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

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.

# Declarations

void unique();

(until C++20)

size_type unique();

(since C++20)

template< class BinaryPredicate >
void unique( BinaryPredicate p );

(until C++20)

template< class BinaryPredicate >
size_type unique( BinaryPredicate p );

(since C++20)

# 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.

# Return value

(none)

# Notes

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

# Example

#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";
}

# Defect reports

# See also

• unique