std::unique_copy() algorithm

since C++20

// (1)
template< class InputIt, class OutputIt >
constexpr OutputIt unique_copy( InputIt first, InputIt last, OutputIt d_first );

// (2)
template< class InputIt, class OutputIt, class BinaryPredicate >
constexpr OutputIt unique_copy( InputIt first, InputIt last,
                      OutputIt d_first, BinaryPredicate p );

// (3)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 >
ForwardIt2 unique_copy( ExecutionPolicy&& policy, ForwardIt1 first,
                        ForwardIt1 last, ForwardIt2 d_first );

// (4)
template< class ExecutionPolicy, class ForwardIt1,
          class ForwardIt2, class BinaryPredicate >
ForwardIt2 unique_copy( ExecutionPolicy&& policy,
                        ForwardIt1 first, ForwardIt1 last,
                        ForwardIt2 d_first, BinaryPredicate p );

since C++17

// (1)
template< class InputIt, class OutputIt >
OutputIt unique_copy( InputIt first, InputIt last, OutputIt d_first );

// (2)
template< class InputIt, class OutputIt, class BinaryPredicate >
OutputIt unique_copy( InputIt first, InputIt last,
                      OutputIt d_first, BinaryPredicate p );

// (3)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 >
ForwardIt2 unique_copy( ExecutionPolicy&& policy, ForwardIt1 first,
                        ForwardIt1 last, ForwardIt2 d_first );

// (4)
template< class ExecutionPolicy, class ForwardIt1,
          class ForwardIt2, class BinaryPredicate >
ForwardIt2 unique_copy( ExecutionPolicy&& policy,
                        ForwardIt1 first, ForwardIt1 last,
                        ForwardIt2 d_first, BinaryPredicate p );

until C++17

// (1)
template< class InputIt, class OutputIt >
OutputIt unique_copy( InputIt first, InputIt last, OutputIt d_first );

// (2)
template< class InputIt, class OutputIt, class BinaryPredicate >
OutputIt unique_copy( InputIt first, InputIt last,
                      OutputIt d_first, BinaryPredicate p );

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==.

• (2) Elements are compared using the given binary predicate p.

• (3 - 4) Same as (1 - 2), but executed according to policy.

  Overload Resolution

  These overloads participate in overload resolution only if std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>  (until C++20) std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>  (since C++20) is true.

Undefined Behaviour

The behavior is undefined

if it is not an equivalence relation.

Parameters

first last	The range of elements to process.
d_first	The beginning of the destination range.
policy	The execution policy to use. See execution policy for details.
p	Binary predicate which returns true if the elements should be treated as equal. The signature of the function should be equivalent to the following: bool fun(const Type1& a, const Type2& b); The signature does not need to have const&. The function must not modify the objects passed to it. Must accept all values of type (possibly const) Type1 and Type2, regardless of value category (so 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 them.

Type requirements

InputIt	LegacyInputIterator
OutputIt	LegacyOutputIterator
ForwardIt1 ForwardIt2	LegacyForwardIterator

The type of dereferenced InputIt must meet the requirements of CopyConstructible, if:

• neither InputIt, nor OutputIt satisfies LegacyForwardIterator, or
• InputIt doesn't satisfy LegacyForwardIterator and the value type of InputIt differs from that of OutputIt

Return value

Output iterator to the element past the last written element.

Complexity

For nonempty ranges, exactly std::distance(first, last) - 1 applications of the corresponding predicate.

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.

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.

For the overloads with an ExecutionPolicy, there may be a performance cost if the value type of ForwardIt1 is not both CopyConstructible and CopyAssignable.

Examples

The following copy-prints a vector, replacing all values over 5 with 99 on the fly.

Main.cpp

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

int main()
{
    std::string s1 {"The      string    with many       spaces!"};
    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 == ' ' && c2 == ' '; });

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

Output

before: The      string    with many       spaces!
after:  The string with many spaces!