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

Removes all consecutive duplicate elements from the container. Only the first element in each group of equal elements is left. The first version uses operator== to compare the elements, the second version uses the given binary predicate p.


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);

The signature does not need to have const &, but the function must not modify the objects passed to it.
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



Linear in the size of the container.


#include <iostream>
#include <list>
int main()
  std::list<int> x = {1, 2, 2, 3, 3, 2, 1, 1, 2};
  std::cout << "contents before:";
  for (auto val : x)
    std::cout << ' ' << val;
  std::cout << '\n';
  std::cout << "contents after unique():";
  for (auto val : x)
    std::cout << ' ' << val;
  std::cout << '\n';
  return 0;


contents before: 1 2 2 3 3 2 1 1 2
contents after unique(): 1 2 3 2 1 2

See also

removes consecutive duplicate elements in a range
(function template)

© cppreference.com
Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.