactly N-1 comparisons using operator == .
2) Exactly 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

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;
    const auto 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;
    const auto 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 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

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

See also

removes consecutive duplicate elements in a range
(function template)