std::forward_list<T,Allocator>::unique
(until C++20)
(constexpr since C++26)
void unique( BinaryPred p );
(until C++20)
size_type unique( BinaryPred p );
(constexpr since C++26)
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.
[edit] Parameters
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 forward_list<T,Allocator>::const_iterator can be dereferenced and then implicitly converted to both of them.
BinaryPred must meet the requirements of BinaryPredicate.
[edit] Return value
(none)
(until C++20)The number of elements removed.
(since C++20)[edit] Complexity
If empty() is true, no comparison is performed.
Otherwise, given \(\scriptsize N\)N as std::distance (begin(), end()):
[edit] Notes
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_list_remove_return_type |
201806L |
(C++20) | Change the return type |
[edit] Example
#include <iostream> #include <forward_list> std::ostream & operator<< (std::ostream & os, std::forward_list <int> const& container) { for (int val : container) os << val << ' '; return os << '\n'; } int main() { std::forward_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