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

    From cppreference.com

    [edit template]
     
    C++
     
    Containers library
     
    std::list
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    Non-member functions
    (until C++20)(until C++20)(until C++20)(until C++20)(until C++20)
    Deduction guides(C++17)
     
    void unique();
    		(1) (until C++20) 
    size_type unique();
    		(since C++20) 
    template< class BinaryPredicate >
void unique( BinaryPredicate p );
    		(2) (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.

    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

    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
    -
    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 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
    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

    See also

    		 removes consecutive duplicate elements in a range
    (function template & algorithm function object)[edit]
    (C++20)