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

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< cpp‎ | container‎ | list

| (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. Invalidates only the iterators and references to the removed elements.

1) Uses operator== to compare the elements.
2) Uses p to compare the elements.

The behavior is undefined if the corresponding comparator does not establish an equivalence relation.

### 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 elements removed. | (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 | Comment
--- | --- | --- | ---
`__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

unique | removes consecutive duplicate elements in a range (function template)

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