# std::unique_copy

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< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
template< class InputIt, class OutputIt > OutputIt unique_copy( InputIt first, InputIt last, OutputIt d_first ); | (1) | (constexpr since C++20)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 > ForwardIt2 unique_copy( ExecutionPolicy&& policy, ForwardIt1 first, ForwardIt1 last, ForwardIt2 d_first ); | (2) | (since C++17)
template< class InputIt, class OutputIt, class BinaryPred > OutputIt unique_copy( InputIt first, InputIt last, OutputIt d_first, BinaryPred p ); | (3) | (constexpr since C++20)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class BinaryPred > ForwardIt2 unique_copy( ExecutionPolicy&& policy, ForwardIt1 first, ForwardIt1 last, ForwardIt2 d_first, BinaryPred p ); | (4) | (since C++17)

Copies the elements from the range `[`first`,` last`)`, to another range beginning at d_first in such a way that there are no consecutive equal elements. Only the first element of each group of equal elements is copied.

1) Elements are compared using operator==.
If operator== does not establish an equivalence relation, the behavior is undefined.
3) Elements are compared using the given binary predicate p.
If p does not establish an equivalence relation, the behavior is undefined.
2,4) Same as (1,3), but executed according to policy.
These overloads participate in overload resolution only if all following conditions are satisfied:

std::is_execution_policy_v < std::decay_t <ExecutionPolicy>> is true. | (until C++20)
std::is_execution_policy_v < std::remove_cvref_t <ExecutionPolicy>> is true. | (since C++20)

If *d_first = *first is invalid(until C++20)*first is not writable to d_first(since C++20), the program is ill-formed.

If source and destination ranges overlap, the behavior is undefined.

Given `T` as the value type of `InputIt`, if overload (1) or (3) does not satisfy all of the following conditions, the behavior is undefined:

- `InputIt` meets the requirements of LegacyForwardIterator.

(until C++20)

- `InputIt` models `forward_iterator`.

(since C++20)

- `T` is both CopyConstructible and CopyAssignable.
- All following conditions are satisfied:

  - `OutputIt` meets the requirements of LegacyForwardIterator.
  - The value type of `OutputIt` is also `T`.
  - `T` is CopyAssignable.

### Parameters

first, last | - | the range of elements to process
d_first | - | the beginning of the destination range
policy | - | the execution policy to use

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 InputIt can be dereferenced and then implicitly converted to both of them. ​

Type requirements
- `InputIt` must meet the requirements of LegacyInputIterator.
- `OutputIt` must meet the requirements of LegacyOutputIterator.
- `ForwardIt1, ForwardIt2` must meet the requirements of LegacyForwardIterator.

### Return value

Output iterator to the element past the last written element.

### Complexity

Given \(\scriptsize N\)N as std::distance (first, last):

1,2) Exactly \(\scriptsize max(0,N-1)\)max(0,N-1) comparisons using operator==.
3,4) Exactly \(\scriptsize max(0,N-1)\)max(0,N-1) applications of the predicate p.

For overloads (2,4), there may be a performance cost if the value type of `ForwardIt1` is not both CopyConstructible and CopyAssignable.

### Exceptions

The overloads with a template parameter named `ExecutionPolicy` report errors as follows:

- If execution of a function invoked as part of the algorithm throws an exception and `ExecutionPolicy` is one of the standard policies, std::terminate is called. For any other `ExecutionPolicy`, the behavior is implementation-defined.
- If the algorithm fails to allocate memory, std::bad_alloc is thrown.

### Possible implementation

See also the implementations in libstdc++ and libc++.

### Notes

If `InputIt` satisfies LegacyForwardIterator, this function rereads the input in order to detect duplicates.

Otherwise, if `OutputIt` satisfies LegacyForwardIterator, and the value type of `InputIt` is the same as that of `OutputIt`, this function compare *d_first to *first.

Otherwise, this function compares *first to a local element copy.

### Example

Run this code

    #include <algorithm>
    #include <iostream>
    #include <iterator>
    #include <string>

    int main()
    {
        std::string s1 {"A string with mmmany letters!"};
        std::cout << "Before: " << s1 << '\n';

        std::string s2;
        std::unique_copy(s1.begin(), s1.end(), std::back_inserter(s2),
                         [](char c1, char c2) { return c1 == 'm' && 'm' == c2; });

        std::cout << "After:  " << s2 << '\n';
    }

Output:

    Before: A string with mmmany letters!
    After:  A string with many letters!

### 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 239 | C++98 | the predicate was applied std::distance (first, last) times | applied one time fewer (for non-empty ranges)
LWG 241 | C++98 | the value type of `InputIt` was not required to be CopyConstructible | conditionally required
LWG 538 | C++98 | the value type of `InputIt` was not required to be CopyAssignable | conditionally required
LWG 2439 | C++98 | the value type of `InputIt` was not required to be CopyConstructible if `OutputIt` is a LegacyForwardIterator | conditionally required

### See also

adjacent_find | finds the first two adjacent items that are equal (or satisfy a given predicate) (function template)
unique | removes consecutive duplicate elements in a range (function template)
copycopy_if (C++11) | copies a range of elements to a new location (function template)
ranges::unique_copy (C++20) | creates a copy of some range of elements that contains no consecutive duplicates (algorithm function object)

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